Pass by value vs pass by reference for methods in Java

In Java, when you talk about “pass by value” vs “pass by reference” for methods, the key point is:

Java is always pass-by-value — but what is passed by value depends on the type of variable.

1. Pass-by-Value (for primitives)

When you pass a primitive type (int, double, char, boolean, etc.) to a method:

• Java passes a copy of the actual value.
• Changes inside the method do not affect the original variable.

Example:

public class PassByValueExample {
    public static void main(String[] args) {
        int x = 5;
        changeValue(x);
        System.out.println("After method call: " + x); // 5
    }

    static void changeValue(int num) {
        num = 10; // Only changes the local copy
    }
}

public class PassByValueExample {
    public static void main(String[] args) {
        int x = 5;
        changeValue(x);
        System.out.println("After method call: " + x); // 5
    }

    static void changeValue(int num) {
        num = 10; // Only changes the local copy
    }
}

Explanation:
x stays 5 because num in changeValue() is just a copy.

2. Pass-by-Value of Object References

When you pass an object to a method:

• The value being passed is a copy of the reference (memory address) to the object.
• Both the original variable and the method parameter refer to the same object.
• If you change the object’s internal state via the reference, the change is visible outside.
• But if you make the parameter point to a new object, it does not change the original variable’s reference.

Example:

class Box {
    int value;
}

public class PassByReferenceExample {
    public static void main(String[] args) {
        Box b = new Box();
        b.value = 5;

        modifyObject(b);
        System.out.println("After modifyObject: " + b.value); // 10

        changeReference(b);
        System.out.println("After changeReference: " + b.value); // still 10
    }

    static void modifyObject(Box box) {
        box.value = 10; // modifies the actual object
    }

    static void changeReference(Box box) {
        box = new Box(); // changes local copy of reference
        box.value = 20;  // affects new object only, not original
    }
}

class Box {
    int value;
}

public class PassByReferenceExample {
    public static void main(String[] args) {
        Box b = new Box();
        b.value = 5;

        modifyObject(b);
        System.out.println("After modifyObject: " + b.value); // 10

        changeReference(b);
        System.out.println("After changeReference: " + b.value); // still 10
    }

    static void modifyObject(Box box) {
        box.value = 10; // modifies the actual object
    }

    static void changeReference(Box box) {
        box = new Box(); // changes local copy of reference
        box.value = 20;  // affects new object only, not original
    }
}

3. Summary Table