Shadowing of reference variables

[Vivek Alampally]

Hello,

Kindly look at the following program and explain why the fourth line in output is 99 instead of 420

class Bar {
	int barNum = 28;
	}
class Foo {
	Bar myBar = new Bar();
	
	void changeIt(Bar myBar) {
	   myBar.barNum = 99;
	   System.out.println("myBar.barNum in changeIt is " + myBar.barNum);
	   myBar = new Bar();
	   myBar.barNum = 420;
	   System.out.println("myBar.barNum in changeIt is now " + myBar.barNum);
	}
	public static void main (String [] args) {
	Foo f = new Foo();
	System.out.println("f.myBar.barNum is " + f.myBar.barNum);
	f.changeIt(f.myBar);
	System.out.println("f.myBar.barNum after changeIt is "
	+ f.myBar.barNum);
	}
	}

Output
-----------
f.myBar.barNum is 28
myBar.barNum in changeIt is 99
myBar.barNum in changeIt is now 420
f.myBar.barNum after changeIt is 99

Thanks
Vivek.

[Christophe Verré]

Please UseCodeTags the next time you post some code.

[Christophe Verré]

In changeIt, myBar = new Bar(); is assigning a new instance to the method parameter myBar, not to the instance variable. myBar.barNum = 99 sets the instance variable to 99, because the instance variable was passed to the method (i.e. method parameter = instance variable).

[Christophe Verré]

You can see that the instance variable doesn't change by outputting its value :

class Bar {
	int barNum = 28;
}

public class Foo {
	Bar myBar = new Bar();

	void changeIt(Bar myBar) {
		myBar.barNum = 99;
		System.out.println("myBar.barNum in changeIt is " + myBar.barNum);
		System.out.println("method param:" + myBar + ", instance var:" + this.myBar) ;
		myBar = new Bar();
		myBar.barNum = 420;
		System.out.println("myBar.barNum in changeIt is now " + myBar.barNum);
		System.out.println("method param:" + myBar + ", instance var:" + this.myBar) ;
	}

	public static void main(String[] args) {
		Foo f = new Foo();
		System.out.println("Instance var:" + f.myBar);
		System.out.println("f.myBar.barNum is " + f.myBar.barNum);
		f.changeIt(f.myBar);
		System.out
				.println("f.myBar.barNum after changeIt is " + f.myBar.barNum);
	}
}