Java Class and Object - Object Creation, Reference and Examples

⏱️ 9 min read • Beginner Level • Lesson 44

In Java, a class is a blueprint used to create objects, and an object is a real instance of a class. Classes define data and behavior, while objects use that data and behavior in a program.

Before learning class and objects, you should understand Java OOP concepts, Java methods, and Java variables.

What is Class in Java?

A class in Java is a template or blueprint that defines the properties (data) and behaviors (methods) of objects.

Syntax of Class in Java

class ClassName {
    // fields
    // methods
}

Here, ClassName should be a valid Java identifier. The class body can contain fields, methods, constructors, blocks, and nested classes.

Example: Class with Fields and Method

class Test {
    int a;
    private int b;
    protected int c;
    public int d;

    void show() {
        System.out.println("show");
    }
}

What is Object in Java?

An object is an entity that has state, behavior, and identity.

Object Creation in Java

Creating an object from a class is known as object instantiation. It is done using the new keyword.

ClassName objectName = new ClassName();

Example: Object Creation and Member Access

This program creates an Employee object and accesses its fields and method.

class Employee {
    int id;
    String name;

    void showDetails() {
        System.out.println(id);
        System.out.println(name);
    }

    public static void main(String[] args) {
        Employee e = new Employee();

        e.id = 123;
        e.name = "Ayan Khan";

        e.showDetails();
    }
}

Output:

123
Ayan Khan

main() Method in Another Class

In real application development, every class does not need to have a main() method. We can create one class and use it from another class.

class Employee {
    int id;
    String name;

    void showDetails() {
        System.out.println(id);
        System.out.println(name);
    }
}

public class MainClass {
    public static void main(String[] args) {
        Employee e = new Employee();

        e.id = 123;
        e.name = "Ayan Khan";

        e.showDetails();
    }
}

Output:

123
Ayan Khan

Object Memory Representation

In Java, the reference variable is stored in stack memory, while the actual object is stored in heap memory.

Instance Variable vs Reference Variable

Example: Multiple Objects of Same Class

We can create multiple objects of the same class. Each object has its own copy of instance variables.

class Employee {
    int id;
    String name;

    void showDetails() {
        System.out.println(id);
        System.out.println(name);
    }

    public static void main(String[] args) {
        Employee e1 = new Employee();
        Employee e2 = new Employee();

        e1.id = 123;
        e1.name = "Ayan";

        e2.id = 1234;
        e2.name = "Haaziq";

        e1.showDetails();
        e2.showDetails();
    }
}

Output:

123
Ayan
1234
Haaziq

Object Alias in Java

An object alias means another reference variable points to the same object. In this case, both references access the same object in heap memory.

class Employee {
    int id;

    public static void main(String[] args) {
        Employee e1 = new Employee();
        e1.id = 101;

        Employee e2 = e1; // alias

        e2.id = 202;

        System.out.println(e1.id);
        System.out.println(e2.id);
    }
}

Output:

202
202

Anonymous Object in Java

An object without a reference variable is called an anonymous object. It is usually used when an object is needed only once.

class Employee {
    void showDetails() {
        System.out.println("Employee details displayed");
    }

    public static void main(String[] args) {
        new Employee().showDetails();
    }
}

Output:

Employee details displayed

Difference Between Class and Object

Common Mistakes

Interview Questions

Frequently Asked Questions

🧠 Test your understanding with a quick quiz

class Employee {
    int id;
    String name;

    void showDetails() {
        System.out.println(id);
        System.out.println(name);
    }

    public static void main(String[] args) {
        Employee e = new Employee();
        e.id = 123;
        e.name = "Ayan Khan";
        e.showDetails();
    }
}

class Employee {
    int id;

    public static void main(String[] args) {
        Employee e1 = new Employee();
        e1.id = 101;

        Employee e2 = e1;
        e2.id = 202;

        System.out.println(e1.id);
        System.out.println(e2.id);
    }
}

🎉 Great job! Continue learning Java OOP step by step.