Java实现List去重的方法详解
简介
本文用示例介绍Java的List（ArrayList、LinkedList等）的去重的方法。
List去重的方法
一共有这几种方法（按推荐顺序排列）：
jdk8的stream的distinct
- 转为HashSet（分不保持顺序和保持顺序两种）
- 转为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("源数据： " + 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（不保持顺序）
private static List<Integer> hashSetWithoutOrder(List<Integer> list) {
HashSet<Integer> h = new HashSet<Integer>(list);
return new ArrayList<>(h);
}
// 删除ArrayList中重复元素，保持顺序
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（自动排序）
public static List<Integer> treeSet(List<Integer> list) {
TreeSet<Integer> set = new TreeSet<Integer>(list);
return new ArrayList<>(set);
}
}执行结果
源数据： [1, 3, 2, 3]
stream： [1, 3, 2]
hashSetWithoutOrder：[1, 3, 2]
hashSetWithOrder： [1, 3, 2]
treeSet： [1, 2, 3]
根据对象属性去重
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'}]所有代码
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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