How to "attach" to an Object

Question

Assume here is a Button object which I cannot modify, and I want to add some function to it, making it draggable for example, so I create a class Draggable as follows:

public class Draggable {

    private Button button;

    private Draggable(Button button) {this.button = button;}

    // attach to a button
    public static Draggable attachTo(Button button) {
        return new Draggable(button);
    }

    // retrieve Draggable object attached to the button
    public static Draggable of(Button button) {
        // ...
    }

    // detatch from button
    public static void detachFrom(Button button) {
        Draggable d = of(button);
        if (d != null) {d.button = null;}
    }
}

Whenever the Button object is destroyed and garbage collected, the Draggable object attached to it should also be garbage collectable automatically. So is there a design pattern or something that can help me implement this of() method?

Answer to your quesiton is here: https://stackoverflow.com/questions/2506488/when-is-the-finalize-method-called-in-java — Azodious, Aug 17 '17 at 06:08
@Sweeper these objects are provided by others, so creating a sub class for every one of these types would be inextensible. — JSPDeveloper01, Aug 17 '17 at 06:57
@GhostCat The first further reading link is fully helpful that I have solved the problem with WeakHashMap. Thanks again! — JSPDeveloper01, Aug 17 '17 at 07:33

GhostCat · Accepted Answer · 2017-08-17T07:06:09.390

You probably need an internal cache based on WeakReferences.

In other words: your Draggalbe can only be subject to garbage collection when no "hard" alive reference is pointing to it.

In other words:

every time you create a Draggabe you put that into your cache
you periodically check the cache if the weak references cached in there are still valid
when you find a weak reference that turned null - you delete the entry from the cache

And of course - you have to make sure that no other "life" object keeps a hard reference to a Draggable object.

See here and there for further reading.

How to "attach" to an Object

1 Answers1