WeakHashMap

[Isabel Wanderley]

Can anyone explain when WeakHashMap is used and how it works?? I'm studying it here but I didn't understand what's used for...
Thanks

[Thomas Paul]

Items in a WeakHashMap are eligible for garbage collection if the WeakHashMap has the only reference.

[Jose Botella]

I wrote this some time ago:

import java.lang.ref.*;
import java.util.WeakHashMap;
class References4 {
public static void main(String[] args) throws
InterruptedException {
String key1 = new String("1");
WeakHashMap whm = new WeakHashMap();
whm.put(key1, new Integer(1));
whm.put(new String("2"), new Integer(2));
whm.put(new String("3"), new Integer(3));
System.out.println(whm.get("2"));
System.out.println(whm.remove("2"));
System.out.println(whm);
System.gc();
System.out.println(whm);
}
}
/*
C:\Java\TIJ2\Test>java References4
2
2
{3=3, 1=1}
{1=1}
*/

[John Dale]

I think items (keys and values) in a WeakHashMap are removed from the map when the key is no longer referenced outside the map. The value then eligible for garbage collection only if it is not referenced elsewhere (perhaps by another key in the map).
I've not had an occasion in which I knew needed WeakHashMap yet, so there is a chance I don't have this right.
I suspect WeakHashMap may be useful when a subsystem needs to provide a handle by which other subsystems can refer to an object. The handle could be key in a WeakHashMap. When no other subsystem (and nothing besides the WeakHashMap) contains a reference to a particular key, that key and its value are removed from the map. At that point, if nothing else refers to the value object, the value object is unreferenced and hence eligable for garbage collection.

[Barry Gaunt]

me too