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Thread: weakHashMap

  1. #1
    Contributing Member
    Join Date
    Sep 2006


    what do you mean by weakHashMap? which situation we are using this?


  2. #2
    Junior Member
    Join Date
    Jan 2007

    Re: weakHashMap

    This comes directly from the java api

    public class WeakHashMap
    extends AbstractMap
    implements Map
    A hashtable-based Map implementation with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently than other Map implementations.

    Both null values and the null key are supported. This class has performance characteristics similar to those of the HashMap class, and has the same efficiency parameters of initial capacity and load factor.

    Like most collection classes, this class is not synchronized. A synchronized WeakHashMap may be constructed using the Collections.synchronizedMap method.

    This class is intended primarily for use with key objects whose equals methods test for object identity using the == operator. Once such a key is discarded it can never be recreated, so it is impossible to do a lookup of that key in a WeakHashMap at some later time and be surprised that its entry has been removed. This class will work perfectly well with key objects whose equals methods are not based upon object identity, such as String instances. With such recreatable key objects, however, the automatic removal of WeakHashMap entries whose keys have been discarded may prove to be confusing.

    -Hope it helps

  3. #3
    Expert Member
    Join Date
    Mar 2012

    Re: weakHashMap

    A WeakHashMap is a special Map implementation where the keys of the map are stored in a java.lang.ref.WeakReference. By storing the keys in a weak reference, key-value pairs can dynamically be dropped from the map when the only reference to the key is from the weak reference.

    Elements in a weak hashmap can be reclaimed by the garbage collector if there are no other strong references to the object, this makes them useful for caches/lookup storage.

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