HashMap,LinkedHashMap,TreeMap,HashTable,ConcurrentHashMap,ConcurrentSkipListMap 关于k,v是否为null，以及输出排序

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HashMap:k,v可为null,LinkedHashMap:k,v可为null,TreeMap:v可为null,HashTable:k,v都不可为null,ConcurrentHashMap:k,v都不可为null,ConcurrentSkipListMap:k,v都不可为null。

HashMap,HashTable,ConcurrentHashMap 为无序输出;TreeMap为key排序输出;LinkedHashMap put顺序先后输;ConcurrentSkipListMap为key排序输出。

Map类型	KEY	VALUE
HashMap	null	null
LinkedHashMap	null	null
TreeMap	not null	null
HashTable	not null	not null
ConcurrentHashMap	not null	not null
ConcurrentSkipListMap	not null	not null

下面代码测试：

public class MapKVIsNullTest {public static void main(String[] args) {// HashMapSystem.out.println("------HashMap无序输出------");HashMap<String, String> hashMap = new HashMap<String, String>();hashMap.put("3", null);hashMap.put("b", "Value1");hashMap.put("2", "Value2");hashMap.put("a", "ValueB");hashMap.put("ee", "ValueA");Iterator<Entry<String, String>> it = hashMap.entrySet().iterator();while (it.hasNext()) {Entry<String, String> e = it.next();System.out.println("Key: " + e.getKey() + "--Value: " + e.getValue());}// TreeMapSystem.out.println("------TreeMap按Key排序输出------");TreeMap<String, String> teMap = new TreeMap<String, String>();teMap.put("3", null);teMap.put("1", "Value1");teMap.put("2", "Value2");teMap.put("b", "ValueB");teMap.put("a", "ValueA");Iterator<Entry<String, String>> tit = teMap.entrySet().iterator();while (tit.hasNext()) {Entry<String, String> e = tit.next();System.out.println("Key: " + e.getKey() + "--Value: " + e.getValue());}// LinkedHashMapSystem.out.println("--LinkedHashMap根据输入的顺序输出--");LinkedHashMap<String, String> lhsMap = new LinkedHashMap<String, String>();lhsMap.put(null, null); //lhsMap.put("1", "Value1");lhsMap.put("2", "Value2");lhsMap.put("b", "ValueB");lhsMap.put("a", "ValueA");Iterator<Entry<String, String>> lit = lhsMap.entrySet().iterator();while (lit.hasNext()) {Entry<String, String> e = lit.next();System.out.println("Key: " + e.getKey() + "--Value: " + e.getValue());}Collections.emptyMap();// HashTableSystem.out.println("--Hashtable无序输出--");Hashtable<String, String> hstable = new Hashtable<String, String>();hstable.put("3", "Value3");hstable.put("1", "Value1");hstable.put("2", "Value2");hstable.put("b", "ValueB");hstable.put("a", "ValueA");Iterator<Entry<String, String>> ithstable = hstable.entrySet().iterator();while (ithstable.hasNext()) {Entry<String, String> e = ithstable.next();System.out.println("Key: " + e.getKey() + "--Value: " + e.getValue());}// ConcurrentHashMapSystem.out.println("--ConcurrentHashMap无序输出--");ConcurrentHashMap<String, String> concurrentHashMap = new ConcurrentHashMap<String, String>();concurrentHashMap.put("3", "Value3");concurrentHashMap.put("b", "Value1");concurrentHashMap.put("2", "Value2");concurrentHashMap.put("a", "ValueB");concurrentHashMap.put("ee", "ValueA");for (Entry<String, String> v : concurrentHashMap.entrySet()) {System.out.println("Key: " + v.getKey() + "--Value: " + v.getValue());}// ConcurrentSkipListMapSystem.out.println("--ConcurrentHashMap无序输出--");ConcurrentSkipListMap<String, String> concurrentSkipListMap = new ConcurrentSkipListMap<String, String>();concurrentSkipListMap.put("11", "Value3");concurrentSkipListMap.put("b", "Value1");concurrentSkipListMap.put("2", "Value2");concurrentSkipListMap.put("a", "ValueB");concurrentSkipListMap.put("ee", "ValueA");for (Entry<String, String> v : concurrentSkipListMap.entrySet()) {System.out.println("Key: " + v.getKey() + "--Value: " + v.getValue());}}
}