探究Hashtable、TreeMap、HashMap、LinkedHashMap四个Map的keySet()顺序问题。
package com.jxp;import java.util.HashMap;import java.util.Hashtable;import java.util.LinkedHashMap;import java.util.TreeMap;import org.junit.Before;import org.junit.Test;public class MapTest {private Hashtable<String , String> hashTable;private TreeMap<String , String> treeMap;private HashMap<String , String> hashMap;private LinkedHashMap<String, String> linkedHashMap;@Beforepublic void init() {hashTable = new Hashtable<String , String>();treeMap = new TreeMap<String , String>();hashMap = new HashMap<String , String>();linkedHashMap = new LinkedHashMap<String , String>();for(int i = 0; i < 31; i++) {hashTable.put(String.valueOf(i), i + "" + i);treeMap.put(String.valueOf(i), i + "" + i);hashMap.put(String.valueOf(i), i + "" + i);linkedHashMap.put(String.valueOf(i), i + "" + i);}}@Testpublic void indexTest() {System.out.println("Hashtable: " + hashTable.keySet());System.out.println("TreeMap: " + treeMap.keySet());System.out.println("HashMap: " + hashMap.keySet());System.out.println("LinkedHashMap: " + linkedHashMap.keySet());}}运行indexTest方法，结果如下
Hashtable: [30, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 29, 8, 28, 7, 27, 6, 26, 5, 25, 4, 24, 3, 23, 2, 22, 1, 21, 0, 20]TreeMap: [0, 1, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 2, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 3, 30, 4, 5, 6, 7, 8, 9]HashMap: [22, 23, 24, 25, 26, 27, 28, 29, 30, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 20, 21]LinkedHashMap: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]上述打印的时候，尽量从keySet()得到Iterator进行取值。可以看出LinkedHashMap.keySet()拿到的Set是按照顺序的，其余三个都是乱序。因此，当遇到对数据排序有严格要求的问题时，要用LinkedHashMap。
深层原因，后期自行查看源码。