In-depth explanation of WeakHashMap in Java Collections.

WeakHashMap is a type of Map collection implementation class in the Java collection framework, it extends the AbstractMap class and implements the Map interface. WeakHashMap is similar to HashMap, both used to store key-value pairs, but its characteristic is that keys that are no longer referenced will be automatically removed.

WeakHashMap has the following characteristics:

1. WeakHashMap uses weak references to store keys in key-value pairs, and automatically removes a key when it is no longer referenced.
2. No guarantee on order: The WeakHashMap does not guarantee the order of key-value pairs, and its iteration order may be arbitrary.
3. WeakHashMap is not thread-safe, meaning that if multiple threads access and modify it simultaneously, unexpected results may occur.
4. WeakHashMap may have poor performance when there is frequent adding and removing of key-value pairs due to its use of weak references.

The usage of WeakHashMap is as follows:

1. Create object.

Map<Key, Value> map = new WeakHashMap<>();

1. 添加键值对：

map.put(key1, value1);
map.put(key2, value2);
...

1. Retrieve value:

Value value = map.get(key);

1. Remove key-value pair:

map.remove(key);

1. Check if a key is included:

boolean containsKey = map.containsKey(key);

WeakHashMap is primarily used in memory-sensitive caching scenarios where an object will be automatically removed from the WeakHashMap when it is no longer referenced by other objects, helping to save memory space.