HashMap, LinkedHashMap和TreeMap在Java中的区别是什么?
我在输出中没有看到任何不同，因为这三个都有keySet和values。什么是哈希表?
Map m1 = new HashMap();
m1.put("map", "HashMap");
m1.put("schildt", "java2");
m1.put("mathew", "Hyden");
m1.put("schildt", "java2s");
print(m1.keySet());
print(m1.values());
SortedMap sm = new TreeMap();
sm.put("map", "TreeMap");
sm.put("schildt", "java2");
sm.put("mathew", "Hyden");
sm.put("schildt", "java2s");
print(sm.keySet());
print(sm.values());
LinkedHashMap lm = new LinkedHashMap();
lm.put("map", "LinkedHashMap");
lm.put("schildt", "java2");
lm.put("mathew", "Hyden");
lm.put("schildt", "java2s");
print(lm.keySet());
print(lm.values());
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());
}
这三个都表示从唯一键到值的映射，因此实现了Map接口。
HashMap is a map based on hashing of the keys. It supports O(1) get/put operations. Keys must have consistent implementations of hashCode() and equals() for this to work.
LinkedHashMap is very similar to HashMap, but it adds awareness to the order at which items are added (or accessed), so the iteration order is the same as insertion order (or access order, depending on construction parameters).
TreeMap is a tree based mapping. Its put/get operations take O(log n) time. It requires items to have some comparison mechanism, either with Comparable or Comparator. The iteration order is determined by this mechanism.
