Comparable接口和Comparator接口

1、一个类在设计之初就要实现对该类对象的排序功能，那么这个类要实现Comparable接口，实现public int compareTo(T t)方法。如代码中的Student类。
对于实现Comparable接口的类，

调用java.util.Arrays.sort(Object[] a)对包含对象实例的数组进行排序，

调用java.util.Collections.sort(List<T> list)对包含对象实例的list进行排序。

2、若一个类在设计之初并没有对排序功能的需求，而是在后续的使用中想要对这个类添加排序的功能，如Teacher类。这时的办法是实现一个比较器类，
如TeacherComparator类，实现public int compare(T t1, T t2)让该类实现Comparator接口。

调用java.util.Arrays.sort(T[] a, Comparator<? super T> c) 对包含对象实例的数组进行排序，
调用java.util.Collections.sort(List<T> list, Comparator<? super T> c) 对包含对象实例的list进行排序。

3、a.compareTo(b)方法和compare(a,b)方法，在方法中判断两个对象的某个或某些属性的大小关系，若a的某个属性比b的属性小，返回负整数，若相等，返回0，否则返回正整数。

if a.attr < b.attr

return -1 

else 

return 1

上面即为按照某属性从小到大排序。

 

if a.attr < b.attr

return 1 

else 

return -1

上面为按照某属性从大到小排序。

以下代码按照Score从高到底排序，若Score相同，则按照age从小到大排序。

public class Student implements Comparable<Student>{
    
    private String name;
    private int id;
    private int score;
    private int age;
    
    
    public Student(String name, int id, int score, int age) {
        super();
        this.name = name;
        this.id = id;
        this.score = score;
        this.age = age;
    }
    
    @Override
    public int compareTo(Student s) {
        if (this.score < s.getScore()) {
            return 1;
        } else if (this.score > s.getScore()) {
            return -1;
        } else {
            if (this.age < s.getAge()) {
                return -1;
            } else {
                return 1;
            }
        }
    }
    
    public String getName() {
        return name;
    }



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



    public int getId() {
        return id;
    }



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



    public int getScore() {
        return score;
    }



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



    public int getAge() {
        return age;
    }



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



    public static void main(String[] args) {
        
        System.out.println("use Interface Comparable");
        System.out.println();
        
        System.out.println("Array ");
        System.out.println();
        
        Student[] stu = {new Student("a", 1, 88, 18),
                new Student("b", 2, 78, 16),
                new Student("c", 3, 88, 17),
                new Student("d", 4, 78, 16),
                new Student("e", 5, 100, 18)};
        
        Arrays.sort(stu);
        for (Student s : stu) {
            System.out.println(s.getScore() + "  " + s.getAge());
        } 
        
        System.out.println("***");
        System.out.println("List");
        System.out.println();
        
        List<Student> list = new ArrayList<Student>();
        list.add(new Student("a", 1, 88, 18));
        list.add(new Student("b", 2, 78, 16));
         list.add(new Student("c", 3, 88, 17));
         list.add(new Student("d", 4, 78, 16));
         list.add(new Student("e", 5, 100, 18));
         Collections.sort(list);
         for (Student s : list) {
             System.out.println(s.getScore() + "  " + s.getAge());
         }
         System.out.println();
         System.out.println("use Interface Comparator");
         System.out.println();
         System.out.println("Array");
         System.out.println();
         
         Teacher[] teach = {
                 new Teacher("a", 1, 88, 18),
                    new Teacher("b", 2, 78, 16),
                    new Teacher("c", 3, 88, 17),
                    new Teacher("d", 4, 78, 16),
                    new Teacher("e", 5, 100, 18)};
        Arrays.sort(teach, new TeacherComparator());     
        for (Teacher t : teach) {
            System.out.println(t.getScore() + "  " + t.getAge());
        } 
        
        System.out.println();
        System.out.println("List");
        System.out.println();
        
        List<Teacher> tList = new ArrayList<Teacher>();
        tList.add(new Teacher("a", 1, 88, 18));
        tList.add(new Teacher("b", 2, 78, 16));
        tList.add(new Teacher("c", 3, 88, 17));
        tList.add(new Teacher("d", 4, 78, 16));
         tList.add(new Teacher("e", 5, 100, 18));
         Collections.sort(tList, new TeacherComparator());
         for (Teacher t : tList) {
             System.out.println(t.getScore() + "  " + t.getAge());
         }
    }
}

class Teacher {
    
    private String name;
    private int id;
    private int score;
    private int age;
    

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


    public String getName() {
        return name;
    }


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


    public int getId() {
        return id;
    }


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


    public int getScore() {
        return score;
    }


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


    public int getAge() {
        return age;
    }


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

class TeacherComparator implements Comparator<Teacher> {

    @Override
    public int compare(Teacher t1, Teacher t2) {
        
        if (t1.getScore() < t2.getScore()) {
            return 1;
        } else if (t1.getScore() > t2.getScore()) {
            return -1;
        } else {
            if (t1.getAge() < t2.getAge()) {
                return -1;
            } else {
                return 1;
            }
        }
    }
    
}