原連結地址  https://blog.csdn.net/chengyabingfeiqi/article/details/49906875
 

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 -統計'a'出現的頻率");
	System.out.println("a : " + Collections.frequency(list, "a"));
 
	System.out.println("\n例子 2 - 統計每一個元素出現的頻率");
	//將List轉換為Set
	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統計每個元素出現的頻率");
	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("\nSorted Map");
	Map treeMap = new TreeMap(map);
	printMap(treeMap);
 
  }
 
  public static void printMap(Map map){
 
	for (Map.Entry entry : map.entrySet()) {
		System.out.println("Key : " + entry.getKey() + " Value : "
			+ entry.getValue());
	}
 
  }
 
}

	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 -統計'a'出現的頻率");
	System.out.println("a : " + Collections.frequency(list, "a"));
 
	System.out.println("\n例子 2 - 統計每一個元素出現的頻率");
	//將List轉換為Set
	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統計每個元素出現的頻率");
	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("\nSorted Map");
	Map treeMap = new TreeMap(map);
	printMap(treeMap);
 
  }
 
  public static void printMap(Map map){
 
	for (Map.Entry entry : map.entrySet()) {
		System.out.println("Key : " + entry.getKey() + " Value : "
			+ entry.getValue());
	}
 
  }
 
}

	for (String temp : uniqueSet) {
		System.out.println(temp + ": " + Collections.frequency(list, temp));
	}
 
	System.out.println("\n例子 3 - 用Map統計每個元素出現的頻率");
	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("\nSorted Map");
	Map treeMap = new TreeMap(map);
	printMap(treeMap);
 
  }
 
  public static void printMap(Map map){
 
	for (Map.Entry entry : map.entrySet()) {
		System.out.println("Key : " + entry.getKey() + " Value : "
			+ entry.getValue());
	}
 
  }
 
}

 
	for (String temp : list) {
		Integer count = map.get(temp);
		map.put(temp, (count == null) ? 1 : count + 1);
	}
	printMap(map);
 
	System.out.println("\nSorted Map");
	Map treeMap = new TreeMap(map);
	printMap(treeMap);
 
  }
 
  public static void printMap(Map map){
 
	for (Map.Entry entry : map.entrySet()) {
		System.out.println("Key : " + entry.getKey() + " Value : "
			+ entry.getValue());
	}
 
  }
 
}

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

 
	for (Map.Entry entry : map.entrySet()) {
		System.out.println("Key : " + entry.getKey() + " Value : "
			+ entry.getValue());
	}
 
  }
 
}

		System.out.println("Key : " + entry.getKey() + " Value : "
			+ entry.getValue());
	}
 
  }
 
}

結果：
例子 1 -統計'a'出現的頻率
a : 4

例子 2 - 統計每一個元素出現的頻率
d: 1
b: 2
c: 2
a: 4

例子 3 - 用Map統計每個元素出現的頻率
Key : d Value : 1
Key : b Value : 2
Key : c Value : 2
Key : a Value : 4

Sorted Map
Key : a Value : 4
Key : b Value : 2
Key : c Value : 2
Key : d Value : 1