java api -- 对列表和数组排序

一. 简单排序

package test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class Test {
	public static void main(String[] args) throws Exception {
        //对列表排序
		List list = new ArrayList();
        list.add("aaa");
        list.add("ccc");
        list.add("bbb");
        System.out.println("列表排序前");
        System.out.println(list.get(0) + " " + list.get(1) + " " + list.get(2));
        //对list进行字典排序
        Collections.sort(list);
        System.out.println("列表排序后");
        System.out.println(list.get(0) + " " + list.get(1) + " " + list.get(2));

        //对数组排序
        String s[] = {"aaa", "ccc", "bbb"};
        System.out.println("数组排序前");
        System.out.println(s[0] + " " + s[1] + " " + s[2]);
        Arrays.sort(s);
        System.out.println("数组排序后");
        System.out.println(s[0] + " " + s[1] + " " + s[2]);
	}
}

 

二.稍微复杂点的

下面是对列表中的一个自实现hash表根据其key1字段值进行降序排列的例子排序。注意实现的关键在于被排序的对象类型实现了java.lang.Comparable接口。 看了下面的例子，相信列表中在存放其他类型，如javabean实现排序也就知道怎么做了。

package test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;

public class Test {
	public static void main(String[] args) throws Exception {
        //对列表排序
		List list = new ArrayList();
		MyMap map1 = new MyMap();
		map1.put("key1", 1);
		MyMap map2 = new MyMap();
		map2.put("key1", 2);
		list.add(map1);
		list.add(map2);
		System.out.println("排序前");
		System.out.println(list.get(0) + " " + list.get(1));
		Collections.sort(list);
		System.out.println("排序后（降序排列）");
		System.out.println(list.get(0) + " " + list.get(1));
	}
}

class MyMap extends HashMap implements Comparable {
	public MyMap(){
		super();
	}
	
	/**
	 * 降序排列
	 */
	public int compareTo(Object b) {
		MyMap m = (MyMap) b;
		int i = Integer.parseInt(this.get("key1") + "");
		int you = Integer.parseInt(m.get("key1") + "");
		if(i > you)
			return -1;
		else if(i < you) 
			return 1;
		else
			return 0;
	}
}

  当然也可以用下面的方法来排序，这就是引用匿名类的技巧

package test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

@SuppressWarnings("unchecked")
public class Test {
	public static void main(String[] args) throws Exception {
        //对列表排序
		List list = new ArrayList();
		
		list = new ArrayList();
		Map map11 = new HashMap();
		map11.put("key1", 1);
		Map map12 = new HashMap();
		map12.put("key1", 2);
		list.add(map11);
		list.add(map12);
		System.out.println("排序前");
		System.out.println(list.get(0) + " " + list.get(1));
		Collections.sort(list, new Comparator() {
			public int compare(Object a, Object b) {
				Map aa = (Map) a;
				Map bb = (Map) b;
				int i = Integer.parseInt(aa.get("key1") + "");
				int you = Integer.parseInt(bb.get("key1") + "");
				if(i > you)
					return -1;
				else if(i < you) 
					return 1;
				else
					return 0;
			}
		});
		System.out.println("排序后（降序排列）");
		System.out.println(list.get(0) + " " + list.get(1));
	}
}