Java collection Map

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Map is one of the commonly used collections in Java, and its key implementation classes include HashMap, LinkedHashMap, TreeMap, and Hashtable.

Note: Map is a Collection, but not a Collection.

HashMap

The underlying HashMap uses arrays + linked lists/red-black trees (JDK 1.8). The uniqueness of the HashMap keys is guaranteed by hashcode, so if the keys are entities, you need to override equals() and hashcode().

LinkedHashMap

LinkedHashMap is a special form of HashMap, where the entries add before and after to make the underlying form an array + two-way list. A bidirectional linked list records the order of the entire table, so the LinkedHashMap is ordered.

static class Entry<K.V> extends HashMap.Node<K.V> {
    Entry<K,V> before, after;
    Entry(int hash, K key, V value, Node<K,V> next) {
        super(hash, key, value, next); }}Copy the code

The structure diagram of HashMap and LinkedHashMap is as follows:

Hashtable

Hashtable is thread-safe, but has been deprecated due to performance issues. In a multithreaded environment, you can use the Collections. SynchronizedMap or ConcurrentHashMap, the former more lock, so the performance of the latter is better.

Other Map

1. IdentityHashMap allows null keys and does not guarantee consistent order of insert traversals. The special thing is to use == instead of equals to compare keys for equality.
2. WeakHashMap key misreports the weak reference of the real object, so the key-value pair will be deleted automatically as the real object is reclaimed, which is not commonly used.
3. TreeM is non-thread-safe and is implemented based on a red-black tree, because the tree is always in balance and therefore has no tuning preferences. However, you can override the comparator to implement custom sorting.

conclusion