Java實現List去重的方法詳解

2022-05-24 22:05:10

簡介

本文用範例介紹Java的List（ArrayList、LinkedList等）的去重的方法。

List去重的方法

一共有這幾種方法（按推薦順序排列）：

JDK8的stream的distinct

轉為HashSet（分不保持順序和保持順序兩種）
轉為TreeSet
使用retainAll/removeAll、contains、equals等基本方法

直接去重

package com.example.a;
 
import java.util.*;
import java.util.stream.Collectors;
 
public class Demo {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(3);
        list.add(2);
        list.add(3);
 
        System.out.println("源資料：             " + list);
        System.out.println("stream：             " + stream(list));
        System.out.println("hashSetWithoutOrder：" + hashSetWithOrder(list));
        System.out.println("hashSetWithOrder：   " + hashSetWithOrder(list));
        System.out.println("treeSet：            " + treeSet(list));
    }
 
    //JDK1.8的stream去重
    private static List<Integer> stream(List<Integer> list) {
        return list.stream().distinct().collect(Collectors.toList());
    }
 
    //HashSet（不保持順序）
    private static List<Integer> hashSetWithoutOrder(List<Integer> list) {
        HashSet<Integer> h = new HashSet<Integer>(list);
        return new ArrayList<>(h);
    }
 
    // 刪除ArrayList中重複元素，保持順序
    public static List<Integer> hashSetWithOrder(List<Integer> list) {
        Set<Integer> set = new HashSet<Integer>();
        List<Integer> newList = new ArrayList<Integer>();
        for (Integer element : list) {
            if (set.add(element))
                newList.add(element);
        }
        return newList;
    }
 
    //TreeSet（自動排序）
    public static List<Integer> treeSet(List<Integer> list) {
        TreeSet<Integer> set = new TreeSet<Integer>(list);
        return new ArrayList<>(set);
    }
}

執行結果

源資料： [1, 3, 2, 3]
stream： [1, 3, 2]
hashSetWithoutOrder：[1, 3, 2]
hashSetWithOrder： [1, 3, 2]
treeSet： [1, 2, 3]

根據物件屬性去重

法1：TreeSet

Set<User> setByName = new TreeSet<User>((o1, o2) ->
		o1.getName().compareTo(o2.getName()));
setByName.addAll(list);
List<User> listByName = new ArrayList<>(setByName);
System.out.println(listByName);
//[User{name='Pepper', age=20, Phone='123'}, User{name='Tony', age=20, Phone='12'}]
 
Set<User> setByNameAndAge = new TreeSet<User>((o1, o2) -> {
	return (o1.getName() + o1.getAge()).compareTo((o2.getName() + o2.getAge()));
//            return o1.getName().compareTo(o2.getName()) == 0
//                    ? o1.getAge().compareTo(o2.getAge())
//                    : o1.getName().compareTo(o2.getName());
});
setByNameAndAge.addAll(list);
List<User> listByNameAndAge = new ArrayList<>(setByNameAndAge);
System.out.println(listByNameAndAge);
//[User{name='Pepper', age=20, Phone='123'}, 
// User{name='Tony', age=20, Phone='12'}, 
// User{name='Tony', age=22, Phone='1234'}]

法2：stream+TreeSet

List<User> streamByNameList = list.stream().collect(Collectors.collectingAndThen(
		Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(User::getName))), ArrayList::new
));
System.out.println(streamByNameList);
//[User{name='Pepper', age=20, Phone='123'}, User{name='Tony', age=20, Phone='12'}]
 
List<User> streamByNameAndAgeList = list.stream().collect(Collectors.collectingAndThen(
		Collectors.toCollection(
            () -> new TreeSet<>(Comparator.comparing(o -> o.getName() + o.getAge()))), ArrayList::new
));
System.out.println(streamByNameAndAgeList);
//[User{name='Pepper', age=20, Phone='123'}, 
// User{name='Tony', age=20, Phone='12'}, 
// User{name='Tony', age=22, Phone='1234'}]

所有程式碼

package org.example.a;
 
import java.util.*;
import java.util.stream.Collectors;
 
class User {
    private String name;
    private Integer age;
    private String Phone;
 
    public User(String name, Integer age, String phone) {
        this.name = name;
        this.age = age;
        Phone = phone;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public Integer getAge() {
        return age;
    }
 
    public void setAge(Integer age) {
        this.age = age;
    }
 
    public String getPhone() {
        return Phone;
    }
 
    public void setPhone(String phone) {
        Phone = phone;
    }
 
    @Override
    public String toString() {
        return "User{" +
                "name='" + name + ''' +
                ", age=" + age +
                ", Phone='" + Phone + ''' +
                '}';
    }
}
 
public class Demo {
    public static void main(String[] args) {
        List<User> list = new ArrayList<User>() {{
            add(new User("Tony", 20, "12"));
            add(new User("Pepper", 20, "123"));
            add(new User("Tony", 22, "1234"));
        }};
 
        Set<User> setByName = new TreeSet<User>((o1, o2) ->
                o1.getName().compareTo(o2.getName()));
        setByName.addAll(list);
        List<User> listByName = new ArrayList<>(setByName);
        System.out.println(listByName);
        //[User{name='Pepper', age=20, Phone='123'}, User{name='Tony', age=20, Phone='12'}]
 
        Set<User> setByNameAndAge = new TreeSet<User>((o1, o2) -> {
            return (o1.getName() + o1.getAge()).compareTo((o2.getName() + o2.getAge()));
//            return o1.getName().compareTo(o2.getName()) == 0
//                    ? o1.getAge().compareTo(o2.getAge())
//                    : o1.getName().compareTo(o2.getName());
        });
        setByNameAndAge.addAll(list);
        List<User> listByNameAndAge = new ArrayList<>(setByNameAndAge);
        System.out.println(listByNameAndAge);
        //[User{name='Pepper', age=20, Phone='123'}, 
        // User{name='Tony', age=20, Phone='12'}, 
        // User{name='Tony', age=22, Phone='1234'}]
 
        List<User> streamByNameList = list.stream().collect(Collectors.collectingAndThen(
                Collectors.toCollection(
                    () -> new TreeSet<>(Comparator.comparing(User::getName))), ArrayList::new
        ));
        System.out.println(streamByNameList);
        //[User{name='Pepper', age=20, Phone='123'}, User{name='Tony', age=20, Phone='12'}]
 
        List<User> streamByNameAndAgeList = list.stream().collect(Collectors.collectingAndThen(
                Collectors.toCollection(
                  () -> new TreeSet<>(Comparator.comparing(o -> o.getName() + o.getAge()))), ArrayList::new
        ));
        System.out.println(streamByNameAndAgeList);
        //[User{name='Pepper', age=20, Phone='123'}, 
        // User{name='Tony', age=20, Phone='12'}, 
        // User{name='Tony', age=22, Phone='1234'}]
    }
}

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