How to get all instances of a class

[Bat Haar]

Hello everybody,
anyone knows how to get all instances of a class;
something like ArrayList.getInstances() and I get all the instances of class ArrayList.
Thanx
Bat

[Mikael Jonasson]

By default there isn't a way (I think). But you could always write a static method that does this for your own classes.
/Mike

[Bat Haar]

Thanx Mikael,
but how should this static method work?
Bat

[Nathan Pruett]

Here's one way of doing this...

import java.util.Vector;
 
public class TrackedClass
{
    private static Vector allInstances;
 
    static
    {
        allInstances = new Vector();
    }
 
    public TrackedClass()
    {
        allInstances.add( this );
    }
 
    public static synchronized Vector getAllInstances()
    {
        return( ( Vector )allInstances.clone() );
    }
 
    protected void finalize()
    {
        allInstances.removeElement( this );
    }
 
    public static void main( String[] args )
    {
        TrackedClass t1 = new TrackedClass();
        TrackedClass t2 = new TrackedClass();
        TrackedClass t3 = new TrackedClass();
        TrackedClass t4 = t3;
        TrackedClass t5 = t4;
        Vector v = TrackedClass.getAllInstances();
        System.out.println( v.size() );
    }
}

[William Barnes]

What does "finalize" do that isn't done by the gc when the class gets cleaned up? Thanks.

[Nathan Pruett]

Yes, finalize is run by the garbage collector whenever an object has no more active references... actually, that brings up a good question... I added the finalize() in so that the reference in the hashmap would be removed, but I think the very fact that the object exists in the hashmap means that finalize will never be run because the hashmap will always contain an active reference... I guess I didn't think it through that well... So either all objects of this class never get garbage collected until the program is over, or you have to add a special method to remove the reference from the hashmap to get rid of all active references...

-Nate