集合排序（List

Question

我找不到使用此方法的任何示例，所有示例都给出了第二个参数“ null”。我听说此方法用于根据多个标准对类进行排序，但找不到示例。

public class Student implements Comparable<Student> {
String name;
int age;

public Student(String name, int age) {
    this.name = name;
    this.age = age;
}

@Override
public String toString() {
    return name + ":" + age;
}

@Override
public int compareTo(Student o) {
    Integer myAge = age;
    Integer oAge = o.age;
    return myAge.compareTo(oAge);
}
}

对于本课程，如果我想根据学生的姓名和年龄对学生列表进行排序，如何使用方法Collections sort（List，Comparator）

Answer 1

在你现有的学生班级的基础上，这通常是我的工作方式，尤其是当我需要多个比较器时。

public class Student implements Comparable<Student> {

    String name;
    int age;

    public Student(String name, int age) {
       this.name = name;
       this.age = age;
    }

    @Override
    public String toString() {
        return name + ":" + age;
    }

    @Override
    public int compareTo(Student o) {
        return Comparators.NAME.compare(this, o);
    }


    public static class Comparators {

        public static Comparator<Student> NAME = new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.name.compareTo(o2.name);
            }
        };
        public static Comparator<Student> AGE = new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.age - o2.age;
            }
        };
        public static Comparator<Student> NAMEANDAGE = new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                int i = o1.name.compareTo(o2.name);
                if (i == 0) {
                    i = o1.age - o2.age;
                }
                return i;
            }
        };
    }
}

用法：

List<Student> studentList = new LinkedList<>();
Collections.sort(studentList, Student.Comparators.AGE);

编辑

自Java 8发行以来，Comparators使用lambda可以大大简化内部类。Java 8还为该Comparator对象引入了一种新方法，该方法thenComparing无需在嵌套每个比较器时进行手动检查。下面是Student.Comparators该类的Java 8实现，其中考虑了这些更改。

public static class Comparators {
    public static final Comparator<Student> NAME = (Student o1, Student o2) -> o1.name.compareTo(o2.name);
    public static final Comparator<Student> AGE = (Student o1, Student o2) -> Integer.compare(o1.age, o2.age);
    public static final Comparator<Student> NAMEANDAGE = (Student o1, Student o2) -> NAME.thenComparing(AGE).compare(o1, o2);
}