获取两个List中的不同元素，4种方法，逐步优化，学习使用

完全复制https://www.cnblogs.com/czpblog/archive/2012/08/06/2625794.html

先上测试结果

代码

 package com.syl.test;

 import java.util.*;

 /**
  * 获取两个List的不同元素（假设List自身不存在重复元素）
  * Created by syl on 2017/12/26 0026.
  */
 public class TestCompareList {
     public static void main(String[] args) {
         List<String> list1 = new ArrayList<String>();
         List<String> list2 = new ArrayList<String>();
         for (int i = 0; i < 30000; i++) {
             list1.add("test" + i);
         }
         for (int i = 0; i < 80000; i++) {
             list2.add("test" + i * 2);
         }

         getDiffrent1(list1, list2);
         getDiffrent2(list1, list2);
         getDiffrent3(list1, list2);
         getDiffrent4(list1, list2);
     }

     // 方法1，两层遍历查找，遍历次数为list1.size()*list2.size()，有点蠢
     private static List<String> getDiffrent1(List<String> list1, List<String> list2) {
         // diff 存放不同的元素
         List<String> diff = new ArrayList<String>();
         // 开始查找的时间，用于计时
         long start = System.currentTimeMillis();
         for (String str : list1) {
             if (!list2.contains(str)) {
                 diff.add(str);
             }
         }
         // 计时
         System.out.println("方法1 耗时：" + (System.currentTimeMillis() - start) + " 毫秒");
         return diff;
     }

     // 方法2，两层遍历查找，用retainAll()方法查找，也很蠢，方法底层依旧是两层遍历
     private static List<String> getDiffrent2(List<String> list1, List<String> list2) {
         long start = System.currentTimeMillis();
         list1.retainAll(list2);// 返回值是boolean
         System.out.println("方法2 耗时：" + (System.currentTimeMillis() - start) + " 毫秒");
         return list1;
     }

     // 方法3，用Map存放List1和List2的元素作为key，value为其在List1和List2中出现的次数
     // 出现次数为1的即为不同元素，查找次数为list1.size() + list2.size()，较方法1和2，是极大简化
     private static List<String> getDiffrent3(List<String> list1, List<String> list2) {
         List<String> diff = new ArrayList<String>();
         long start = System.currentTimeMillis();
         Map<String, Integer> map = new HashMap<String, Integer>(list1.size() + list2.size());
         // 将List1元素放入Map，计数1
         for (String string : list1) {
             map.put(string, 1);
         }
         // 遍历List2，在Map中查找List2的元素，找到则计数+1；未找到则放入map，计数1
         for (String string : list2) {
             Integer count = map.get(string);
             if (count != null) {
                 map.put(string, ++count);// 此处可优化，减少put次数，即为方法4
                 continue;
             }
             map.put(string, 1);
         }
         for (Map.Entry<String, Integer> entry : map.entrySet()) {
             if (entry.getValue() == 1) {
                 diff.add(entry.getKey());
             }
         }
         System.out.println("方法3 耗时：" + (System.currentTimeMillis() - start) + " 毫秒");
         return diff;
     }

     // 优化方法3，减少put次数
     private static List<String> getDiffrent4(List<String> list1, List<String> list2) {
         List<String> diff = new ArrayList<String>();
         long start = System.currentTimeMillis();
         Map<String, Integer> map = new HashMap<String, Integer>(list1.size() + list2.size());
         List<String> maxList = list1;
         List<String> minList = list2;
         if (list2.size() > list1.size()) {
             maxList = list2;
             minList = list1;
         }
         for (String string : maxList) {
             map.put(string, 1);
         }
         for (String string : minList) {
             Integer count = map.get(string);
             if (count != null) {
                 map.put(string, ++count);
                 continue;
             }
             map.put(string, 1);
         }
         for (Map.Entry<String, Integer> entry : map.entrySet()) {
             if (entry.getValue() == 1) {
                 diff.add(entry.getKey());
             }
         }
         System.out.println("方法4 耗时：" + (System.currentTimeMillis() - start) + " 毫秒");
         return diff;

     }

 }