JAVA实体类集合该如何去重？

最近在工作中经常遇到需要去重的需求，所以特意系统的来梳理一下

单元素去重

方法一：利用Set去重

 public static void main(String[] args) {// 生成含重复元素的集合并打印List<Integer> list = new ArrayList<>();list.add(1);list.add(2);list.add(2);list.add(3);System.out.println("去重前：" + list);// 利用HashSet去重后并打印HashSet<Integer> hashSet = new HashSet<>(list);list.clear();list.addAll(hashSet);System.out.println("去重后：" + list);}

执行：

方法二：利用java 8的stream写法，方便优雅快捷高效

 public static void main(String[] args) {// 生成含重复元素的集合并打印List<Integer> list = new ArrayList<>();list.add(2);list.add(3);list.add(3);list.add(4);System.out.println("去重前：" + list);// 利用java 8的stream写法list = list.stream().distinct().collect(Collectors.toList());System.out.println("去重后：" + list);}

执行：

实体类对象去重

先新建实体类：

/*** @Author: guqueyue* @Description: 学生类* @Date: 2023/12/12**/
public class Student {/** 姓名 */private String name;/** 年龄 */private Integer age;public Student() {}public Student(String name, Integer age) {this.name = name;this.age = age;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +'}';}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;}
}

单属性去重

方法一：利用map去重

public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 22));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 利用map根据姓名去重Map<String, Student> map = new HashMap<>();for (Student student : studentList) {map.put(student.getName(), student);}studentList = new ArrayList<>(map.values());System.out.println("去重后: " + studentList);}

执行得：

方法二：利用map去重，java 8语法

比方法一代码看似简洁了，但实际上好似更加复杂了

 public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 利用map根据姓名去重，java 8语法studentList = studentList.stream().collect(Collectors.toMap(student -> student.getName(), Function.identity(), (o, n) -> n)).values().stream().collect(Collectors.toList());System.out.println("去重后: " + studentList);}

执行：

方法三：利用Set去重

public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 利用Set去重
//        Set<Student> set = new TreeSet<>(((o1, o2) -> o1.getName().compareTo(o2.getName())));Set<Student> set = new TreeSet<>((Comparator.comparing(Student::getName)));set.addAll(studentList);studentList.clear();studentList = new ArrayList<>(set);System.out.println("去重后: " + studentList);}

执行：

方法四： 利用Set去重，java 8写法

public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 利用Set去重，java 8写法studentList = studentList.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Student::getName))),ArrayList::new));System.out.println("去重后: " + studentList);}

执行：

方法五：定义过滤器

public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 定义过滤器studentList = studentList.stream().filter(distinctKey(Student::getName)).collect(Collectors.toList());System.out.println("去重后: " + studentList);}/*** @Description 自定义过滤器* @Param [keyExtractor]* @return java.util.function.Predicate<T>**/public static <T> Predicate<T> distinctKey(Function<? super T, Object> keyExtractor) {Map<Object, Boolean> map = new ConcurrentHashMap<>();return o -> map.putIfAbsent(keyExtractor.apply(o), Boolean.TRUE) == null;}

执行：

补充

补充一点，如何提取去重后的单元素集合：

public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 提取去重后的姓名List<String> nameList = studentList.stream().map(Student::getName).distinct().collect(Collectors.toList());System.out.println("去重后: " + nameList);}

执行：

多属性去重

方法一：利用map根据姓名、年龄去重

public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 利用map根据姓名、年龄去重Map<String, Student> map = new HashMap<>();for (Student student : studentList) {map.put(student.getName() + "_" + student.getAge(), student);}studentList = new ArrayList<>(map.values());System.out.println("去重后: " + studentList);}

执行：

方法二：利用map根据姓名、年龄去重，java 8写法

public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 利用map根据姓名、年龄去重，java 8写法studentList = studentList.stream().collect(Collectors.toMap(s -> s.getName() + "_" + s.getAge(), Function.identity(), (o, n) -> n)).values().stream().collect(Collectors.toList());System.out.println("去重后: " + studentList);}

执行：

方法三：利用Set根据姓名、年龄去重，java 8写法

public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 利用Set根据姓名、年龄去重，java 8写法studentList = studentList.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(o -> o.getName() + "_" + o.getAge()))), ArrayList::new));System.out.println("去重后: " + studentList);}

执行：

重写equals()和hashCode()方法

下面的几种方法首先需要在实体类中重写equals()和hashCode()方法

@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Student student = (Student) o;return Objects.equals(name, student.name) && Objects.equals(age, student.age);}@Overridepublic int hashCode() {return Objects.hash(name, age);}

方法四：利用java 8 的distinct()，最推荐

 public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 利用java 8 的distinct()，根据姓名和年龄去重studentList = studentList.stream().distinct().collect(Collectors.toList());System.out.println("去重后: " + studentList);}

执行：

方法五：通过contains()方法来调用equals()方法来对比

 public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 通过contains()方法来调用equals()方法来对比，根据姓名和年龄去重List<Student> newStudentList = new ArrayList<>();for (Student student : studentList) {if (!newStudentList.contains(student)) {newStudentList.add(student);}}System.out.println("去重后: " + newStudentList);}

执行：

方法六：重写方法后，通过Set去重

 public static void main(String[] args) {// 生成学生集合List<Student> studentList = new ArrayList<>();studentList.add(new Student("张三", 18));studentList.add(new Student("张三", 19));studentList.add(new Student("张三", 18));studentList.add(new Student("李四", 18));System.out.println("去重前: " + studentList);// 通过Set，根据姓名和年龄去重Set<Student> set = new HashSet<>(studentList);studentList.clear();studentList.addAll(set);System.out.println("去重后: " + studentList);}

执行：

完结撒花！！！