统计List中对象重复次数【转】 需JDK1.5+

import java.util.ArrayList;  
import java.util.Collections;  
import java.util.HashMap;  
import java.util.HashSet;  
import java.util.List;  
import java.util.Map;  
import java.util.Set;  
import java.util.TreeMap;  

public class CountDuplicatedList {  

    public static void main(String[] args) {  

        List list = new ArrayList();  
        list.add("a");  
        list.add("b");  
        list.add("c");  
        list.add("d");  
        list.add("b");  
        list.add("c");  
        list.add("a");  
        list.add("a");  
        list.add("a");  

        System.out.println("\n例子1 - 计算'a'出现的次数");  
        System.out.println("a : " + Collections.frequency(list, "a"));  

        System.out.println("\n例子2 - 计算所有对象出现的次数");  
        Set uniqueSet = new HashSet(list);  
        for (String temp : uniqueSet) {  
            System.out.println(temp + ": " + Collections.frequency(list, temp));  
        }  

        System.out.println("\n例子3 -用Map来计算对象出现的次数");  
        Map map = new HashMap();  

        for (String temp : list) {  
            Integer count = map.get(temp);  
            map.put(temp, (count == null) ? 1 : count + 1);  
        }  
        printMap(map);  

        System.out.println("\nMap排序-以key排序");  
        Map treeMap = new TreeMap(map);  
        printMap(treeMap);  

    }  

    public static void printMap(Map map) {  
        for (Map.Entry entry : map.entrySet()) {  
            System.out.println("Key-value : " + entry.getKey() + "- "  
                    + entry.getValue());  
        }  
    }  
}