JDK8 List排序详解

一、组合字段

public class Person {
    private String name;
    private int age;
    private String gender;

    // 构造函数、getter和setter等省略
}

List personList = new ArrayList();
personList.add(new Person("Alice", 20, "female"));
personList.add(new Person("Bob", 25, "male"));
personList.add(new Person("Charlie", 22, "male"));
personList.add(new Person("Diana", 20, "female"));

// 按照age升序排列，相同age根据gender降序排列，相同age和gender根据name升序排列
personList.sort(
        Comparator.comparingInt(Person::getAge)
                .thenComparing(Person::getGender, Comparator.reverseOrder())
                .thenComparing(Person::getName)
);

personList.forEach(System.out::println);

当需要按照多个字段进行排序时，需要使用thenComparing方法进行拼接。例子中先按照age进行排序，如果age相同，则按照gender进行降序排列，如果gender相同，则按照name升序排列。

二、JDK8 List分组

public class User {
    private String name;
    private int age;

    // 构造函数、getter和setter等省略
}

List userList = new ArrayList();
userList.add(new User("Alice", 20));
userList.add(new User("Bob", 25));
userList.add(new User("Charlie", 22));
userList.add(new User("Diana", 20));

Map<Integer, List> ageMap = userList.stream()
        .collect(Collectors.groupingBy(User::getAge));

ageMap.forEach((age, list) -> {
    System.out.println("Age: " + age);
    list.forEach(user -> System.out.println("    " + user.getName()));
});

使用stream和groupingBy方法可以对List进行分组操作。例子中按照age字段进行分组，结果为一个Map，key为age，value为User对象的List。

三、JDK8 List转Map

public class Student {
    private String name;
    private int id;

    // 构造函数、getter和setter等省略
}

List studentList = new ArrayList();
studentList.add(new Student("Alice", 1));
studentList.add(new Student("Bob", 2));
studentList.add(new Student("Charlie", 3));
studentList.add(new Student("Diana", 4));

Map idNameMap = studentList.stream()
        .collect(Collectors.toMap(Student::getId, Student::getName));

idNameMap.forEach((id, name) -> System.out.println(id + ": " + name));

使用stream和toMap方法可以将List转换成Map。例子中将List转换成一个Map，key为id，value为name。