Java Class and Object

Introduction

Java Classes and Objects are essential when it comes to understanding Java programming. Java objects are similar to real-world objects. They have states, behaviors, and attributes. Java classes allow programmers to model their code after a metaphor of the real world. Java classes also contain variables and methods; variables capture the state of an object, while methods dictate its behavior. Java Classes and Objects work together to form the core of Java's powerful object-oriented programming capabilities that make coding more straightforward and intuitive. Java's strong focus on both Objects and Classes makes it a sound choice for anyone looking to learn a new programming language!

Java Class

A java class is a building block of java programming language that encapsulates a set of objects with the same characteristics. It defines the data and behavior associated with the class, serving as an outline of what the objects it creates will contain. Java classes provide structure to java programs and are essential for java developers to be able to use the language accurately and efficiently. An object is an abstract concept in java; it has no physical representation like variables or other types of data but instead acts as a template for entities being modeled within a program. It is via java classes that these models can be made by introducing structure and other elements such as inheritance, polymorphism, abstraction, and encapsulation. Ultimately, java classes will form the script from which objects are created: without them, java development would be virtually impossible.

Syntax:

access_modifier class<class_name>
{ 
    data member; 
    method; 
    constructor;
    nested class;
    interface;
}

Example

class Student {
 int id; // data member (also instance variable)
 String name; // data member (also instance variable)

 public static void main(String args[])
 {
  Student s1 = new Student(); // creating an object of
         // Student
  System.out.println(s1.id);
  System.out.println(s1.name);
 }
}

How to create a class in java

Object in java

How to create an object in java

Define a class:

public class Car {
   // Class variables
   String color;
   int modelYear;
   // Class constructor
   public Car(String color, int modelYear) {
      this.color = color;
      this.modelYear = modelYear;
   }
   // Class methods
   public void start() {
      System.out.println("The car is starting.");
   }
   public void stop() {
      System.out.println("The car is stopping.");
   }
}

Create an instance of the class:

Access the object:

Different ways to create objects in java

Using the new keyword: 

Example:

ClassName objectName = new ClassName();

Using Class.forName() method: 

Example:

Class className = Class.forName("ClassName");
ClassName objectName = (ClassName) className.newInstance();

Using clone() method:

Example:

ClassName object1 = new ClassName();
ClassName object2 = object1.clone();

Using object deserialization: 

Example:

ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);
ClassName objectName = (ClassName) objectInputStream.readObject();

Using Factory Method:

Example:

public static ClassName createObject() {
    return new ClassName();
}
ClassName objectName = ClassName.createObject();

Using Dependency Injection:

Example:

public class MyClass {
    private Dependency dependency;
    public MyClass(Dependency dependency) {
        this.dependency = dependency;
    }
}
MyClass objectName = new MyClass(new Dependency());

Anonymous object in java

btn.setOnAction(new EventHandler()
{
    public void handle(ActionEvent event)
    {
        System.out.println("Hello World!");
    }
});

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