In Java, WeakHashMap is a class that implements the Map interface and provides a key-value based data structure. It is part of the java.util package and is similar to the HashMap class, except that it uses weak references to its keys.

A weak reference is a type of reference that does not prevent the object that it refers to from being garbage collected. When the key of a mapping in a WeakHashMap is no longer strongly reachable from the application code (i.e., there are no strong references to the key), the mapping is automatically removed from the WeakHashMap.

Here are some of the most commonly used methods in the WeakHashMap class:

1. void clear(): This method removes all the key-value mappings from the WeakHashMap.

2. boolean containsKey(Object key): This method returns true if the WeakHashMap contains a mapping for the specified key, otherwise false.

3. boolean containsValue(Object value): This method returns true if the WeakHashMap contains one or more mappings to the specified value, otherwise false.

4. Set<Map.Entry<K, V>> entrySet(): This method returns a Set view of the key-value mappings in the WeakHashMap.

5. V get(Object key): This method returns the value to which the specified key is mapped, or null if the key is not mapped to any value.

6. boolean isEmpty(): This method returns true if the WeakHashMap contains no key-value mappings, otherwise false.

7. Set<K> keySet(): This method returns a Set view of the keys in the WeakHashMap.

8. V put(K key, V value): This method associates the specified value with the specified key in the WeakHashMap. If the WeakHashMap previously contained a mapping for the key, the old value is replaced with the new value and the old value is returned.

9. void putAll(Map<? extends K, ? extends V> m): This method copies all of the mappings from the specified Map to this WeakHashMap.

10. V remove(Object key): This method removes the mapping for the specified key from the WeakHashMap, if it is present.

11. int size(): This method returns the number of key-value mappings in the WeakHashMap.

12. Collection<V> values(): This method returns a Collection view of the values in the WeakHashMap.

The use of weak references in WeakHashMap can be useful in certain situations where the keys are temporary or the application code does not need to maintain strong references to them. However, it can also make the behavior of WeakHashMap harder to predict, since the mappings can be removed at any time by the garbage collector.