Java 集合元素排序接口Comparable

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);
}

• 是一个接口，定制排序规则
• 对实现它的每个类的对象进行整体排序，里面compateTo方法是实现排序的具体方法
• 比如TreeSet、SortedSet、Collections.sort()方法调用进行排序
• String、Integer等类默认实现了这个接口，所以可以排序

 

 

compareTo方法

• 用于比较次对象和指定对象的顺序，o为要比较的对象
• 返回int类型
  • 大于0，表示this大于传进来的对象o，则往后排，即升序
  • 等于0，表示this等于传进来的对象o
  • 小于0，表示this小于传进来的对象o

案例

　　根据学生的年龄进行排序

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;
    }
}