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Please pardon me, I am trying to understand this concept, don't want to just accept it without knowing what is happening. I read that only object of class X can modify itself, from the code below, Class ModifyX as a matter of fact can change X.x.num by calling it setNum method.

My questions are:

How come [ModifyX object "mx"] is able to change [ X object "x" ] value out of X?

The value of y passed as argument is changed in X, but why is it not changed in main(String[] args)?

public class X { private int num; public void setNum(int num) { this.num = num; } public void getNum() { System.out.println("X is " + num); } public static void main(String[] arsgs) { int y = 10; X x = new X(); x.setNum(y); //sets the value of num in X to y; x.getNum(); // print the value of num in X ModifyX mx = new ModifyX(); // A new class that is inteded to modify X mx.changeX(x); //calls the set method of X with new Value 5 x.getNum(); // returns the new Value passed in changeX instead of y System.out.println("Integer Y is not changed = " + y); // but y still remains the same } } class ModifyX { public void changeX(X num) { num.setNum(5); // changes y to 5 } }
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  • You use a lot of delicate terminology in improper manner that's hints lack of understanding. I suggest you study some basics first to help yourself out. Commented Feb 13, 2016 at 21:10
  • Where did you read that? It's wrong. If a class has an accessible setter method or the field is accessible directly, then any object can modify that class's instances. As for your other question, where exactly is it changed in X? Commented Feb 13, 2016 at 21:11
  • I found you post ambigious. And what you get is what must happen. Be clear about what you want to know. For more help, see topics on public methods access and Passing an object as argument to a method. Commented Feb 13, 2016 at 21:15
  • @MohamedBathaoui the question may seem ambigious to you, because you have more knowledge about this than I do. Peter Lawrence pointed out "pass by value" which I had no idea about just like how you pointed that I should research more on passing object as args. Please don't assume everyone know as much as you do Commented Feb 13, 2016 at 21:51

2 Answers 2

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This method

s.setNum(y);

takes a copy of the value y and pass it to the method. Java is always pass-by-value so it will always pass a shallow copy of the value. This means you can change either copy of this value without effecting the other.

This method

num.setNum(5);

is the same as

num.num = 5;

so exactly one value is changed here which the the num fields of the X num.

Your local variable y is 10 and this variable isn't changed anywhere so there is no reason believe it should change.

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@Nexima360 A common source of confusion is that X num is a reference and this reference is passed by value, but it is a shallow copy allowing you to change the X object which has been referenced. This means x.num can be changed but x cannot be changed for the caller.
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So 'num' is a non-public member of class X, so only the class itself is able to modify this directly. However, class X also provides the 'public' accessor function "setNum", which is a method which anyone using the class can use to "indirectly" set the value of the property 'num'.

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