集合排序（List< T>，Comparator<？super T>）方法示例 [英] Collections sort(List<T>,Comparator<? super T>) method example

问题描述

可能重复：

使用多个键排序Java对象

我可以找不到使用此方法的任何示例，所有示例都给出第二个参数null。
我听说这个方法用于根据多个标准对类进行排序，但没有找到示例。

I can't find any example of using this method, all examples give the second parameter "null". I heard that this method used for sorting classes according to more than one criterion but no example where found.

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);
}

}

如果我想根据他们的名字和学生排序学生列表，请为本课程。年龄如何使用方法集合排序（列表，比较器）

for this class if i want to sort a list of Student according to their names & ages how can i use the method Collections sort(List,Comparator)

推荐答案

以现有学生为基础 class，这就是我通常这样做的方式，特别是如果我需要多个比较器的话。

Building upon your existing Student class, this is how I usually do it, especially if I need more than one comparator.

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发布以来，使用lambdas可以大大简化内部类 Comparators 。 Java 8还为 Comparator 对象 thenComparing 引入了一种新方法，这样就无需手动检查每个比较器嵌套时。下面是 Student.Comparators 类的Java 8实现，其中考虑了这些更改。

Since the release of Java 8 the inner class Comparators may be greatly simplified using lambdas. Java 8 also introduces a new method for the Comparator object thenComparing, which removes the need for doing manual checking of each comparator when nesting them. Below is the Java 8 implementation of the Student.Comparators class with these changes taken into account.

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);
}

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