Java 集合元素排序接口Comparable

什么是Comparable

public interface Comparable<T> {
    /**
     * 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.
     *
     * <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==
     * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
     * implies that <tt>x.compareTo(y)</tt> must throw an exception iff
     * <tt>y.compareTo(x)</tt> throws an exception.)
     *
     * <p>The implementor must also ensure that the relation is transitive:
     * <tt>(x.compareTo(y)&gt;0 &amp;&amp; y.compareTo(z)&gt;0)</tt> implies
     * <tt>x.compareTo(z)&gt;0</tt>.
     *
     * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>
     * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for
     * all <tt>z</tt>.
     *
     * <p>It is strongly recommended, but <i>not</i> strictly required that
     * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>.  Generally speaking, any
     * class that implements the <tt>Comparable</tt> 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."
     *
     * <p>In the foregoing description, the notation
     * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
     * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
     * <tt>0</tt>, or <tt>1</tt> according to whether the value of
     * <i>expression</i> 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<Student> 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<Student> 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;
    }
}