HashMap,LinkedHashMap,TreeMap对比

相同点：

HashMap,LinkedHashMap,TreeMap都属于Map；用于存储键值对（key-value），根据键得到值。键不允许重复，值允许重复。

不同点：

1. HashMap根据键的HashCode为索引存储数据。键值对取出时是随机的。
2. TreeMap键值对取出是排序的。
3. LinkedHashMap是HashMap的子类。键值对输入顺序和取出顺序是相同的。

测试代码：

public class MapTest {public static void main(String[] args) {try {hashMapTest();treeMapTest();likedHashMapTest();} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();}}public static void hashMapTest() throws Exception {System.out.println("--------HashMap--------");Map<String, String> map = new HashMap<String, String>();map.put("1", "map test 1");map.put("2", "map test 2");map.put("3", "map test 3");map.put("a", "map test a");map.put("b", "map test b");map.put("c", "map test c");Iterator<Entry<String, String>> it = map.entrySet().iterator();while (it.hasNext()) {Entry<String, String> entry = it.next();System.out.println("Key: " + entry.getKey() + "   Value: "+ entry.getValue());}}public static void treeMapTest() throws Exception {System.out.println("--------TreeMap--------");Map<String, String> map = new TreeMap<String, String>();map.put("1", "map test 1");map.put("2", "map test 2");map.put("3", "map test 3");map.put("a", "map test a");map.put("b", "map test b");map.put("c", "map test c");Iterator<Entry<String, String>> it = map.entrySet().iterator();while (it.hasNext()) {Entry<String, String> entry = it.next();System.out.println("Key: " + entry.getKey() + "   Value: "+ entry.getValue());}}public static void likedHashMapTest() throws Exception {System.out.println("--------LikedHashMap--------");Map<String, String> map = new LinkedHashMap<String, String>();map.put("1", "map test 1");map.put("2", "map test 2");map.put("3", "map test 3");map.put("a", "map test a");map.put("b", "map test b");map.put("c", "map test c");Iterator<Entry<String, String>> it = map.entrySet().iterator();while (it.hasNext()) {Entry<String, String> entry = it.next();System.out.println("Key: " + entry.getKey() + "   Value: "+ entry.getValue());}}}

结果：

--------HashMap--------
Key: 1   Value: map test 1
Key: a   Value: map test a
Key: 2   Value: map test 2
Key: b   Value: map test b
Key: 3   Value: map test 3
Key: c   Value: map test c
--------TreeMap--------
Key: 1   Value: map test 1
Key: 2   Value: map test 2
Key: 3   Value: map test 3
Key: a   Value: map test a
Key: b   Value: map test b
Key: c   Value: map test c
--------LikedHashMap--------
Key: 1   Value: map test 1
Key: 2   Value: map test 2
Key: 3   Value: map test 3
Key: a   Value: map test a
Key: b   Value: map test b
Key: c   Value: map test c