什么是Comparable

public interface Comparable
    
      {    /**     * Compares this object with the specified object for order.  Returns a     * negative integer, zero, or a positive integer as this object is less     * than, equal to, or greater than the specified object.     *     * 
     
The implementor must ensure sgn(x.compareTo(y)) ==     * -sgn(y.compareTo(x)) for all x and y.  (This     * implies that x.compareTo(y) must throw an exception iff     * y.compareTo(x) throws an exception.)     *     * 
     
The implementor must also ensure that the relation is transitive:     * (x.compareTo(y)>0 && y.compareTo(z)>0) implies     * x.compareTo(z)>0.     *     * 
     
Finally, the implementor must ensure that x.compareTo(y)==0     * implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for     * all z.     *     * 
     
It is strongly recommended, but not strictly required that     * (x.compareTo(y)==0) == (x.equals(y)).  Generally speaking, any     * class that implements the Comparable interface and violates     * this condition should clearly indicate this fact.  The recommended     * language is "Note: this class has a natural ordering that is     * inconsistent with equals."     *     * 
     
In the foregoing description, the notation     * sgn(expression) designates the mathematical     * signum function, which is defined to return one of -1,     * 0, or 1 according to whether the value of     * expression is negative, zero or positive.     *     * @param   o the object to be compared.     * @return  a negative integer, zero, or a positive integer as this object     *          is less than, equal to, or greater than the specified object.     *     * @throws NullPointerException if the specified object is null     * @throws ClassCastException if the specified object's type prevents it     *         from being compared to this object.     */    public int compareTo(T o);}
    

import java.util.ArrayList;import java.util.Collections;import java.util.List;import java.util.Set;import java.util.TreeSet;public class TestCom {    public static void main(String[] args) {        Set
    
      studentSet = new TreeSet<>();        studentSet.add(new Student("jack", 32));        studentSet.add(new Student("tom", 22));        studentSet.add(new Student("mary", 35));        studentSet.add(new Student("tim", 11));        studentSet.add(new Student("tony", 49));        studentSet.add(new Student("dd", 30));        System.out.println(studentSet);        System.out.println("==============================");        List
     
       studentList = new ArrayList<>();        studentList.add(new Student("jack", 32));        studentList.add(new Student("tom", 22));        studentList.add(new Student("mary", 35));        studentList.add(new Student("tim", 11));        studentList.add(new Student("tony", 49));        studentList.add(new Student("dd", 30));        System.out.println(studentList);        Collections.sort(studentList);        System.out.println(studentList);    }}class Student implements Comparable {    private int age;    private String name;    public void setAge(int age) {        this.age = age;    }    public int getAge() {        return age;    }    public void setName(String name) {        this.name = name;    }    public String getName() {        return name;    }    public Student(String name, int age) {        this.name = name;        this.age = age;    }    @Override    public String toString() {        return "Student{" + "age=" + age + ", name='" + name + '\'' + '}';    }    @Override    public int compareTo(Object o) {        if (o instanceof Student) {            Student student = (Student) o;            return this.age - student.age;        }        // 返回的数是0代表两个对象相同        return 0;    }}