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。

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) {        // HashMap        System.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());        }        // TreeMap        System.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());        }        // LinkedHashMap        System.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();        // HashTable        System.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());        }        // ConcurrentHashMap        System.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());        }        // ConcurrentSkipListMap        System.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());        }    }}