Java Instance Members - Variables, Methods and Blocks

⏱️ 15 min read • Beginner Level • Lesson 46

Instance members in Java are non-static members that belong to an object. Every object gets its own copy of instance variables, and instance methods work with object-level data.

This lesson deeply explains instance variables, instance methods, instance initializer blocks, object memory behavior, tricky cases, common mistakes, and interview questions. Before this lesson, review Java Class Members and Java Class and Objects.

What are Instance Members in Java?

Instance members are members of a class that belong to objects. They are called instance members because they are related to an instance of a class. In Java, an object is also called an instance.

Real-World Analogy

Think about a Student class. Every student has a different name, roll number, and marks.

These are object-specific values, so they should be instance variables.

Types of Instance Members

Instance Variables

An instance variable is declared inside a class but outside any method, constructor, or block. It is not declared with the static keyword. Each object gets a separate copy.

class Student {
    String name; // instance variable

    public static void main(String[] args) {
        Student s1 = new Student();
        Student s2 = new Student();

        s1.name = "Ayan";
        s2.name = "Sarah";

        System.out.println(s1.name);
        System.out.println(s2.name);
    }
}

Output:

Ayan
Sarah

Instance Methods

An instance method belongs to an object. It can directly access instance variables and instance methods of the same class. To call an instance method from main(), create an object first.

class Student {
    String name = "Ayan";

    void showName() {
        System.out.println(name);
    }

    public static void main(String[] args) {
        Student s = new Student();
        s.showName();
    }
}

Output:

Ayan

Instance Initializer Block

An instance initializer block executes every time an object is created. It executes before the constructor body. It is useful when multiple constructors need common initialization code.

class InstanceBlockExample {
    int number;

    {
        number = 100;
        System.out.println("Instance block executed");
    }

    InstanceBlockExample() {
        System.out.println("Constructor executed");
    }

    public static void main(String[] args) {
        InstanceBlockExample obj = new InstanceBlockExample();
        System.out.println(obj.number);
    }
}

Output:

Instance block executed
Constructor executed
100

Object Memory Diagram

Each object has a separate copy of instance variables.

Tricky Cases: If This, Then What Happens?

1. If you call an instance method directly from static main?

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

    public static void main(String[] args) {
        show(); // Error
    }
}

Output:

Compilation Error

2. If object reference is null and you access instance member?

class Test {
    int number = 10;

    public static void main(String[] args) {
        Test obj = null;
        System.out.println(obj.number);
    }
}

Output:

Runtime Error: NullPointerException

Instance Members vs Static Members

Common Mistakes

Interview Questions ⭐

🧠 Test your understanding with a quick quiz

class Demo {
    {
        System.out.println("Instance block");
    }

    Demo() {
        System.out.println("Constructor");
    }

    public static void main(String[] args) {
        new Demo();
    }
}

class Test {
    int number;

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

    public static void main(String[] args) {
        Test t = new Test();
        t.show();
    }
}

class Student {
    int marks = 50;

    public static void main(String[] args) {
        Student s1 = new Student();
        Student s2 = s1;

        s2.marks = 90;
        System.out.println(s1.marks);
    }
}

class Student {
    String name;

    public static void main(String[] args) {
        Student s1 = new Student();
        Student s2 = new Student();

        s1.name = "Ayan";
        s2.name = "Sarah";

        System.out.println(s1.name);
        System.out.println(s2.name);
    }
}

class Student {
    int marks = 50;

    public static void main(String[] args) {
        Student s1 = new Student();
        Student s2 = new Student();

        s1.marks = 90;
        System.out.println(s2.marks);
    }
}

class Test {
    int number = 10;

    public static void main(String[] args) {
        System.out.println(number);
    }
}

class Test {
    int number = 10;

    public static void main(String[] args) {
        Test obj = null;
        System.out.println(obj.number);
    }
}

class Test {
    void show() {
        int number;
        System.out.println(number);
    }
}

class Demo {
    {
        System.out.println("Block");
    }

    Demo() {
        System.out.println("Constructor");
    }

    public static void main(String[] args) {
        new Demo();
        new Demo();
    }
}

class Student {
    String name = "Ayan";

    void showName() {
        System.out.println(name);
    }

    public static void main(String[] args) {
        Student s = new Student();
        s.showName();
    }
}

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