java中Collections的两种排序方法

在编程中时不时需要进行排序，在java中有内置的函数可以简便高效地进行排序，这就是Collections.sort方法。这个方法有两个重载版本，一个是只有一个参数的List<T>，此时要求这个列表的所有对象都是可以比较的。也就是说需要实现Comparable接口，同时实现接口下的compareTo函数。另一个sort方法有两个参数：List<T>和Comparator<? super T>。此时不要求List中的对象实现Comparable接口，但是却要求传人另一个比较对象，这个对象必须实现Comparator接口，同时实现接口下的compare函数。

下面是一个运用两种排序方法的小程序

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

class Student implements Comparable<Student>
{
	private String name;
	private int id;
	private int score;

	public Student()
	{
		// empty
	}

	public Student(String name, int id, int score)
	{
		this.name = name;
		this.id = id;
		this.score = score;
	}

	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;
	}

	@Override
	public String toString()
	{
		return "Student [name=" + name + ", id=" + id + ", score=" + score + "]";
	}

	@Override
	public int compareTo(Student o)
	{
		/*
		 * 如果当前对象小于另一个对象，则返回负数 
		 * 如果当前对象大于另一个对象，则返回正数
		 * 如果当前对象等于（比较意义上的等于）另一个对象，则返回0
		 */
		if (id < o.getId())
			return -1;
		if (id > o.getId())
			return 1;
		return 0;
	}
}

class SortStudentByScore implements Comparator<Student>
{
	@Override
	public int compare(Student o1, Student o2)
	{
		/*
		 * 如果第一个对象小于第二个对象，则返回负数 
		 * 如果第一个对象大于第二个对象，则返回正数
		 * 如果第一个对象等于（比较意义上的等于）第二个对象，则返回0
		 */
		if (o1.getScore() < o2.getScore())
			return -1;
		if (o1.getScore() > o2.getScore())
			return 1;
		return 0;
	}
	
}

class Teacher
{
	private String name;
	private int id;

	public Teacher()
	{
		// empty
	}

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

	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;
	}

	@Override
	public String toString()
	{
		return "Teacher [name=" + name + ", id=" + id + "]";
	}

}

class SortTeacherById implements Comparator<Teacher>
{
	@Override
	public int compare(Teacher o1, Teacher o2)
	{
		return o1.getId() - o2.getId();
	}
	
}

public class Test
{
	private ArrayList<Student> students = new ArrayList<Student>();
	private ArrayList<Teacher> teachers = new ArrayList<Teacher>();

	private void getStudents()
	{
		Student stu1 = new Student("Tom", 1, 70);
		Student stu2 = new Student("Amy", 3, 60);
		Student stu3 = new Student("Sunny", 2, 100);
		Student stu4 = new Student("Tony", 4, 80);

		students.add(stu1);
		students.add(stu2);
		students.add(stu3);
		students.add(stu4);
	}
	
	private void getTeachers()
	{
		Teacher t1 = new Teacher("Li", 1);
		Teacher t2 = new Teacher("Wang", 4);
		Teacher t3 = new Teacher("Zhang", 2);
		Teacher t4 = new Teacher("Zhao", 3);

		teachers.add(t1);
		teachers.add(t2);
		teachers.add(t3);
		teachers.add(t4);
	}

	private void printStudents()
	{
		for (Student stu : students)
			System.out.println(stu);
	}
	
	private void printTeachers()
	{
		for (Teacher t : teachers)
			System.out.println(t);
	}

	private void testSort()
	{
		getStudents();
		System.out.println("-----Student Class implements Comparable------");
		System.out.println("-----------init, before sort------------");
		printStudents();

		Collections.sort(students);
		System.out.println("-------after sort by Comparable, sort by id-------");
		printStudents();

		Collections.sort(students, new SortStudentByScore());
		System.out.println("-------after sort by Comparator, sort by score-----");
		printStudents();
		
		System.out.println();
		
		getTeachers();
		System.out.println("-----Teacher Class not implements Comparable------");
		System.out.println("-----------init, before sort------------");
		printTeachers();
		
		Collections.sort(teachers, new SortTeacherById());
		System.out.println("-----------after sort by Comparator-------------");
		printTeachers();
	}

	public static void main(String[] argv)
	{
		Test test = new Test();
		test.testSort();
	}
}

程序运行结果如下图