java 类排序

参考文档：Java排序: Comparator vs Comparable 入门

java类经常面临排序问题，据我所知：java提供comparable和comparator两种比较方式：

1. comparable方法需要排序的类继承comparable接口，重写compareTo函数，但是只能实现一种对比方法（重写compareTo一次），例如:

类定义如下：

class Student implements Comparable<Student> {
    private String name;
    private String key;
    private float score;
    private int age;

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

    @Override
    /*
    public int compareTo(Student ano) {
        if (this.score < ano.score)
            return -1;
        if (this.score > ano.score)
            return 1;
        else {
            if (this.age > ano.age)
                return 1;
            if (this.age < ano.age)
                return -1;
            else
                return 0;
        }//else
    }//compareTo
    */
    public int compareTo(Student ano){
        return (int)(this.score - ano.score);
    }
}

可以看到compareTo被我们重写，接下来我们可以运行排序，然后观察运行结果：

        Student stus[] = {new Student("lee","9527",22,80)
                ,new Student("zhang","1839",23,83)
                ,new Student("zhao","1284",21,80)};
        print(stus);
        Arrays.sort(stus);
        System.out.println("after sorted:");
        print(stus);

运行结果如下：

Student{name='9527', key='lee', score=22.0, age=80}
Student{name='1839', key='zhang', score=23.0, age=83}
Student{name='1284', key='zhao', score=21.0, age=80}
after sorted:
Student{name='1284', key='zhao', score=21.0, age=80}
Student{name='9527', key='lee', score=22.0, age=80}
Student{name='1839', key='zhang', score=23.0, age=83}

扩展：对元素是自定义类的List进行排序，自定List定义如下：

   public static List<Student> getStudents(){

        List<Student> col = new ArrayList<Student>();

        Random rand = new Random();
        String valkey;
        int valAge, valScore;
        String valName;
        for (int i = 0; i < 10; i++) {
            valkey = UUID.randomUUID().toString();
            valName = RandomStringUtils.random(10, 20, 110, true, true);
            valAge = rand.nextInt(80);
            valScore = rand.nextInt(80);
            col.add(new Student(valkey,valName, valAge, valScore));
        }

        return col;
    }

通过Collections.sort(List)方法进行排序：

　　　　　List<Student> grade = Student.getStudents();
        Iterator<Student> iter = grade.iterator();
        Collections.sort(grade);
        while(iter.hasNext()) {
            //iter.next();
            System.out.println(iter.next().toString());
        }

2. 有时候我们需要对多种关键字进行排序，那么我们需要通过实现java.util.Comparator来实现，我们可以对类Student任何字段进行排序。我们不需要改变类Student，类本身不用实现java.lang.Comparable或者是java.util.Comparator接口，代码如下：

class studSortByName implements Comparator<Student> {
    public int compare(Student st1, Student st2){
        return st1.getName().compareTo(st2.getName());
    }
}

接下来我们通过排序进行测验，我们必须使用Collections.sort(List, Comparator)这个方法而不是Collections.sort(List)方法：

public class ClassSort {
    public static void main(String []args) {
        //Student stu1 = new Student("wen", 90, 17);
        //Student stu2 = new Student("mike", 90, 18);
        //System.out.print("res:" + stu1.compareTo(stu2));
        List<Student> grade = Student.getStudents();
        Iterator<Student> iter = grade.iterator();
        //Collections.sort(grade);
        Collections.sort(grade,new StudSortByName());
        while(iter.hasNext()) {
            //iter.next();
            System.out.println(iter.next().toString());
        }

        /*Student stus[] = {new Student("lee","9527",22,80)
                ,new Student("zhang","1839",23,83)
                ,new Student("zhao","1284",21,80)};
        print(stus);
        Arrays.sort(stus);
        System.out.println("after sorted:");
        print(stus);
        */

    }

    public static void print(Student [] stus){
        for (int i = 0; i < stus.length; i++) {
            System.out.println(stus[i].toString());
        }
    }

}

附注可执行代码如下:

package comparable;

import java.util.*;

import org.apache.commons.lang3.RandomStringUtils;
import org.apache.commons.lang3.builder.Diff;

/**
 * Created by carl on 11/7/15.
 */
class Student implements Comparable<Student> {
    private String name;
    private String key;
    private float score;
    private int age;

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

    @Override
    /*
    public int compareTo(Student ano) {
        if (this.score < ano.score)
            return -1;
        if (this.score > ano.score)
            return 1;
        else {
            if (this.age > ano.age)
                return 1;
            if (this.age < ano.age)
                return -1;
            else
                return 0;
        }//else
    }//compareTo
    */
    public int compareTo(Student ano){
        return (int)(this.score - ano.score);
    }

    public static List<Student> getStudents(){

        List<Student> col = new ArrayList<Student>();

        Random rand = new Random();
        String valkey;
        int valAge, valScore;
        String valName;
        for (int i = 0; i < 10; i++) {
            valkey = UUID.randomUUID().toString();
            valName = RandomStringUtils.random(10, 20, 110, true, true);
            valAge = rand.nextInt(80);
            valScore = rand.nextInt(80);
            col.add(new Student(valkey,valName, valAge, valScore));
        }

        return col;
    }

    public String getName(){
        return name;
    }

    public float getScore() {
        return score;
    }

    public int getAge() {
        return age;
    }

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

class StudSortByName implements Comparator<Student> {
    public int compare(Student st1, Student st2){
        return st1.getName().compareTo(st2.getName());
    }
}

public class ClassSort {
    public static void main(String []args) {
        //Student stu1 = new Student("wen", 90, 17);
        //Student stu2 = new Student("mike", 90, 18);
        //System.out.print("res:" + stu1.compareTo(stu2));
        List<Student> grade = Student.getStudents();
        Iterator<Student> iter = grade.iterator();
        //Collections.sort(grade);
        Collections.sort(grade,new StudSortByName());
        while(iter.hasNext()) {
            //iter.next();
            System.out.println(iter.next().toString());
        }

        /*Student stus[] = {new Student("lee","9527",22,80)
                ,new Student("zhang","1839",23,83)
                ,new Student("zhao","1284",21,80)};
        print(stus);
        Arrays.sort(stus);
        System.out.println("after sorted:");
        print(stus);
        */

    }

    public static void print(Student [] stus){
        for (int i = 0; i < stus.length; i++) {
            System.out.println(stus[i].toString());
        }
    }

}