List去除重复元素

import java.util.*;
import java.util.stream.Collectors;

public class ListDistinct {
    public static void main(String[] args) {

        List<Integer> list1 = Arrays.asList(1, 2, 3, 3, 4, 2, 6, 3, 7, 6);
        ArrayList<Integer> list = new ArrayList<>(list1);

        List distinct = distinct3(list);
        for (Object o : distinct) {
            System.out.print(o + " ");
        }
    }
    //双重循环遍历list，去除重复值
    public static List distinct1(List list){
        for (int i = 0; i < list.size()-1; i++) {//每次拿第i个元素
            for (int j = list.size()-1; j > i ; j--) {//从最后一个元素开始往前比较
                if (list.get(j).equals(list.get(i))){//删除相同元素
                    list.remove(j);
                }
            }
        }
        return list;
    }

    //通过Set的不重复特性去除重复元素
    public static List distinct2(List list){
        HashSet set = new HashSet<>(list);
        list.clear();//删除之前所有元素
        list.addAll(set);//搬运set中的所有元素
        return list;
    }

    //去重顺序不乱
    public static List distinct3(List list){
        HashSet set = new HashSet<>();
        List<Object> newList = new ArrayList<>();
        Iterator iterator = list.iterator();//用迭代器遍历
        while (iterator.hasNext()){
            Object element = iterator.next();
            if (set.add(element)){//如果set添加了这个元素新列表也添加进去，set 的add()源码解释如下图
                newList.add(element);
            }
        }
        list.clear();
        list.addAll(newList);

        return list;
    }
    //把list里的对象遍历一遍，用list.contains()，如果不存在就放入到另外一个list集合中
    public static List distinct4(List list){
        List<Object> newList = new ArrayList<>();
        for (int i = 0; i < list.size()-1; i++) {
            if (!newList.contains(list.get(i))){
                newList.add(list.get(i));
            }
        }
        list.clear();
        list.addAll(newList);
        return list;
    }

    //用jdk1.8新特性stream
    public static List distinct5(List list){
        return  (List) list.stream().distinct().collect(Collectors.toList());

    }

}