Objects eligible for GC

[Kamil Wojcik]

Hi,

Source: K&B Self Test Ch3 Q11

  
package pl.selftest3;

public class Q11 {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Beta b1 = new Beta();
		Beta b2 = new Beta();
		Alpha a1 = new Alpha();
		Alpha a2 = new Alpha();
		
		a1.b1 = b1;
		a1.b2 = b1;
		a2.b2 = b2;
		a1 = null;
		b1 = null;
		b2 = null;
	}

}

class Beta {
	
}

class Alpha {
	static Beta b1;
	Beta b2;
}

Please, I need clear explanation why only ONE object is eligible for GC?

[Palash Kumar]

There are created 4 objects ( using 'new' )

Two of them are of type Beta : b1,b2
Two of them are of type Alpha : a1,a2

Look here, a2 is not null, so you can access its members b1 and b2 ( those which were created above using new )
[a2.b1 is same as a1.b1, since class alpha's b1 is static ]

Now it is clear that, you can access a2,b1 and b2 but you cant access a1 and that is eligible for GC.


Try it with paper and pen digramatically,

[Aakash Goel]

see the image attached... its a bit hazy though.

X -> reference that have been nulled
boxes -> the objects

so the object originally referred by a1 can now not be reached, all others can be reached through a2.

Hope this helps.

[Kamil Wojcik]

Aakash,
Thank You for that pic, it's really helpful. Now it's clear!