常用去重方法

1. 常规元素去重
   碰到List去重的问题，除了遍历去重，我们常常想到利用Set集合不允许重复元素的特点，通过List和Set互转，来去掉重复元素。

 // 遍历后判断赋给另一个List集合，保持原来顺序
    public static void ridRepeat1(List<String> list) {
        System.out.println("list = [" + list + "]");
        List<String> listNew = new ArrayList<String>();
        for (String str : list) {
            if (!listNew.contains(str)) {
                listNew.add(str);
            }
        }
        System.out.println("listNew = [" + listNew + "]");
    }

    // Set集合去重，保持原来顺序
    public static void ridRepeat2(List<String> list) {
        System.out.println("list = [" + list + "]");
        List<String> listNew = new ArrayList<String>();
        Set set = new HashSet();
        for (String str : list) {
            if (set.add(str)) {
                listNew.add(str);
            }
        }
        System.out.println("listNew = [" + listNew + "]");
    }

    // Set去重     由于Set(HashSet)的无序性，不会保持原来顺序
    public static void ridRepeat3(List<String> list) {
        System.out.println("list = [" + list + "]");
        Set set = new HashSet();
        List<String> listNew = new ArrayList<String>();
        set.addAll(list);
        listNew.addAll(set);
        System.out.println("listNew = [" + listNew + "]");
    }

    // Set通过HashSet去重（将ridRepeat3方法缩减为一行） 无序
    public static void ridRepeat4(List<String> list) {
        System.out.println("list = [" + list + "]");
        List<String> listNew = new ArrayList<String>(new HashSet(list));
        System.out.println("listNew = [" + listNew + "]");
    }

    // Set通过TreeSet去重   会按字典顺序重排序
    public static void ridRepeat5(List<String> list) {
        System.out.println("list = [" + list + "]");
        List<String> listNew = new ArrayList<String>(new TreeSet<String>(list));
        System.out.println("listNew = [" + listNew + "]");
    }

    // Set通过LinkedHashSet去重  保持原来顺序
    public static void ridRepeat6(List<String> list) {
        System.out.println("list = [" + list + "]");
        List<String> listNew = new ArrayList<String>(new LinkedHashSet<String>(list));
        System.out.println("listNew = [" + listNew + "]");
    }

除此之外，可以利用java8的stream来实现去重

//利用java8的stream去重
List uniqueList = list.stream().distinct().collect(Collectors.toList());
System.out.println(uniqueList.toString());
上面的方法在List元素为基本数据类型及String类型时是可以的，但是如果List集合元素为对象，却不会奏效

public static void main(String[] args) {
    List<User> userList = new ArrayList<User>();
    userList.add(new User("小黄",10));
    userList.add(new User("小红",23));
    userList.add(new User("小黄",78));
    userList.add(new User("小黄",10));
    
    //使用HashSet,无序
    Set<User> userSet = new HashSet<User>();
    userSet.addAll(userList);
    System.out.println(userSet);
    
    //使用LinkedHashSet，有序
    List<User> listNew = new ArrayList<User>(new LinkedHashSet(userList));
    System.out.println(listNew.toString()); }

User类结构如下：

输出如下：(没有去重)

2. 对象去重
   //实现

public static class Person {

        private int id;
 
        private String name;
 
        public Person(int id, String name) {
            this.id = id;
            this.name = name;
        }
 
        public int getId() {
            return id;
        }
 
        public void setId(int id) {
            this.id = id;
        }
 
        public String getName() {
            return name;
        }
 
        public void setName(String name) {
            this.name = name;
        }
        }
        

//测试

public class Test {
        public static void main(String[] args) {
            Person p1 = new Person(1, "11");
            Person p2 = new Person(1, "22");
            Person p3 = new Person(2, "11");
            List<Person> persons = Arrays.asList(p1,p2,p3);
            List<Person> unique = persons.stream().collect(
                    Collectors.collectingAndThen(
                            Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Person::getId))), ArrayList::new)
            );
            unique.forEach(p -> System.out.println(p.getId()));
        }
}