• Java Arrays
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Overview of Java

  • Introduction to Java
  • The Complete History of Java Programming Language
  • C++ vs Java vs Python
  • How to Download and Install Java for 64 bit machine?
  • Setting up the environment in Java
  • How to Download and Install Eclipse on Windows?
  • JDK in Java
  • How JVM Works - JVM Architecture?
  • Differences between JDK, JRE and JVM
  • Just In Time Compiler
  • Difference between JIT and JVM in Java
  • Difference between Byte Code and Machine Code
  • How is Java platform independent?

Basics of Java

  • Java Basic Syntax
  • Java Hello World Program
  • Java Data Types
  • Primitive data type vs. Object data type in Java with Examples
  • Java Identifiers

Operators in Java

  • Java Variables
  • Scope of Variables In Java

Wrapper Classes in Java

Input/output in java.

  • How to Take Input From User in Java?
  • Scanner Class in Java
  • Java.io.BufferedReader Class in Java
  • Difference Between Scanner and BufferedReader Class in Java
  • Ways to read input from console in Java
  • System.out.println in Java
  • Difference between print() and println() in Java
  • Formatted Output in Java using printf()
  • Fast I/O in Java in Competitive Programming

Flow Control in Java

  • Decision Making in Java (if, if-else, switch, break, continue, jump)
  • Java if statement with Examples
  • Java if-else
  • Java if-else-if ladder with Examples
  • Loops in Java
  • For Loop in Java
  • Java while loop with Examples
  • Java do-while loop with Examples
  • For-each loop in Java
  • Continue Statement in Java
  • Break statement in Java
  • Usage of Break keyword in Java
  • return keyword in Java
  • Java Arithmetic Operators with Examples
  • Java Unary Operator with Examples
  • Java Assignment Operators with Examples
  • Java Relational Operators with Examples
  • Java Logical Operators with Examples
  • Java Ternary Operator with Examples
  • Bitwise Operators in Java
  • Strings in Java
  • String class in Java
  • Java.lang.String class in Java | Set 2
  • Why Java Strings are Immutable?
  • StringBuffer class in Java
  • StringBuilder Class in Java with Examples
  • String vs StringBuilder vs StringBuffer in Java
  • StringTokenizer Class in Java
  • StringTokenizer Methods in Java with Examples | Set 2
  • StringJoiner Class in Java
  • Arrays in Java
  • Arrays class in Java
  • Multidimensional Arrays in Java
  • Different Ways To Declare And Initialize 2-D Array in Java
  • Jagged Array in Java
  • Final Arrays in Java
  • Reflection Array Class in Java
  • util.Arrays vs reflect.Array in Java with Examples

OOPS in Java

  • Object Oriented Programming (OOPs) Concept in Java
  • Why Java is not a purely Object-Oriented Language?

Classes and Objects in Java

  • Naming Conventions in Java
  • Java Methods

Access Modifiers in Java

  • Java Constructors
  • Four Main Object Oriented Programming Concepts of Java

Inheritance in Java

Abstraction in java, encapsulation in java, polymorphism in java, interfaces in java.

  • 'this' reference in Java
  • Inheritance and Constructors in Java
  • Java and Multiple Inheritance
  • Interfaces and Inheritance in Java
  • Association, Composition and Aggregation in Java
  • Comparison of Inheritance in C++ and Java
  • abstract keyword in java
  • Abstract Class in Java
  • Difference between Abstract Class and Interface in Java
  • Control Abstraction in Java with Examples
  • Difference Between Data Hiding and Abstraction in Java
  • Difference between Abstraction and Encapsulation in Java with Examples
  • Difference between Inheritance and Polymorphism
  • Dynamic Method Dispatch or Runtime Polymorphism in Java
  • Difference between Compile-time and Run-time Polymorphism in Java

Constructors in Java

  • Copy Constructor in Java
  • Constructor Overloading in Java
  • Constructor Chaining In Java with Examples
  • Private Constructors and Singleton Classes in Java

Methods in Java

  • Static methods vs Instance methods in Java
  • Abstract Method in Java with Examples
  • Overriding in Java
  • Method Overloading in Java
  • Difference Between Method Overloading and Method Overriding in Java
  • Differences between Interface and Class in Java
  • Functional Interfaces in Java
  • Nested Interface in Java
  • Marker interface in Java
  • Comparator Interface in Java with Examples
  • Need of Wrapper Classes in Java
  • Different Ways to Create the Instances of Wrapper Classes in Java
  • Character Class in Java
  • Java.Lang.Byte class in Java
  • Java.Lang.Short class in Java
  • Java.lang.Integer class in Java
  • Java.Lang.Long class in Java
  • Java.Lang.Float class in Java
  • Java.Lang.Double Class in Java
  • Java.lang.Boolean Class in Java
  • Autoboxing and Unboxing in Java
  • Type conversion in Java with Examples

Keywords in Java

  • Java Keywords
  • Important Keywords in Java
  • Super Keyword in Java
  • final Keyword in Java
  • static Keyword in Java
  • enum in Java
  • transient keyword in Java
  • volatile Keyword in Java
  • final, finally and finalize in Java
  • Public vs Protected vs Package vs Private Access Modifier in Java
  • Access and Non Access Modifiers in Java

Memory Allocation in Java

  • Java Memory Management
  • How are Java objects stored in memory?
  • Stack vs Heap Memory Allocation
  • How many types of memory areas are allocated by JVM?
  • Garbage Collection in Java
  • Types of JVM Garbage Collectors in Java with implementation details
  • Memory leaks in Java
  • Java Virtual Machine (JVM) Stack Area

Classes of Java

  • Understanding Classes and Objects in Java
  • Singleton Method Design Pattern in Java
  • Object Class in Java
  • Inner Class in Java
  • Throwable Class in Java with Examples

Packages in Java

  • Packages In Java
  • How to Create a Package in Java?
  • Java.util Package in Java
  • Java.lang package in Java
  • Java.io Package in Java
  • Java Collection Tutorial

Exception Handling in Java

  • Exceptions in Java
  • Types of Exception in Java with Examples
  • Checked vs Unchecked Exceptions in Java
  • Java Try Catch Block
  • Flow control in try catch finally in Java
  • throw and throws in Java
  • User-defined Custom Exception in Java
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  • Null Pointer Exception In Java
  • Exception Handling with Method Overriding in Java
  • Multithreading in Java
  • Lifecycle and States of a Thread in Java
  • Java Thread Priority in Multithreading
  • Main thread in Java
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  • Runnable interface in Java
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  • What does start() function do in multithreading in Java?
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  • Thread.sleep() Method in Java With Examples
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File Handling in Java

  • Java.io.File Class in Java
  • Java Program to Create a New File
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  • Java.io.RandomAccessFile Class Method | Set 1
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  • java.lang.Character class methods | Set 1
  • Java IO : Input-output in Java with Examples
  • Java.io.Reader class in Java
  • Java.io.Writer Class in Java
  • Java.io.FileInputStream Class in Java
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  • Java Networking
  • TCP/IP Model
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  • Differences between IPv4 and IPv6
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  • Socket Programming in Java
  • java.net.ServerSocket Class in Java
  • URL Class in Java with Examples

JDBC - Java Database Connectivity

  • Introduction to JDBC (Java Database Connectivity)
  • JDBC Drivers
  • Establishing JDBC Connection in Java
  • Types of Statements in JDBC
  • JDBC Tutorial

In Java, classes and objects are basic concepts of Object Oriented Programming (OOPs) that are used to represent real-world concepts and entities. The class represents a group of objects having similar properties and behavior. For example, the animal type Dog is a class while a particular dog named Tommy is an object of the Dog class.

In this article, we will discuss Java objects and classes and how to implement them in our program.

Java Classes

A class in Java is a set of objects which shares common characteristics/ behavior and common properties/ attributes. It is a user-defined blueprint or prototype from which objects are created. For example, Student is a class while a particular student named Ravi is an object.

Properties of Java Classes

  • Class is not a real-world entity. It is just a template or blueprint or prototype from which objects are created.
  • Class does not occupy memory.
  • Class is a group of variables of different data types and a group of methods.
  • Data member
  • Constructor
  • Nested Class

Class Declaration in Java

Example of java class, components of java classes.

 In general, class declarations can include these components, in order: 

Modifiers : A class can be public or has default access (Refer this for details). Class keyword: class keyword is used to create a class. Class name:  The name should begin with an initial letter (capitalized by convention). Superclass(if any): The name of the class’s parent (superclass), if any, preceded by the keyword extends. A class can only extend (subclass) one parent. Interfaces(if any): A comma-separated list of interfaces implemented by the class, if any, preceded by the keyword implements. A class can implement more than one interface. Body: The class body is surrounded by braces, { }.

Constructors are used for initializing new objects. Fields are variables that provide the state of the class and its objects, and methods are used to implement the behavior of the class and its objects. There are various types of classes that are used in real-time applications such as  nested classes , anonymous classes , and lambda expressions .

Java Objects

An object in Java is a basic unit of Object-Oriented Programming and represents real-life entities. Objects are the instances of a class that are created to use the attributes and methods of a class.  A typical Java program creates many objects, which as you know, interact by invoking methods. An object consists of : 

  • State : It is represented by attributes of an object. It also reflects the properties of an object.
  • Behavior : It is represented by the methods of an object. It also reflects the response of an object with other objects.
  • Identity : It gives a unique name to an object and enables one object to interact with other objects.

Example of an object: dog

Objects in Java

Objects correspond to things found in the real world. For example, a graphics program may have objects such as “circle”, “square”, and “menu”. An online shopping system might have objects such as “shopping cart”, “customer”, and “product”.  

Note: When we create an object which is a non primitive data type, it’s always allocated on the heap memory.

Declaring Objects (Also called instantiating a class)

When an object of a class is created, the class is said to be instantiated . All the instances share the attributes and the behavior of the class. But the values of those attributes, i.e. the state are unique for each object. A single class may have any number of instances.

Declaring Objects in Java

Java Object Declaration

As we declare variables like (type name;). This notifies the compiler that we will use the name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable. So for reference variables , the type must be strictly a concrete class name. In general, we can’t create objects of an abstract class or an interface.  

If we declare a reference variable(tuffy) like this, its value will be undetermined(null) until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object.

Initializing a Java object

The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the class constructor. 

Initialize by using method/function:

This class contains a single constructor. We can recognize a constructor because its declaration uses the same name as the class and it has no return type. The Java compiler differentiates the constructors based on the number and the type of the arguments. The constructor in the Dog class takes four arguments. The following statement provides “tuffy”, “papillon”,5, and “white” as values for those arguments:

The result of executing this statement can be illustrated as :

Memory Allocation in Java objects

Memory Allocation of Java Objects

Note: All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, also called the default constructor. This default constructor calls the class parent’s no-argument constructor (as it contains only one statement i.e super();), or the Object class constructor if the class has no other parent (as the Object class is the parent of all classes either directly or indirectly). 

Ways to Create an Object of a Class

There are four ways to create objects in Java. Strictly speaking, there is only one way(by using a new keyword), and the rest internally use a new keyword. 

1. Using new keyword

It is the most common and general way to create an object in Java. 

2. Using Class.forName(String className) method

There is a pre-defined class in java.lang package with name Class. The forName(String className) method returns the Class object associated with the class with the given string name. We have to give a fully qualified name for a class. On calling the new Instance() method on this Class object returns a new instance of the class with the given string name.

3. Using clone() method

clone() method is present in the Object class. It creates and returns a copy of the object.

4. Deserialization

De-serialization is a technique of reading an object from the saved state in a file. Refer to Serialization/De-Serialization in Java

Creating multiple objects by one type only (A good practice) 

In real-time, we need different objects of a class in different methods. Creating a number of references for storing them is not a good practice and therefore we declare a static reference variable and use it whenever required. In this case, the wastage of memory is less. The objects that are not referenced anymore will be destroyed by the Garbage Collector of Java. 

In the inheritance system, we use a parent class reference variable to store a sub-class object. In this case, we can switch into different subclass objects using the same referenced variable. 

Anonymous Objects in Java

Anonymous objects are objects that are instantiated but are not stored in a reference variable.  

  • They are used for immediate method calls.
  • They will be destroyed after method calling.
  • They are widely used in different libraries. For example, in AWT libraries, they are used to perform some action on capturing an event(eg a key press).
  • In the example below, when a key button(referred to by the btn) is pressed, we are simply creating an anonymous object of EventHandler class for just calling the handle method.

Difference between Java Class and Objects

The differences between class and object in Java are as follows: 

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Java Classes: writing your own classes, constructors

Creating your own classes. abstraction, constructors.

Museum of Antiquities.

Your task is to design an Artifact class. There are three types of artifacts kept at the museum. We know nothing about the first type except the serial number assigned by the museum (for example: 212121). For the second type, we know the serial number and the culture that created it (for example: 212121, "Aztecs"). For the third type, we know the serial number, the culture that created it, and the century it was created in (for example: 212121, "Aztecs", 12).

Create an Artifact class that describes the antiquities kept at the museum, and write the required set of constructors for the class. Then, in the main() method, create one artifact of each kind.

Dating website

Let's create a user database for a dating website. But here's the problem: you forgot the required order of the arguments, and there's no technical documentation available. Design a User class, which will have the following fields: name ( String ), age ( short ), and height ( int ). Create the appropriate number of constructors, so that the name, age, and height can be specified in any order.

writing classes in java

The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available. See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases. See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.

Questions and Exercises: Classes

Consider the following class:

What are the class variables?

What are the instance variables?

What is the output from the following code:

Write a class whose instances represent a single playing card from a deck of cards. Playing cards have two distinguishing properties: rank and suit. Be sure to keep your solution as you will be asked to rewrite it in Enum Types .

You can use the assert statement to check your assignments. You write:

If the boolean expression is false, you will get an error message. For example,

should return true , so there will be no error message.

If you use the assert statement, you must run your program with the ea flag:

Write a class whose instances represent a full deck of cards. You should also keep this solution.

3. Write a small program to test your deck and card classes. The program can be as simple as creating a deck of cards and displaying its cards.

Check your answers.

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Writing Classes

Now that we have learned about what classes are... but wouldn't it be great if we could write our own classes? Well, it turns out that we can! In this section, we will go over the different parts that make up a typical class in Java, parts that you will have to write yourself in order to make your class.

As a reminder, a class is a general blueprint for a certain type of object. For example, we could have a class named Dog that serves as a general blueprint for objects of the type Dog .

What Goes Inside a Class?

Usually, most classes will include the following features:

  • Instance variables

Constructors

Instance variables.

Hopefully, by now, you know what a variable is. As a reminder, a variable is something that stores information. An instance variable (sometimes also named field is a kind of variable that stores a certain piece information about an object created from a class. It essentially represents a kind of information that is shared among other objects of the same class. The kinds of instance variables a class contains will depend on what pieces of information you would want the class's objects to store! A class will declare what kinds of instance variables it would like its objects to have, and each object will have its own copy (also known as instance ) of each instance variable declared in the class.

So let's use our Dog class example from before... What kind of instance variables would we want to put into a Dog class? Well, let's first think about what kinds of information that different Dog objects would share. All dogs (we are not considering wild ones here) seem to have names. That's one instance variable! What else other kinds of things are similar in dogs? Age seems to be another property that all dogs have. That's another instance variable! These instance variables (as well as other instance variables) can be declared in the Dog class like this:

Constructors are essentially object creators . We use constructors to create new objects using a class. When we create new objects using constructors, we also set any initial values of instance variables if necessary. These initial values of instance variables can be set either with or without the use of parameters.

Here is the continuation of the above example showing how you would write a constructor:

In the above example, the variables in the parenthesis are the parameters that are passed in when you create a new Dog object. This is how you would create a Dog object using the Dog class from above:

That will create a new Dog named Johnny that has an age of 17, a weight of 110.5, and is alive.

It is good to know that you you are not forced to always have a parameter corresponding to an instance variable. You are also allowed to set an instance variable for all objects to the same value, like this:

In the above example, there are only two parameters (the parameters theName and theAge ), but we decided to set the weight of ALL dogs to 122.5 and make ALL dogs alive. So if you wanted to create a new dog using this constructor, you would only need to write this:

This example will create will create a new Dog named Johnny that has an age of 17, a weight of 122.5, and is alive.

In the Karel Java exercises, we said that methods were like ways of teaching Karel new words/commands. Here in non-Karel Java, methods are still essentially the same: they teach the program new commands.

We will go into more detail about methods in later sections. For now, you should know about a specific kind of method that you can write into your classes: the toString method. The toString just "converts" a class's object into something that you will be able to print out using System.out.println .

So for example, you could add a toString method to the Dog class from above, like this:

Notice how the toString method returns a String . So to print out a Dog object, you would go like this:

That above snippet would print out:

What Comes Next?

In this section, we went over the basic elements that make up a class in Java. In the next sections, we will explore the details of writing classes.

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How to write generic classes and methods in Java

Why should we write generic code, 1. writing generic classes in java.

  • T is just a name for a type parameter, like a variable name. That means you can use any name you like for the type parameter. However, there are some conventions for naming type parameters in Java: T for type; E for element; K for key; V for value, etc. And we should follow this convention.  
  • The type parameter can be used as an actual type in the body of the class. As you see in the GeneralDAO class, T is used as parameter type in the save() method and as a return type in the get() method.  
  • Type parameters are never added to the names of constructors or methods. They are used only in the names of classes and interfaces.

* Writing generic classes with more than one type parameter

* using bounded type parameters when writing generic classes, * using multiple bounds, 2. writing generic methods, * using bounded type parameters in generic methods:, * using type wildcards in generic methods:, references:.

  • Defining Simple Generics (The Java Tutorials)
  • Generic Methods (The Java Tutorials)

Related Java Generics Tutorials:

  • What are Generics in Java
  • Generics with extends and super Wildcards and the Get and Put Principle
  • Generics with Subtyping and the Substitution Principle

Other Java Collections Tutorial:

  • Java List Tutorial
  • Java Set Tutorial
  • Java Map Tutorial
  • Java Queue Tutorial
  • Java Stream API Tutorial
  • Understand equals and hashCode in Java
  • Understand object ordering in Java

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  • 6.5 Mutator Methods
  • 6.6 Writing Instance Methods
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6. Writing Classes ¶

In this unit, you will learn to write your own classes and objects.
  • 6.1.1. Creating a Class
  • 6.1.2. Instance Variables
  • 6.1.3. Methods
  • 6.1.4. Object-Oriented Design
  • 6.1.5. Programming Challenge : Riddle Class
  • 6.1.6. Practice
  • 6.1.7. Summary
  • 6.2.1. Practice
  • 6.2.2. Programming Challenge : Student Class
  • 6.2.3. Practice
  • 6.2.4. Summary
  • 6.3.1. Comments
  • 6.3.2. Preconditions and Postconditions
  • 6.3.3. Software Validity and Use-Case Diagrams
  • 6.3.4. Agile Software Development
  • 6.3.5. Programming Challenge : Comments and Conditions
  • 6.3.6. Summary
  • 6.4.1. toString()
  • 6.4.2. Programming Challenge : Class Pet
  • 6.4.3. Practice
  • 6.4.4. Summary
  • 6.5.1. Programming Challenge : Class Pet Setters
  • 6.5.2. Practice
  • 6.5.3. Summary
  • 6.6.1. Practice
  • 6.6.2. Summary
  • 6.7.1. Programming Challenge : Static Song and counter
  • 6.7.2. Summary
  • 6.8.1. Programming Challenge : Debugging
  • 6.8.2. Practice
  • 6.8.3. Summary
  • 6.9.1. Programming Challenge : Bank Account
  • 6.9.2. Practice
  • 6.9.3. Summary
  • 6.10.1. Impacts of CS
  • 6.10.2. Summary
  • 6.11.1. Concept Summary
  • 6.11.2. Java Keyword Summary
  • 6.11.3. Vocabulary Practice
  • 6.11.4. Common Mistakes
  • 6.12. Exercises
  • 6.13. Lesson Workspace

Learn Java practically and Get Certified .

Popular Tutorials

Popular examples, reference materials, learn java interactively, java introduction.

  • Java Hello World
  • Java JVM, JRE and JDK
  • Java Variables and Literals
  • Java Data Types
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  • Java Expressions & Blocks
  • Java Comment

Java Flow Control

  • Java if...else
  • Java switch Statement
  • Java for Loop
  • Java for-each Loop
  • Java while Loop
  • Java break Statement
  • Java continue Statement
  • Java Arrays
  • Multidimensional Array
  • Java Copy Array

Java OOP (I)

  • Java Class and Objects
  • Java Methods
  • Java Method Overloading
  • Java Constructor
  • Java Strings
  • Java Access Modifiers
  • Java this keyword
  • Java final keyword
  • Java Recursion
  • Java instanceof Operator

Java OOP (II)

  • Java Inheritance
  • Java Method Overriding
  • Java super Keyword
  • Abstract Class & Method
  • Java Interfaces
  • Java Polymorphism
  • Java Encapsulation

Java OOP (III)

  • Nested & Inner Class
  • Java Static Class
  • Java Anonymous Class
  • Java Singleton
  • Java enum Class
  • Java enum Constructor
  • Java enum String
  • Java Reflection
  • Java Exception Handling
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  • Java ArrayList
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  • Java PriorityQueue
  • Java Deque Interface
  • Java LinkedList
  • Java ArrayDeque
  • Java BlockingQueue Interface
  • Java ArrayBlockingQueue
  • Java LinkedBlockingQueue
  • Java Map Interface
  • Java HashMap
  • Java LinkedHashMap
  • Java WeakHashMap
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  • Java OutputStreamWriter Class

Java Writer Class

The Writer class of the java.io package is an abstract superclass that represents a stream of characters.

Since Writer is an abstract class, it is not useful by itself. However, its subclasses can be used to write data.

  • Subclasses of Writer

In order to use the functionality of the Writer , we can use its subclasses. Some of them are:

  • BufferedWriter
  • OutputStreamWriter
  • StringWriter

BufferedWriter, InputStreamWriter, and StringWriter are subclasses of Writer.

We will learn about all these subclasses in the next tutorial.

  • Create a Writer

In order to create a Writer , we must import the java.io.Writer package first. Once we import the package, here is how we can create the writer.

Here, we have created a writer named output using the FileWriter class. It is because the Writer is an abstract class. Hence we cannot create an object of Writer .

Note : We can also create writers from other subclasses of the Writer class.

  • Methods of Writer

The Writer class provides different methods that are implemented by its subclasses. Here are some of the methods:

  • write(char[] array) - writes the characters from the specified array to the output stream
  • write(String data) - writes the specified string to the writer
  • append(char c) - inserts the specified character to the current writer
  • flush() - forces to write all the data present in the writer to the corresponding destination
  • close() - closes the writer
  • Example: Writer Using FileWriter

Here is how we can implement the Writer using the FileWriter class.

In the above example, we have created a writer using the FileWriter class. The writer is linked with the file output.txt .

To write data to the output.txt file, we have implemented these methods.

When we run the program, the output.txt file is filled with the following content.

To learn more, visit Java Writer (official Java documentation) .

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‘HelloWorld’ and ‘main()’ meet minimalistic

Mala Gupta

“Just ignore the terms class , public , static and args[] for now; we’ll talk about it later”. If you have ever mentioned these lines to a new Java student, who is about to execute their first ‘HelloWorld’ program, know that this is changing. If you are that student, well, congratulations, getting started is simpler in Java :-).

Initially released as a preview language feature in Java 21, Implicitly Declared Classes and Instance Main Methods is currently being previewed for the second time in Java 22. This feature is for folks starting to learn Java. It simplifies the initial steps for students when they start learning basics, such as variable assignment, sequence, conditions and iteration. Students no longer need to declare an explicit class to develop their code, or write their main() method using this signature – public static void main(String []) . With this feature, classes could be declared implicitly and the main() method can be created with a shorter list of keywords.

In this blog post, I’ll cover why and how this feature is helpful for students using hands-on coding examples. I’ll also cover some details and frequently asked questions relevant to educators or experienced programmers. Let’s get started.

Hi Students – Your HelloWorld code is changing

HelloWorld refers to the first piece of executable code that anyone writes when they start learning a new language or a framework. It usually includes executing bare minimum code, such as printing ‘Hello World’ on the console to ensure the setup is okay. In Java, it meant writing a class with a method main() . As the name suggests, main() is one of the most important methods. It defines the entry point of execution, when a class executes.

What happens when you see that your ‘HelloWorld’ code executed as expected? It brings a lot of joy and a sense of winning to the one executing it. I still remember the time when I was able to run my first piece of code in Java. I believe we kind of never forget our firsts, both in life, love and coding!

Let’s see how your HelloWorld code is changing.

Class ‘HelloWorld’ before and after Java 21

Before Java 21, you would need to define a class, say, HelloWorld , that defined a main() method with a specific list of keywords, to print any text, say, ‘Hello World’ to the console, as follows:

With Java 21, this initial step has been shortened. You can define a source code file, say, HelloWorld.java, with the following code, to print a message to the console (it doesn’t need to define a class; it has a shorter signature for method main() ):

The preceding code is simpler than what was required earlier. Let’s see how this change could help you focus on what you need, rather than what you don’t.

Reducing cognitive complexity

One of the main benefits of the minimalistic main() method and not having to write an explicit class is – the code is concise and to the point. It introduces less concepts or keywords that could otherwise overwhelm new students.

Of course, you could ignore the terms class , public , static , args[] as suggested by your teacher, but they do use space in your memory every time you look at it. These little things pile up, in coding or life, overwhelming one. The minimalist main() and implicit enclosing class reduce cognitive complexity for you.

Compiling and executing your code

Once you are done writing your code, the next step is to execute it.

On the command prompt, you could use the javac and java commands to compile and execute your code. Assuming you have defined your code in a source code file HelloWorld.java, you could use the following commands to run and execute it:

Since Java 11, it is possible to skip the compilation process for code defined in a single source code file, so you could use just the second command (by specifying the name of the source code file, as follows):

However, since instance main methods and implicit classes is a preview language feature, you should add the flag --enable-preview with --source 22 with these commands, as follows:

Sooner or later, you might switch to using an IDE to write your code. If you wish to use IntelliJ IDEA for creating instance main methods, here’s a quick list of steps to follow. Create a new Java project, select the build system as IntelliJ (so you could use Java compiler and runtime tools), create a new file, say, HelloWorld.java with your instance main method and set the properties to use Java 22, before you run your code, as shown in the following gif (It could save you from typing out the compilation/ execution commands on the command prompt each time you want to execute your code):

Are you wondering if it would be better to create a ‘Java class’ instead of a ‘File’ in the ‘src’ folder? The option of selecting a Java class would generate the body of a bare minimum class, say, public class HelloWorld { } . Since we are trying to avoid unnecessary keywords in the beginning, I recommended creating a new ‘File’ which wouldn’t include any code.

If you plan to use IntelliJ IDEA, here are its configuration details. Java 22 support is available in IntelliJ IDEA 2024.1 EAP . The final release of this version is planned for March 2024.

In your Project Settings, set the SDK to Java 22. For the language level, select ‘22 (Preview) – Statements before super(), string templates (2nd preview etc.)’ on both the Project and Modules tab, as shown in the below settings screenshot:

writing classes in java

Let’s move beyond the simple ‘Hello World’ code and explore further.

What else can main() do apart from printing messages to the console?

It is an interesting question. Let’s check out a few code examples below to learn more about it.

Example 1. Variable declarations, assignments and simple calculations

The initial steps in learning programming start with basics, such as declaring variables, assigning values, simple calculations and checking the values (by printing them to the console). Here’s a common example that covers them all – calculating the total amount returned by a bank for a specified interest rate and principal amount. Let’s define this code in a source code file, say, CalcInterest.java:

I know, the previous example seems like configuring your project-it is important but boring. Let’s talk about something more challenging but that could still be accomplished by a single main() method – like printing a pattern.

Example 2. Print patterns, such as, big letters using a specified character

Here’s another interesting problem that includes basics, such as, variable assignments, conditional statements (via if-else ) and iteration (via for ).

The following code prints a letter P, using the letter X (as specified by the variable charToPrint ), that is 10 lines in height (as specified by the variable size ):

The following gif shows the output of the preceding code:

Do you think you would be up for creating methods that could include logic to print other letters of the alphabet (A – Z), or even other alphabets (like the Devnagri)? Just asking :-)

Before we move forward, did you know you could overload methods, that is, define multiple methods with the same name, such as, ‘main’ in your source code file? Which of these methods main() method would execute when you run your source code file? IntelliJ IDEA displays the run icon next to the main() method in your source code. The following gif shows how that icon moves when you change the signature of a main method:

Let’s take a step forward in the next example by animating words spanning multiple lines.

Example 3. Animating multiline text – one word at a time

Soon, you’ll find yourself using other concepts like, say, iterating words in a String, single and multiple line comments, using other classes from core Java API like Threads, perhaps also some exception handling. Here’s an example of a multiline String that is displayed like animated text, one word at a time (all of which can be coded within the main() method):

If you are feeling adventurous, perhaps you could try other animations too – like animating each letter or decreasing the speed by which the words are printed to console. Are you up for this challenge :-)

Example 4. Data structure problems

I’ve seen that both students and experienced programmers enjoying solving problems that involve data structures. So, here’s one for you (which can be coded using just the main() method): Given an array of integers, find the maximum subarray sum. The subarray must be contiguous, but could be of any length.

Below is the code for your reference:

Example 5. Text based Hangman game

Here’s another example that shows code for a text based hangman game – a simple game that asks you to guess one letter at a time. If the letter appears in the target guess word, it reveals itself in the word. This example uses the Scanner class from the Java API, which could be accessed either by using the full name while defining its variable, or by including an import statement outside the main() method (this helps you to get familiar with using concepts like importing other classes in an incremental way):

Here’s another challenge for you – I just realized that the input in the preceding code is case sensitive. It doesn’t work if you type ‘A’ instead of ‘a’. Could you make it ignore the case of the letters that are typed?

The intent with all these examples is that you could use minimum ceremonial code to start learning basics in Java.

Changing an implicit class to a regular class

When you are ready to level up and work with other concepts like user defined classes, you could also covert the implicit classes and code that we used in the previous examples, to regular classes, as shown below:

In the next section, I’ll answer a few questions that might seem more interesting for folks who are not new to Java and are interested in knowing more about this feature.

Some details and FAQs

Let’s start by sharing that the name of this feature changed from its initial release, which is not common.

A little bit of history

The feature implicitly declared classes and instance main method was initially previewed in Java 21, with a slightly different name, that is, Unnamed Classes and Instance Main Methods . It is not usual for Java features to change their name across their preview versions. However, in this case the name was changed because there have been changes to it in its second review.

In its second preview, when a main method is defined without an enclosing class, it is implicitly declared as a final class during the compilation process.

What do you mean by ‘instance’ main() method?

This feature refers to the main method as an instance main method since it doesn’t use the keywords static with its declaration. In the past, the main method was defined using the static keyword, so that it could be accessed even without the need of creating any objects of the class in which it was defined.

Behind the scenes, a source code file with an instance main method is compiled to a regular class with a no-arg constructor. Its instance main method is executed by creating an instance of this class, followed by calling main() on it.

What happens when you create a source code file with method main(), but no class declaration?

Behind the scenes, the Java compiler creates an implicit top level class, with a no-argument constructor, so that these classes don’t need to be treated in a way that is different to the regular classes.

Here’s a gif that shows a decompiled class for you for the source code file AnimateText.java:

Does this feature introduce a separate beginner’s toolchain?

I’m an educator and I fear that if we introduce simpler processes to students that are not mainstream, that could impact how they transition to learning other Java concepts later. I’m glad to share that this feature doesn’t do that.

With implicit classes and instance main methods, students can still compile and run their code using the same tools and process, as are required when the size of their sample code or count for their source code files increase.

Variations of the main method in the implicit class

As we are aware, a method can be overloaded. Does that imply an implicit class can define multiple main methods? If yes, which one of them qualifies as the ‘main’ method? This is an interesting question. First of all, know that you can’t define a static and non-static main method with the same signature, that is, with the same method parameters. The following method are considered valid main() methods in an implicit class:

If there is no valid main method detected, IntelliJ IDEA could add one for you, as shown in the following gif:

Educators could use this feature to introduce other concepts to the students in an incremental way

If you are an educator, you could introduce your students to other commonly used programming practices, such as creating methods- that is delegating part of your code to another method and calling it from the main method. You could also talk about passing values vs. variables to these methods.

The following gif shows how to do it:

There are other practices too that I would have loved to mention like implementing the abstract methods of an interface, but, I think it would be better to leave them for another blog post.

Preview Features

Implicitly Declared Classes and Instance Main Methods is in its second preview in Java 22. With Java’s new release cadence of six months, new language features are released as preview features. They may be reintroduced in later Java versions in the second or more preview, with or without changes. Once they are stable enough, they may be added to Java as a standard language feature.

Preview language features are complete but not permanent, which essentially means that these features are ready to be used by developers, although their finer details could change in future Java releases depending on developer feedback. Unlike an API, language features can’t be deprecated in the future. So, if you have feedback about any of the preview language features, feel free to share it on the JDK mailing list (free registration required).

Because of how these features work, IntelliJ IDEA is committed to only supporting preview features for the current JDK. Preview language features can change across Java versions, until they are dropped or added as a standard language feature. Code that uses a preview language feature from an older release of the Java SE Platform might not compile or run on a newer release.

Java language designers are reducing the ceremony that is required to write the first HelloWorld code for Java students, by introducing implicitly declared classes and instance main methods. New students can start with bare minimum main() method, such as, void main() and build strong programming foundation by polishing their skills with basics like sequence, selection and iteration.

Some folks might argue that this is a simple change, or that other languages already have it. For the first point, I’d say, it is better to be late than never. Also, in life and coding, it is always better to focus on what you want to do, rather than focusing on what you don’t want to. For the latter point, I’d say, good for those languages :-)

writing classes in java

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Practice questions on Java Classes and Objects

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Java Tutorial

Java methods, java classes, java file handling, java how to, java reference, java examples, java class methods.

You learned from the Java Methods chapter that methods are declared within a class, and that they are used to perform certain actions:

Create a method named myMethod() in Main:

myMethod() prints a text (the action), when it is called . To call a method, write the method's name followed by two parentheses () and a semicolon ;

Inside main , call myMethod() :

Try it Yourself »

Method Parameters

Methods can also have parameters, which is used to pass information:

Create a method that accepts an int parameter called x. In the main() method, we call myMethod() and set an int parameter of 10:

Static vs. Public

You will often see Java programs that have either static or public attributes and methods.

In the example above, we created a static method, which means that it can be accessed without creating an object of the class, unlike public , which can only be accessed by objects:

An example to demonstrate the differences between static and public methods :

Note: You will learn more about these keywords (called modifiers) in the Java Modifiers chapter.

Access Methods With an Object

Create a Car object named myCar . Call the fullThrottle() and speed() methods on the myCar object, and run the program:

Example explained

1) We created a custom Main class with the class keyword.

2) We created the fullThrottle() and speed() methods in the Main class.

3) The fullThrottle() method and the speed() method will print out some text, when they are called.

4) The speed() method accepts an int parameter called maxSpeed - we will use this in 8) .

5) In order to use the Main class and its methods, we need to create an object of the Main Class.

6) Then, go to the main() method, which you know by now is a built-in Java method that runs your program (any code inside main is executed).

7) By using the new keyword we created an object with the name myCar .

8) Then, we call the fullThrottle() and speed() methods on the myCar object, and run the program using the name of the object ( myCar ), followed by a dot ( . ), followed by the name of the method ( fullThrottle(); and speed(200); ). Notice that we add an int parameter of 200 inside the speed() method.

Remember that..

The dot ( . ) is used to access the object's attributes and methods.

To call a method in Java, write the method name followed by a set of parentheses () , followed by a semicolon ( ; ).

A class must have a matching filename ( Main and Main.java ).

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Using Multiple Classes

Like we specified in the Classes chapter , it is a good practice to create an object of a class and access it in another class.

Remember that the name of the java file should match the class name. In this example, we have created two files in the same directory:

Second.java

When both files have been compiled:

Run the Second.java file:

And the output will be:

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Reading and Writing Files in Java

writing classes in java

  • Introduction

In this article, we'll be diving into Reading and Writing Files in Java .

When programming, whether you're creating a mobile app, a web application, or just writing scripts, you often have the need to read or write data to a file. This data could be cache data, data you retrieved for a dataset, an image, or just about anything else you can think of.

In this tutorial, we are going to show the most common ways you can read and write to files in Java.

Java provides several API (also known as Java I/O ) to read and write files since its initial releases. With subsequent releases, Java I/O has been improved, simplified and enhanced to support new features.

Before we get into some actual examples, it would help to understand the classes available to you that will handle the reading and writing of data to files. In the following sections we'll provide a brief overview of the Java I/O classes and explain what they do, then we'll take a look at Java NIO Streams, and finally we'll show some examples of reading and writing data to files.

  • I/O Streams

There are two types of Streams you can use to interact with files:

  • Character Streams
  • Byte Streams

For each of the above stream types, there are several supporting classes shipped with Java, which we'll take a quick look at below.

Character Streams are used to read or write the characters data type. Let's look at the most commonly used classes. All of these classes are defined under the java.io package.

Here are some classes you should know that can be used to read character data:

  • Reader : An abstract class to read a character stream.
  • InputStreamReader : Class used to read the byte stream and converts to character stream.
  • FileReader : A class to read the characters from a file.
  • BufferedReader : This is a wrapper over the Reader class that supports buffering capabilities. In many cases this is the most preferable class to read data because more data can be read from the file in one read() call, reducing the number of actual I/O operations with the file system.

And here are some classes you can use to write character data to a file:

  • Writer : This is an abstract class to write the character streams.
  • OutputStreamWriter : This class is used to write character streams and also convert them to byte streams.
  • FileWriter : A class to actually write characters to the file.
  • BufferedWriter : This is a wrapper over the Writer class, which also supports buffering capabilities. This is the most preferable class to write data to a file since more data can be written to the file in one write() call. And like the BufferedReader , this reduces the number of total I/O operations with the file system.

Byte Streams are used to read or write byte data with files. This is different from before in the way they treat the data. Here you work with raw bytes, which could be characters, image data, Unicode data (which takes 2 bytes to represent a character), etc.

In this section we'll take a look at the most commonly used classes. All of these classes are defined under the java.io package.

Here are the classes used to read the byte data:

  • InputStream : An abstract class to read the byte streams.
  • FileInputStream : A class to simply read bytes from a file.
  • BufferedInputStream : This is a wrapper over InputStream that supports buffering capabilities. As we saw in the character streams, this is a more efficient method than FileInputStream .

And here are the classes used to write the byte data:

  • OutputStream : An abstract class to write byte streams.
  • FileOutputStream : A class to write raw bytes to the file.
  • ByteOutputStream : This class is a wrapper over OutputStream to support buffering capabilities. And again, as we saw in the character streams, this is a more efficient method than FileOutputStream thanks to the buffering.
  • Java NIO Streams

Java NIO is a non-blocking I/O API which was introduced back in Java 4 and can be found in the java.nio package. In terms of performance, this is a big improvement in the API for I/O operations.

Buffers, Selectors, and Channels are the three primary components of Java NIO, although in this article we'll focus strictly on using the NIO classes for interacting with files, and not necessarily the concepts behind the API.

As this tutorial is about reading and writing files, we will discuss only the related classes in this short section:

  • Path : This is a hierarchical structure of an actual file location and is typically used to locate the file you want to interact with.
  • Paths : This is a class that provides several utility methods to create a Path from a given string URI.
  • Files : This is another utility class which has several methods to read and write files without blocking the execution on threads.

Using these few classes, you can easily interact with files in a more efficient way.

  • The Difference Between Java I/O and NIO

The main difference between these two packages is that the read() and write() methods of Java IO are blocking calls. By this we mean that the thread calling one of these methods will be blocked until the data has been read or written to the file.

On the other hand, in the case of NIO, the methods are non-blocking. This means that the calling threads can perform other tasks (like reading/writing data from another source or update the UI) while the read or write methods wait for their operation to complete. This can result in a significant performance increase if you're dealing with many IO requests or lots of data.

  • Examples of Reading and Writing Text Files

In the previous sections, we have discussed the different APIs provided by Java and now it's time to use these API classes in some code.

Check out our hands-on, practical guide to learning Git, with best-practices, industry-accepted standards, and included cheat sheet. Stop Googling Git commands and actually learn it!

The example code below handles reading and writing text files using the various classes we detailed above. To simplify things, and provide a better comparison of the actual methods being used, the input and output are going to remain the same between the examples.

Note : To avoid any confusion on the file path, the example code will read on and write from a file in the user's home directory. The user's home directory can be found using System.getProperty("user.home"); , which is what we use in our examples.

  • Reading and Writing with FileReader and FileWriter

Let's start by using the FileReader and FileWriter classes:

Both classes accept a String, representing the path to the file in their constructors. You can also pass a File object as well as a FileDescriptor .

The write() method writes a valid character sequence - either a String , a char[] . Additionally, it can write a single char represented as an int .

The read() method reads and returns character-by-character, allowing us to use the read data in a while loop for example.

Don't forget to close both of these classes after use!

  • Reading and Writing with BufferedReader and BufferedWriter

Using BufferedReader and BufferedWriter classes:

  • Reading and Writing with FileInputStream and FileOutputStream

Using FileInputStream and FileOutputStream classes:

  • Reading and Writing with BufferedInputStream and BufferedOutputStream

Using BufferedInputStream and BufferedOutputStream classes:

  • Reading and Writing with Java.nio Classes

Using the java.nio classes:

Another way to retrieve the content via the Files class, which is more important if you're not reading text data, is to use the readAllBytes method to read the data in to a byte array:

In case you are interested in using streams with java.nio , you can also use the below methods provided by the Files class, which work just like the streams we covered earlier in the article:

In this article we have covered the most common ways to read and write data to a file using both the Java I/O package and the newer Java NIO package. Whenever possible, we recommend to use the Java NIO classes for file operations due to its non-blocking API, and in addition the code is a bit more maintainable and readable.

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How to Write Unit Tests in Java

Let's say you are developing an application. After long hours of coding, you manage to create some cool features. Now, you want to make sure the features are working as you want.

This involves testing if each and every piece of code works as expected. This procedure is known as Unit Testing. Different languages provide their own frameworks for testing.

In this article, I am going to show you how to write unit tests in Java. I'll first explain what testing involves and some concepts you'll need to know. Then, I'll show a few examples to help you understand better.

For this article, I assume you are familiar with Java and the IntelliJ IDE.

Project Setup

For this project, I'll be using IntelliJ IDE. If you don't have it, follow this guide to install the IDE.

In this project, we'll be using the JUnit and Mockito libraries for testing. These are the most commonly used libraries for testing in Java. You'll understand how these libraries are used as you go through the post.

To setup JUnit, follow these steps as described in this guide :

  • From the main menu, select File > New > Project.
  • Select New Project. Specify a name for the project, I'll give it junit-testing-tutorial.
  • Select Maven as a build tool and in language, select Java.
  • From the JDK list, select the JDK you want to use in the project.
  • Click Create.
  • Open pom.xml in the root directory of your project.
  • In pom.xml, press ⌘ + N , and select Add dependency.
  • This will open a tool window, type org.junit.jupiter:junit-jupiter in the search field. Locate the necessary dependency and click Add next to it.
  • Now, click Load Maven Changes in the notification that appears in the top-right corner in the editor.

Now, to set up Mockito, add these two dependencies in your pom.xml :

Note : The version may differ depending on the time you are reading this post.

To complete the setup, create a class Welcome and define your main function there.

Screenshot-2023-03-12-at-6.14.33-PM

What is Unit Testing?

Unit Testing involves testing each and every component of your code to see if they work as expected. It isolates each individual method of your code and performs tests on it. Unit tests helps make sure that your software is working as expected before releasing it.

As a developer, you'll write unit tests as soon as you finish writing a piece of code. Now, you might ask, isn't that the job of a tester? In a way, yes, a tester is responsible for testing software. But, covering every line of code adds a lot of pressure on the tester. So, it's a best practice for developers to write tests for their own code as well.

The goal of unit testing is to make sure that any new functionality does not break the existing functionality. It also helps identify any issues or bugs earlier in the development process and helps ensure that code meets quality standards set by the organisation.

Do's and Don'ts of Unit Testing

Remember the following guidelines while writing tests for your methods:

  • Do test if the expected output of a method matches the actual one.
  • Do test whether the function calls made inside the method are occurring the desired number of times.
  • Do not try to test code that is not a part of the method under test.
  • Do not make API calls, database connections, or network requests while writing your tests.

Now, let's go over some concepts you need to know before we jump into writing tests.

Assertions determine whether your test passes or fails. They compare the expected return value of a method with the actual one. There are a number of assertions you can make at the end of your test.

The Assertions class in JUnit consists of static methods that provide various conditions deciding whether the test passes or not. We'll see these methods as I walk you through each example.

The class whose methods you are testing may have some external dependencies. As mentioned before, you should not try to test code that is not part of the function under test.

But in cases where your function uses an external class, it is a good practice to mock that class – that is, have mock values instead of the actual ones. We'll use the Mockito library for this purpose.

Method Stubbing

External dependencies may not be limited to just classes, but to certain methods as well. Method stubbing should be done when your function is calling an external function in its code. In this case, you make that function return the value you want instead of calling the actual method.

For instance, the method you're testing (A) is calling an external method (B) in its implementation. B makes a database query, fetching all students with marks greater than 80. Making an actual database call isn't a good practice here. So, you stub the method and make it return a dummy list of students you need for testing.

You'll understand this better with examples. There are many other concepts that are a part of testing in Java. But, for now these three are enough for you to get started.

Steps to Perform While Testing

  • Initialize the necessary parameters you'll need to perform the test.
  • Create mock objects and stub any methods if required.
  • Call the method you are testing with the parameters you initialized in Step 1.
  • Add an assertion to check the outcome of your test. This will decide if the test passes.

You'll understand these steps more with examples. Let's start with a basic test first.

How to Write the First Test

Let's write a simple function that compares two numbers. It returns 1 if the first number is greater than the second and returns -1 otherwise.

We'll put this function inside a Basics class:

Pretty simple! Let's write the test for this class. All your tests should be located inside the test folder.

Screenshot-2023-03-13-at-5.12.04-PM

Inside the test folder, create a class BasicTests where you'll write your tests for this class. The name of the class doesn't matter, but it is a good practice to segregate tests according to each class. Also, follow a similar folder structure as the one in your main code.

Unit tests are basically a set of methods you define that test each method of your class. Inside the above class, create a method compare() with a return type of void . Again, you can name the method anything you want.

The @Test annotation indicates that this method is to be run as a test case.

Now, to test the method, you need to create the object of the above class and call the method by passing some values.

Now, use the assertEquals() method of the Assertions class to check if the expected value matches the expected one.

Our test should pass, as the value returned by the method matches the expected one. To check it, run the test by right clicking the green arrow next to the test method.

Screenshot-2023-03-13-at-5.30.14-PM

Your test results will be shown below.

Screenshot-2023-03-13-at-5.32.06-PM

More Test Examples

In the above test, we only tested one scenario. When there's branching in the function, you need to write tests for each condition. Let's introduce some more branching in the above function.

We have already tested the first branch, so let's write tests for the other two.

The @DisplayName annotation displays the text instead of the method name below. Let's run the test.

Screenshot-2023-03-13-at-6.01.15-PM

For the case where the two numbers are equal:

Sorting an Array

Now, let's write the test for the following code that sorts an array.

To write the test for this, we'll follow a similar procedure: call the method and pass an array to it. Use the assertArrayEquals() to write your assertion.

One challenge for you: write code that reverses a string and write a test case for it.

How to Create Mocks and Stubs for Testing

We have seen a few basic examples of unit tests where you made simple assertions. However, the functions you are testing may contain external dependencies like model classes and database or network connections.

Now, you can't make actual connections in your tests, as it would be very time consuming. In such cases, you mock such implementations. Let's see a few examples of mocking.

Mocking a Class

Let's have a class User with the following properties:

Click on ⌘ + N to generate getters and setters for the above properties.

Screenshot-2023-03-18-at-8.34.34-PM

Let's take a new class Mocking that uses the above object.

This class has a method that assigns certain permissions based on the user's role. It returns 1 if the permission is assigned successfully, else it returns -1 .

For demo purposes, I have only added println() statements. The actual implementation may involve setting certain properties.

In the tests file, we'll add an @ExtendWith annotation at the top since we are using Mockito. I have not shown the imports here as IntelliJ does them automatically.

So how do we write the test for the method? We'll need to mock the User object. You can do this by adding a @Mock annotation while declaring the object.

You can also use the mock() method, as it's similar.

Let's write the test method.

When you run the test, it throws a NullPointerException .

Screenshot-2023-03-18-at-8.51.26-PM

This is because the user object hasn't been initialised yet. The method you called wasn't able to use the mocked object. For this, you'll need to call the setUser method.

Now, the test gives the following error as the mocked object is initially filled with null values.

Screenshot-2023-03-18-at-8.53.53-PM

Does this mean you need to fill actual values in the mock object? No, you just need the getRole() method to return a non-null value. For that, we'll use method stubbing.

Using when()...thenReturn() tells the test to return a value when a method is called. You should stub methods only for mocked objects.

We'll do the same for the getUsername() method.

Now, if you run the test, it will pass.

Screenshot-2023-03-18-at-9.00.21-PM

Method Stubbing Example

In the above example, I simply stubbed getter methods to demonstrate method stubbing. Instead of stubbing getters, you could set the role and username with a parameterised constructor or setter methods if they are available.

But what if the user class has a method that returns all the posts containing a certain word in them?

We want this method to return all posts containing the word "awesome". If you call the actual implementation of this method, it might take a long time as the number of posts could be huge. Also, if you are mocking the User object, the posts array would be null.

In this case, you stub the method and make it return the list you want.

Method Stubbing in Database Queries

Let's see how to test methods that involve making database connections. First, create a class ApplicationDao that contains all the methods performing database queries.

Define a method that fetches the user by id and returns null if the user is not found.

Create another method to save a user into the database. This method throws an exception if the user object you are trying to save is null .

Our Mocking class will use these methods to implement its own functionalities. We'll implement one function that updates the name of a user.

The method implementation is pretty straightforward. First, get the user by id , change its username and save the updated user object. We'll write the test cases for this method.

There are two cases we need to test. The first is when the user is updated successfully. The second is when the update fails, that is when an exception is thrown.

Before writing the tests, create a mock of the ApplicationDao object as we do not want to make actual database connections.

Let's write our first test.

Create a user object for testing.

Since we are calling an external method, let's stub the method so that it returns the above User object.

Pass Mockito.anyString() to the method as we want the stub to work for any string parameter. Now, add an assertion to check if the method is working correctly.

The method returns 1 on successful update, so the test passes.

Now, let's test another scenario where the method fails and throws an exception. Simulate this scenario by making the method getUserById() return null.

This value is then passed to the save() method which in turn throws an exception. In our assertion, we'll use assertThrows() method to test whether an exception was thrown. This method takes the type of the exception and a lambda expression as parameters.

Since the exception is thrown, our test passes.

Screenshot-2023-03-26-at-4.27.07-PM

You can find the complete code here on GitHub .

As a developer, writing unit tests for your code is important. It helps you identify bugs earlier in the development process.

In this post, I started by introducing Unit Testing and explained three important concepts involved in the testing process. These gave you some background before jumping into the code.

After that, I showed you, with examples, how you can test different scenarios by using the same basic techniques in testing. I also showed how to use mock classes and methods for testing complex implementations.

Read more posts .

If you read this far, thank the author to show them you care. Say Thanks

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Writing Classes in Java

There are two meanings of “class”. A class could be:

  • a code library (a collection of methods). For example, java.lang.Math contains methods for performing basic numeric operations.
  • an object data type (a description of a concept). For example, java.lang.String represents character strings.

Classes, Objects, Attributes, Operations

A class represents a concept; an object is a realization of the concept. A class is the blueprint of an object; multiple objects can be created from the same class.

There are two kinds of members inside a class:

  • attributes (data/properties, variables)
  • operations (methods, functions)

Attributes keeps the “state” of an object. Attributes are also called data members, properties, or simply variables. These variables are visible to all methods within the class.

Operations, also called methods, functions, provide controls over the “state” (attributes) of an object (initialize, set, get, update, …).

The following defines a class Account.

Visibility modifiers. Visibility modifiers control the access to the member (attribute or operation) of a class. public: can be referenced from outside of the object private: can be used inside the class definition but not externally

Accessor and mutator. An accessor method (getter) provides read-only access to an attribute. A mutator method (setter) changes an attribute. Accessors and mutators are usually named getX() or setX(), where X denotes the attribute name.

Constructor. A constructor is a special method to initialize a new object when it is created. It has the same name as the class; it has no return type.

Class attributes vs. local variables. A class attribute, declared at the class level, has a scope of the entire class. Any method of the class can refer to it. A local variable, declared inside a method, has scope limited the declaring method. It simply does not exist outside of the method. The parameters in a method header serve as local variables.

Static variables and static methods. These static members are declared using the static modifier. For example, private static int count = 0; A static variable is shared among all instances of a class. There is only one copy of a static variable for all objects of a class. A local variable in a method cannot be static.

Static methods can be called through the class name (instead of an object). A static method cannot access non-static attributes of a class; it can, however, access static variables. Static variables and methods are independent of specific objects.

The following class shows how we can use the Account class defined above.

Class Design

Solving a problem by programming is more than just coding. It usually involves the following steps:

  • Requirement analysis. This is mainly about what the finished program will do, what are the inputs, outputs, or the efficiency requirement. Requirement analysis will not specify “how to do”. Requirements are often incomplete, not accurate, and provided by domain experts (but not computer experts).
  • Designing. This is about the ideas to program to meet the requirements, including the designing of classes, methods, and major algorithms.
  • Implementation. This is on the technical details of the solution — writing code by following the design. Note that, important decision should have been made in the design process. Plan before you try, not after you fail.
  • Test. This is to make sure that the program meet the requirements. Mistakes are inevitable, so we need to build test cases to identify mistakes automatically.

Consider the following problem: Finding the distance between two addresses.

The inputs of this case consists of two addresses, for example 515 W Hastings Street, Vancouver 8888 University Drive, Burnaby The output should be a distance, such as 14.0 km. While the major requirement is clear, there are many details that we may need to clarify or make certain assumption. For example, How accurate should the distance be? What address formats are acceptable?

The idea of a solution is as follows. Each address has a geographical position (latitude, longitude). The distance between two addresses can be computed from the two positions. Thus we would like to obtain the geographical position of the two addresses and then compute the distance.

We would like to design two classes:

  • Location, represents a location, with attributes (address, latitude, longitude) and a method to compute the distance).
  • LocationTool, contains a method to build a Location object by finding the latitude and longitude of an address.

The Location class is implemented as below. It has three attributes and one method:

  • address: String
  • latitude: double
  • longitude: double
  • distanceTo(): distance to another location

The LocationTool class is implemented as below. It has a method makeLocation() which finds the latitude and longitude of an address and builds a Location object. Here we use the Google geocoding service (https://developers.google.com/maps/documentation/geocoding/), for example: http://maps.google.com/maps/api/geocode/json?sensor=false&address=515+W+Hastings+St+Vancouver

Now we are ready to write the main program which uses Location and LocationTool to build the solution. We build a class Distance with a main() method which asks the user for two addresses, builds two Location objects using LocationTool.makeLocation(), computes the distance

Here we separate different tasks into different classes:

  • Location has the information of a location
  • LocationTool uses some external tool to build a Location object

Method Design

After establishing high-level design issues, it’s important to address low-level issues such as the design of key methods. Method design involves the interaction of the methods with the rest of the system: such as parameters, returns, and the control flow.

For example, the method charAt() of the String class is defined as below. public char charAt(int index)

  • Returns the char value at the specified index. An index ranges from 0 to length() – 1. The first char value of the sequence is at index 0
  • Parameters: index – the index of the char value.
  • Returns: the char value at the specified index of this string. http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#charAt(int)

The parameters of a method are local variables. Parameter passing is like an assignment statement. When we pass an object to a method, we are actually passing a reference to the object.

Method decomposition is to divide a complex (big) task into multiple methods. For example, consider the problem of extracting the title from the HTML file specified by a URL. We may split the task into two steps: (1) read the HTML file using the URL (2) search for the “title” element.

Building reusable codes is an important skill for method design. An example is the “progressive tax” calculation. The calculation is common for any government; the only difference is about the threshold numbers. The following method abstracts out the common logic of tax caculation.

Method overloading is to have multiple methods using the same name but different parameter lists (such as different the number of parameters, the types of those parameters, the order of the types). But two overloaded methods cannot differ only by their return type.

For example, the substring method of the String class is overloaded. String substring(int beginIndex, int endIndex) Returns a substring which begins at beginIndex and extends to endIndex – 1

String substring(int beginIndex) Returns a substring which begins at beginIndex and extends to the end of this string When we call the method, the system will decide which method to be invoked based on the parameters. “smiles”.substring(1, 5) returns “mile” “unhappy”.substring(2) returns “happy”

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