HashMap Hashtable LinkedHashMap 和TreeMap区别【转】
???? java为数据结构中的映射定义了一个接口java.util.Map;它有四个实现类,分别是HashMap Hashtable LinkedHashMap 和TreeMap。
????Map主要用于存储健值对，根据键得到值，因此不允许键重复,但允许值重复。
????Hashmap 是一个 最常用的Map,它根据键的HashCode 值存储数据,根据键可以直接获取它的值，具有很快的访问速度，遍历时，取得数据的顺序是完全随机的。HashMap最多只允许一条记录的键为Null;允许多条记录的值Null;HashMap不支持线程的同步，即任一时刻可以有多个线程同时写HashMap;可能会导致数据的不一致。如果需要同步，可以用 Collections的synchronizedMap方法使HashMap具有同步的能力，或者使用ConcurrentHashMap。
????Hashtable 与 HashMap类似,不同的是:它不允许记录的键或者值为空;它支持线程的同步，即任一时刻只有一个线程能写Hashtable,因此也导致了 Hashtable在写入时会比较慢。
????LinkedHashMap保存了记录的插入顺序，在用Iterator遍历LinkedHashMap时，先得到的记录肯定是先插入的.在遍历的时候会比HashMap慢，不过有种情况例外，当HashMap容量很大，实际数据较少时，遍历起来可能会比LinkedHashMap慢，因为 LinkedHashMap的遍历速度只和实际数据有关，和容量无关，而HashMap的遍历速度和他的容量有关。
????TreeMap实现SortMap接口，能够把它保存的记录根据键排序,默认是按键值的升序排序，也可以指定排序的比较器，当用Iterator 遍历TreeMap时，得到的记录是排过序的。
Java代码
?? 1. public static void main(String[] args) {?
?? 2.??
?? 3.???????? Map<String, String> map = new HashMap<String, String>();?
?? 4.???????? for(int i=0;i<100;i++){?
?? 5.???????????? map.put("a"+i,String.valueOf(i));?
?? 6.???????? }?
?? 7.???????? map.put("a3", "dd");?
?? 8.???????? map.put("a2", "bb");?
?? 9.???????? map.put("a1", "cc");?
? 10.???????? for (Iterator iterator = map.values().iterator(); iterator.hasNext();) {?
? 11.???????????? String name = (String) iterator.next();?
? 12.???????????? System.out.println(name);?
? 13.???????? }?
? 14.??
? 15.???????? Map<String, String> map1 = new LinkedHashMap<String, String>();?
? 16.???????? map1.put("a3", "dd");?
? 17.???????? map1.put("a2", "bb");?
? 18.???????? map1.put("a1", "cc");?
? 19.???????? for (Iterator iterator = map1.values().iterator(); iterator.hasNext();) {?
? 20.???????????? String name = (String) iterator.next();?
? 21.???????????? System.out.println(name);?
? 22.???????? }?
? 23.??
? 24.????????? Map<String, String> map2 = new TreeMap<String, String>(new Comparator<Object>(){?
? 25.???????????? Collator collator = Collator.getInstance();?
? 26.???????????? public int compare(Object o1, Object o2) {?
? 27.???????????????? CollationKey key1 = collator.getCollationKey(o1.toString());?
? 28.???????????????? CollationKey key2 = collator.getCollationKey(o2.toString());?
? 29.???????????????? return key2.compareTo(key1);?
? 30.???????????????? //return collator.compare(o1, o2);?
? 31.???????????? }});?
? 32.???????? map2.put("a3", "dd");?
? 33.???????? map2.put("a2", "bb");?
? 34.???????? map2.put("a1", "cc");?
? 35.???????? for (Iterator iterator = map2.values().iterator(); iterator.hasNext();) {?
? 36.???????????? String name = (String) iterator.next();?
? 37.???????????? System.out.println(name);?
? 38.???????? }?
? 39.???? }?
public static void main(String[] args) {
??????? Map<String, String> map = new HashMap<String, String>();
??????? for(int i=0;i<100;i++){
??????????? map.put("a"+i,String.valueOf(i));
??????? }
??????? map.put("a3", "dd");
??????? map.put("a2", "bb");
??????? map.put("a1", "cc");
??????? for (Iterator iterator = map.values().iterator(); iterator.hasNext();) {
??????????? String name = (String) iterator.next();
??????????? System.out.println(name);
??????? }
??????? Map<String, String> map1 = new LinkedHashMap<String, String>();
??????? map1.put("a3", "dd");
??????? map1.put("a2", "bb");
??????? map1.put("a1", "cc");
??????? for (Iterator iterator = map1.values().iterator(); iterator.hasNext();) {
??????????? String name = (String) iterator.next();
??????????? System.out.println(name);
??????? }
???????? Map<String, String> map2 = new TreeMap<String, String>(new Comparator<Object>(){
??????????? Collator collator = Collator.getInstance();
??????????? public int compare(Object o1, Object o2) {
??????????????? CollationKey key1 = collator.getCollationKey(o1.toString());
??????????????? CollationKey key2 = collator.getCollationKey(o2.toString());
??????????????? return key2.compareTo(key1);
??????????????? //return collator.compare(o1, o2);
??????????? }});
??????? map2.put("a3", "dd");
??????? map2.put("a2", "bb");
??????? map2.put("a1", "cc");
??????? for (Iterator iterator = map2.values().iterator(); iterator.hasNext();) {
??????????? String name = (String) iterator.next();
??????????? System.out.println(name);
??????? }
??? }
