2.Java 比较器

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1.比较器

定义一个比较器，需要实现 Comparator 接口的 compare() 方法，compare() 方法的返回值类型为 int

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Comparator<T> nameComparator = new Comparator<T>() {
    public int compare(T o1, T o2) {
        return 0;
    }
};

(1) 升序排序: 

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if(o1 > o2) {
    return >0;
}
if(o1 == o2) {
    return 0;
}
if(o1 < o2) {
    return <0;
}

(2) 降序排序: 

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if(o1 > o2) {
    return <0;
}
if(o1 == o2) {
    return 0;
}
if(o1 < o2) {
    return >0;
}

2.用比较器实现对集合的排序

(1) 定义 Student 类

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public class Student {

    private int id;

    private String name;

    private int age;

    private int score;

    public Student() {

    }

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

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    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 int getScore() {
        return score;
    }

    public void setScore(int score) {
        this.score = score;
    }

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

(2) 对 ArrayList<Student> 集合进行排序

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Student student1 = new Student(1, "d1", 11, 200);
Student student2 = new Student(2, "a2", 12, 300);
Student student3 = new Student(3, "f3", 13, 15);
Student student4 = new Student(4, "f4", 14, 10);
Student student5 = new Student(5, "b5", 15, 50);
Student student6 = new Student(6, "c6", 16, 100);

ArrayList<Student> studentList = new ArrayList<Student>();

studentList.add(student1);
studentList.add(student2);
studentList.add(student3);
studentList.add(student4);
studentList.add(student5);
studentList.add(student6);

// define a name comparator
Comparator<Student> nameComparator = new Comparator<Student>() {
    public int compare(Student s1, Student s2) {
        String name1 = s1.getName();
        String name2 = s2.getName();
        int result = name1.compareTo(name2);
        return result;
    }
};

// 1.按姓名排序(升序)
Collections.sort(studentList, nameComparator);

for (Student student: studentList) {
    System.out.println(student);
}
// Student{id=2, name='a2', age=12, score=300}
// Student{id=5, name='b5', age=15, score=50}
// Student{id=6, name='c6', age=16, score=100}
// Student{id=1, name='d1', age=11, score=200}
// Student{id=3, name='f3', age=13, score=15}
// Student{id=4, name='f4', age=14, score=10}

// 2.按分数排序(升序)
Collections.sort(studentList, new Comparator<Student>() {
    public int compare(Student s1, Student s2) {
        int score1 = s1.getScore();
        int score2 = s2.getScore();
        return score1 - score2;
    }
});

for (Student student: studentList) {
    System.out.println(student);
}
// Student{id=4, name='f4', age=14, score=10}
// Student{id=3, name='f3', age=13, score=15}
// Student{id=5, name='b5', age=15, score=50}
// Student{id=6, name='c6', age=16, score=100}
// Student{id=1, name='d1', age=11, score=200}
// Student{id=2, name='a2', age=12, score=300}

// 3.按年龄排序(降序)
Collections.sort(studentList, new Comparator<Student>() {
    public int compare(Student s1, Student s2) {
        int age1 = s1.getAge();
        int age2 = s2.getAge();
        return age2 - age1;
    }
});

for (Student student: studentList) {
    System.out.println(student);
}
// Student{id=6, name='c6', age=16, score=100}
// Student{id=5, name='b5', age=15, score=50}
// Student{id=4, name='f4', age=14, score=10}
// Student{id=3, name='f3', age=13, score=15}
// Student{id=2, name='a2', age=12, score=300}
// Student{id=1, name='d1', age=11, score=200}