List 去重-蒲公英云

List 去重

傷城~ 2022-10-24 11:43 250阅读 0赞

在日常工作中，需要对 List 进行去重的场景很多。对于我这样一个菜鸟来说，可能能想到的最简单粗暴的方法就是双层循环去重，但是在看到这篇文章之后，果断收藏了。

https://mp.weixin.qq.com/s/N1MRYswi8v1Lh6pdDz8upQ

来源于微信公众号“Java爱好者”，在此表示感谢。若有侵权，请联系删除~

既然刚才说到了简单粗暴的方法，那么我们就先来看看这种暴力方法：

1.双层循环

import java.util.*;
public class ListDemo {
    private void removeRepeat(List<Integer> listInt) {
        List<Integer> list = new ArrayList<>(listInt);
        for (int i = 0; i < list.size(); i++) {
            for (int j = 0; j < list.size(); j++) {
                if (i != j && list.get(i).equals(list.get(j))) {
                    list.remove(list.get(j));
                }
            }
        }
        System.out.println("去重后的list = " + list);
    }
    public static void main(String[] args) {
        ListDemo listDemo = new ListDemo();
        Integer[] array = {1,1,3,5,7,6,4,3,4,5,8,2};
        List<Integer> listInt = Arrays.asList(array);
        System.out.println("去重前的list = " + listInt);
        listDemo.removeRepeat(listInt);
    }
}

此方法不在乎原 List 中的数据的顺序。

2.利用 List 的 contains 方法，遍历循环，重新排序，只添加一次数据

import java.util.*;
public class ListDemo {
    private void removeRepeat(List<Integer> listInt) {
        List<Integer> list = new ArrayList<>(listInt.size());
        for (Integer i : listInt) {
            if (!list.contains(i)) {
                list.add(i);
            }
        }
        System.out.println("去重后的list = " + list);
    }
    public static void main(String[] args) {
        ListDemo listDemo = new ListDemo();
        Integer[] array = {1,1,3,5,7,6,4,3,4,5,8,2};
        List<Integer> listInt = Arrays.asList(array);
        System.out.println("去重前的list = " + listInt);
        listDemo.removeRepeat(listInt);
    }
}

这种方法对于 String 类型的 List 同样适用。

import java.util.*;
public class ListDemo {
    private void removeRepeat(List<String> listStr) {
        List<String> list = new ArrayList<>();
        for (String str : listStr) {
            if (!list.contains(str)) {
                list.add(str);
            }
        }
        System.out.println("去重后的list = " + list);
    }
    public static void main(String[] args) {
        ListDemo listDemo = new ListDemo();
        String[] array = {"A","B","B","C","A","D","E","C","F","D","G"};
        List<String> listStr = Arrays.asList(array);
        System.out.println("去重前的list = " + listStr);
        listDemo.removeRepeat(listStr);
    }
}

3.使用 HashSet

利用 HashSet 不能添加重复数据的特性来对 List 进行去重

import java.util.*;
public class ListDemo {
    private void removeRepeat(List<String> listStr) {
        List<String> list = new ArrayList<>();
        HashSet<String> set = new HashSet<>();
        for (String str : listStr) {
            if (set.add(str)) {
                list.add(str);
            }
        }
        System.out.println("去重后的list = " + list);
    }
    public static void main(String[] args) {
        ListDemo listDemo = new ListDemo();
        String[] array = {"A","B","B","C","A","D","E","C","F","D","G"};
        List<String> listStr = Arrays.asList(array);
        System.out.println("去重前的list = " + listStr);
        listDemo.removeRepeat(listStr);
    }
}

4.使用 java 8 新特性 stream 对 List 进行去重操作

public static void main(String[] args) {
        String[] array = {"A","B","B","C","A","G","E","C","F","D","G"};
        List<String> listStr = Arrays.asList(array);
        System.out.println("去重前的list = " + listStr);
        List<String> result = listStr.stream().distinct().collect(Collectors.toList());
        System.out.println("去重后的list = " + result);
    }

5.使用 LinkedHashSet 来对 List 进行去重操作

public static void main(String[] args) {
        String[] array = {"A","B","B","C","A","G","E","C","F","D","G"};
        List<String> listStr = Arrays.asList(array);
        System.out.println("去重前的list = " + listStr);
        LinkedHashSet<String> linkedHashSet = new LinkedHashSet<>(listStr);
        List<String> result = new ArrayList<>(linkedHashSet);
        System.out.println("去重后的list = " + result);
    }

当需要按照指定的对象属性去去重时，上面这几种方法就不是很适用了，推荐 Stream 的另一种方法：

List<AaDto> collect = list.stream().collect(Collectors.collectingAndThen(
            Collectors.toCollection(() -> new TreeSet<>(
                    Comparator.comparing(p->p.getA() + p.getB()))), ArrayList::new);