Question

Here’s my Code

Class A

public class A {
    private int a;

    public int getA() {
        return a;
    }

    public void setA(int a) {
        this.a = a;
    }
}

Class Change

public class Change {
    public void changeSomething(A a){
        a.setA(13);
    }
}

Class Learn

public class Learn {
    public static  void main(String[] args) {
        A a = new A();
        Change change = new Change();
        change.changeSomething(a);
        System.out.println(a.getA());
    }
}

The output is 13. Now when i am passing an object to the changeSomething method, internally the value of Object A has been changed but why do i see this effect outside that function?

Is not this equivalent to passing by value in C where unless you return that variable/Object you dont get the updated value.

i.e. dont i need to do a=changeSomething(a); and set the return type of this method to be as A?

Thanks

Answer 1

You’re passing a reference to the original object around. When you write a method

void someMethod(A param) { ... }

param is a reference to the original object. The original object isn’t being copied. Consequently when you change this object, the change is visible wherever that object is observed.

When you write:

private A a = new A();

it’s important to realise that the variable is a reference to object type A, not an actual object type A. It’s a fine distinction, granted.

The above behaviour can cause unexpected effects across your system, and it’s an argument for immutability, especially in threaded environments where changes can be triggered from multiple threads.