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本文链接：https://blog.youkuaiyun.com/manbaforever/article/details/136101925

文章介绍了如何在Java中使用Comparator和Comparable接口，通过Student类的实例和Address枚举来筛选出所有地址为北京的学生。展示了两种方法：一种是使用Comparator对列表排序后过滤，另一种是利用Comparable接口直接在List上操作并返回结果。

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enum Address {
    Beijing,
    Shanghai,
    Wuhan
}

class Student implements Comparable {
    String name;
    int age;
    Address address;

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", address=" + address +
                '}';
    }

    @Override
    public int compareTo(Object o) {
        return this.getAge() - ((Student)o).getAge();
    }
}

如图，我定义了一个学生类，学生类包含名字，年龄，地址三个属性，其中地址用的是枚举，现在我想找到所有北京的学生，此时我的代码为:

public class Homework {
    public static void main(String[] args) {

        List<Student> list1 = Arrays.asList(
                new Student("zs",18,Address.Beijing),
                new Student("ls",22,Address.Shanghai),
                new Student("wu",20,Address.Wuhan),
                new Student("zl",24,Address.Wuhan),
                new Student("kongling",23,Address.Shanghai)
        );

//        list1.sort(new Comparator<Student>() {
//            @Override
//            public int compare(Student o1, Student o2) {
//                return o1.getAge() - o2.getAge();
//            }
//        });
//        System.out.println(list1);

//        Collections.sort(list1);
//        System.out.println(list1);
        ArrayList<Student> list2 = new ArrayList<>();
        System.out.println(list1.stream().filter(new Predicate<Student>() {
            @Override
            public boolean test(Student student) {
                return student.getAddress().equals(Address.Beijing);
            }
        }).collect(Collectors.toList()));


    }
}

在这里我equals后面跟的是Address.Beijing，如果跟"Beijing"会报错。

另外，若想实现学生之间的有两种形式，Comparator和Comparable

如果用的是Comparator则：

package com.cskaoyan._kingshomework;

import java.util.*;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class Homework {
    public static void main(String[] args) {

        List<Student> list1 = Arrays.asList(
                new Student("zs",18,Address.Beijing),
                new Student("ls",22,Address.Shanghai),
                new Student("wu",20,Address.Wuhan),
                new Student("zl",24,Address.Wuhan),
                new Student("kongling",23,Address.Shanghai)
        );

        list1.sort(new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.getAge() - o2.getAge();
            }
        });
        System.out.println(list1);

//        Collections.sort(list1);
//        System.out.println(list1);
//        System.out.println(list1.stream().filter(new Predicate<Student>() {
//            @Override
//            public boolean test(Student student) {
//                return student.getAddress().equals(Address.Beijing);
//            }
//        }).collect(Collectors.toList()));


    }
}

此时我只需使用list1返回即可

如果用的是Comparable则代码为:

public class Homework {
    public static void main(String[] args) {

        List<Student> list1 = Arrays.asList(
                new Student("zs",18,Address.Beijing),
                new Student("ls",22,Address.Shanghai),
                new Student("wu",20,Address.Wuhan),
                new Student("zl",24,Address.Wuhan),
                new Student("kongling",23,Address.Shanghai)
        );

//        list1.sort(new Comparator<Student>() {
//            @Override
//            public int compare(Student o1, Student o2) {
//                return o1.getAge() - o2.getAge();
//            }
//        });
//        System.out.println(list1);

        Collections.sort(list1);
        System.out.println(list1);
        System.out.println(list1.stream().filter(new Predicate<Student>() {
            @Override
            public boolean test(Student student) {
                return student.getAddress().equals(Address.Beijing);
            }
        }).collect(Collectors.toList()));


    }
}

enum Address {
    Beijing,
    Shanghai,
    Wuhan
}

class Student implements Comparable{
    String name;
    int age;
    Address address;

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", address=" + address +
                '}';
    }

    @Override
    public int compareTo(Object o) {
        return this.getAge() - ((Student)o).getAge();
    }
}

可以看到此时我使用的是Collection集合进行返回，在Student类中实现接口。