對ArrayList進行排序的兩種方式

根據<c> java.util.Collections重載的兩個sort方法，我們可以用兩種方式來實現對ArrayList進行排序.

 1)  static  void   sort(List<T> list, Comparator<? super T> c)  實現<I>java.util.Comparator，創建實例作爲參數

  2) static <T implements Comparable<? super T>> void sort(List<T> list) 要排序的類繼承<I>java.lang.Comparable,不需創建實例。

這是第一種排序方法示例，類SortByName實現<I>java.util.Comparator接口裏面的compare方法，只能實現按姓名排序。但是，如果我的對象一種排序規則滿足不了怎麼辦呢？例如我的student既想要按年齡排的結果，也想要按姓名排的結果，那就只能在創建一個類SortByAge再重新Comparator裏面的compare方法。你會從排序方式2）的代碼中發現它沒有這種優勢。

package com.lgy.test;
import java.util.*;
public class Test {
//代碼示例 static void	sort(List<T> list, Comparator<? super T> c)
 public static void main(String[] args) {

		Student zlj = new Student("丁曉宇", 60);
		Student dxy = new Student("呂洋", 60);
		Student cjc = new Student("呂峯", 1);
		Student lgc = new Student("劉武", 19);

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

		studentList.add(zlj);
		studentList.add(dxy);
		studentList.add(cjc);
		studentList.add(lgc);

		System.out.println("按年齡升序：");
		Collections.sort(studentList, new SortByAge());//創建實例作爲參數
		
		for (Student student : studentList) {
			System.out.println(student.getName() + "***" + student.getAge());
		}
		System.out.println("按姓名排序：");
		Collections.sort(studentList, new SortByName());
		for (Student student : studentList) {
			System.out.println(student.getName() + "***" + student.getAge());
		}
	}
}

class SortByAge implements Comparator {
	public int compare(Object o1, Object o2) {
		Student s1 = (Student) o1;
		Student s2 = (Student) o2;
		
		 if (s1.getAge() > s2.getAge())
		 return 1;
		 return -1;
		 //不用寫s1.getAge()==s2.getAge()的情況
	}
}

class SortByName implements Comparable {
	public int compare(Object o1, Object o2) {
		Student s1 = (Student) o1;
		Student s2 = (Student) o2;
		return s1.getName().compareTo(s2.getName());
		
	}
}

package com.lgy.test;

public class Student{
	private int age;
	private String name;

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

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

}

下面是第二種排序方式示例，不用創建實例，但是隻能按照一種規則進行排序，也就是說，如果我們的對象只需要一種排序規則的話，可以用這種方式。理由很簡單，實現Comparable接口，重寫Compare方法裏面只能按照一種規則排序。

package com.lgy.test;
import java.util.*;
public class Test {
	public static void main(String[] args) {
//代碼示例 static <T extends Comparable<? super T>> void sort(List<T> list) 
		Student zlj = new Student("丁曉宇", 60);
		Student dxy = new Student("呂洋", 60);
		Student cjc = new Student("呂峯", 1);
		Student lgc = new Student("劉武", 19);

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

		studentList.add(zlj);
		studentList.add(dxy);
		studentList.add(cjc);
		studentList.add(lgc);


		System.out.println("按年齡升序：");
		 Collections.sort(studentList);
		 //不需要創建實例作爲參數，Student類已經實現Comparable,
//		 但是一個類只能有一種排序規則，也就是說，要麼按年齡排，要麼按姓名排。
		for (Student student : studentList) {
			System.out.println(student.getName() + "***" + student.getAge());
		}

	}
}

package com.lgy.test;

public class Student implements Comparable {
	private int age;
	private String name;

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

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@Override
	public int compareTo(Object o) {
		Student s = (Student) o;
		if (s.getAge() > this.getAge())
			return 1;
		else
			return -1;
	}
}