Java集合去重的几种方法

一、基础数据类型集合去重：

例：String

    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("c");
        list.add("c");
        list.add("a");
        list.add("a");
        list.add("b");
        //1.set集合去重，不改变原有的顺序
        pastLeep1(list);
        //2.遍历后判断赋给另一个list集合  不改变原有的顺序
        pastLeep2(list);
        //3.set去重  并按自然顺序排序
        pastLeep3(list);
        //4.set去重（缩减为一行）
        pastLeep4(list);
        //5.去重并按自然顺序排序
        pastLeep5(list);

    }

方法实现：

 //1.set集合去重，不改变原有的顺序
    public static void pastLeep1(List<String> list){
        System.out.println("list = [" + list.toString() + "]");
        List<String> listNew=new ArrayList<>();
        Set set=new HashSet();
        for (String str:list) {
            if(set.add(str)){
                listNew.add(str);
            }
        }
        System.out.println("listNew = [" + listNew.toString() + "]");
    }

    //2.遍历后判断赋给另一个list集合 不改变原有的顺序
    public static void pastLeep2(List<String> list){
        System.out.println("list = [" + list.toString() + "]");
        List<String> listNew=new ArrayList<>();
        for (String str:list) {
            if(!listNew.contains(str)){
                listNew.add(str);
            }
        }
        System.out.println("listNew = [" + listNew.toString() + "]");
    }

    //3.set去重  并按自然顺序排序
    public static void pastLeep3(List<String> list){
        System.out.println("list = [" + list + "]");
        Set set = new HashSet();
        List<String> listNew=new ArrayList<>();
        set.addAll(list);
        listNew.addAll(set);
        System.out.println("listNew = [" + listNew + "]");
    }

    //4.set去重（缩减为一行） 并按自然顺序排序
    public static void pastLeep4(List<String> list){
        System.out.println("list = [" + list + "]");
        List<String> listNew=new ArrayList<>(new HashSet(list));
        System.out.println("listNew = [" + listNew + "]");
    }

    //5.去重并按自然顺序排序
    public static void pastLeep5(List<String> list){
        System.out.println("list = [" + list + "]");
        List<String> listNew=new ArrayList<>(new TreeSet<String>(list));
        System.out.println("listNew = [" + listNew + "]");
    }

二、对象根据指定属性去重

 List<User>userList = new ArrayList<>();
        userList.add(new User(1,"小李","男"));
        userList.add(new User(5,"小李","男"));
        userList.add(new User(7,"小李","男"));
        userList.add(new User(3,"小李","男"));
        userList.add(new User(1,"小李","男"));
        userList.add(new User(5,"小李","男"));
        //6.对象 根据指定属性去重 并按自然顺序排序
        System.out.println(removeDuplicateUser(userList));
        //7. 根据指定属性去重 并按自然顺序排序2
        System.out.println(removeDuplicateUser2(userList));

   //6.对象 根据指定属性去重 并按自然顺序排序
    public static ArrayList<User> removeDuplicateUser(List<User> users) {
        Set<User> set = new TreeSet<User>(new Comparator<User>() {
            @Override
            public int compare(User o1, User o2) {
                return new Integer(o1.getId()).compareTo(new Integer(o2.getId()));
            }
        });
        set.addAll(users);
        return new ArrayList<User>(set);
    }

    //7. 根据指定属性去重 并按自然顺序排序2
    public static List<User> removeDuplicateUser2(List<User> list){
        List<User> unique = list.stream().collect(
                collectingAndThen(
                        toCollection(() -> new TreeSet<>(comparingLong(User::getId))), ArrayList::new)
        );
        return unique;
    }