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

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。

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());}}
}