HashMap 可以包含一个空键。

HashMap没有秩序。

TreeMap

TreeMap不能包含任何空键。

TreeMap保持升序。

LinkedHashMap

LinkedHashMap可用于维护插入顺序，即键被插入到Map中，也可用于维护访问顺序，即键被访问。

例子::

1) HashMap map = new HashMap();

    map.put(null, "Kamran");
    map.put(2, "Ali");
    map.put(5, "From");
    map.put(4, "Dir");`enter code here`
    map.put(3, "Lower");
    for (Map.Entry m : map.entrySet()) {
        System.out.println(m.getKey() + "  " + m.getValue());
    } 

2) TreeMap map = new TreeMap();

    map.put(1, "Kamran");
    map.put(2, "Ali");
    map.put(5, "From");
    map.put(4, "Dir");
    map.put(3, "Lower");
    for (Map.Entry m : map.entrySet()) {
        System.out.println(m.getKey() + "  " + m.getValue());
    }

3) LinkedHashMap map = new LinkedHashMap();

    map.put(1, "Kamran");
    map.put(2, "Ali");
    map.put(5, "From");
    map.put(4, "Dir");
    map.put(3, "Lower");
    for (Map.Entry m : map.entrySet()) {
        System.out.println(m.getKey() + "  " + m.getValue());
    }

我更喜欢视觉呈现:

哈希映射不保留插入顺序。 的例子。Hashmap 如果您正在插入键作为

1  3
5  9
4   6
7   15
3   10

它可以存储为

4  6
5  9
3  10
1  3
7  15

链接Hashmap保留插入顺序。

的例子。 如果您正在插入键

1  3
5  9
4   6
7   15
3   10

它将存储为

1  3
5  9
4   6
7   15
3   10

和我们插入的一样。

树映射以键的递增顺序存储山谷。 的例子。 如果您正在插入键

1  3
5  9
4   6
7   15
3   10

它将存储为

1  3
3  10
4   6
5   9
7   15

这是我自己使用地图的经验，关于我何时使用每种地图:

HashMap - Most useful when looking for a best-performance (fast) implementation. TreeMap (SortedMap interface) - Most useful when I'm concerned with being able to sort or iterate over the keys in a particular order that I define. LinkedHashMap - Combines advantages of guaranteed ordering from TreeMap without the increased cost of maintaining the TreeMap. (It is almost as fast as the HashMap). In particular, the LinkedHashMap also provides a great starting point for creating a Cache object by overriding the removeEldestEntry() method. This lets you create a Cache object that can expire data using some criteria that you define.

请看下图(大图)中每个类在类层次结构中的位置。TreeMap实现了SortedMap和NavigableMap，而HashMap没有。

HashTable已经过时，应该使用相应的ConcurrentHashMap类。

它们都提供一个key->值映射和一种遍历键的方法。最重要的区别 这些类是时间保证和键的顺序。

HashMap offers 0(1) lookup and insertion. If you iterate through the keys, though, the ordering of the keys is essentially arbitrary. It is implemented by an array of linked lists. TreeMap offers O(log N) lookup and insertion. Keys are ordered, so if you need to iterate through the keys in sorted order, you can. This means that keys must implement the Comparable interface.TreeMap is implemented by a Red-Black Tree. LinkedHashMap offers 0(1) lookup and insertion. Keys are ordered by their insertion order. It is implemented by doubly-linked buckets.

假设你将一个空的TreeMap, HashMap和LinkedHashMap传递给下面的函数:

void insertAndPrint(AbstractMap<Integer, String> map) {
  int[] array= {1, -1, 0};
  for (int x : array) {
    map.put(x, Integer.toString(x));
  }
  for (int k: map.keySet()) {
   System.out.print(k + ", ");
  }
}

它们的输出如下所示。

对于HashMap，在我自己的测试中，输出是{0,1，-1}，但它可以是任何顺序。没有任何保证 排序。 Treemap，输出为{- 1,0,1} LinkedList，输出为{1，-1,0}