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package com.syl.test;

import java.util.*;

/**
 * 獲取兩個List的不同元素（假設List自身不存在重複元素）
 * 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1，兩層遍歷查詢，遍歷次數為list1.size()*list2.size()，有點蠢
    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>();
        // 開始查詢的時間，用於計時
        long start = System.currentTimeMillis();
        for (String str : list1) {
            if (!list2.contains(str)) {
                diff.add(str);
            }
        }
        // 計時
        System.out.println("方法1 耗時：" + (System.currentTimeMillis() - start) + " 毫秒");
        return diff;
    }

    // 方法2，兩層遍歷查詢，用retainAll()方法查詢，也很蠢，方法底層依舊是兩層遍歷
    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 耗時：" + (System.currentTimeMillis() - start) + " 毫秒");
        return list1;
    }

    // 方法3，用Map存放List1和List2的元素作為key，value為其在List1和List2中出現的次數
    // 出現次數為1的即為不同元素，查詢次數為list1.size() + list2.size()，較方法1和2，是極大簡化
    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());
        // 將List1元素放入Map，計數1
        for (String string : list1) {
            map.put(string, 1);
        }
        // 遍歷List2，在Map中查詢List2的元素，找到則計數+1；未找到則放入map，計數1
        for (String string : list2) {
            Integer count = map.get(string);
            if (count != null) {
                map.put(string, ++count);// 此處可優化，減少put次數，即為方法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 耗時：" + (System.currentTimeMillis() - start) + " 毫秒");
        return diff;
    }

    // 優化方法3，減少put次數
    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 耗時：" + (System.currentTimeMillis() - start) + " 毫秒");
        return diff;

    }

}