Java How to count duplicated items in List

In this post, I will show you How to count number of duplicated items in a List.

public class CountDuplicatedList {

  public static void main(String[] args) {

 List<String> list = new ArrayList<String>();
 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("\nExample 1 - Count 'a' with frequency");
 System.out.println("a : " + Collections.frequency(list, "a"));

 System.out.println("\nExample 2 - Count all with frequency");
 Set<String> uniqueSet = new HashSet<String>(list);
 for (String temp : uniqueSet) {
  System.out.println(temp + ": " + Collections.frequency(list, temp));
 }

 System.out.println("\nExample 3 - Count all with Map");
 Map<String, Integer> map = new HashMap<String, Integer>();

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

  }

  public static void printMap(Map<String, Integer> map){

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

  }

}

Result:

Example 1 - Count 'a' with frequency
a : 4

Example 2 - Count all with frequency
d: 1
b: 2
c: 2
a: 4

Example 3 - Count all with Map
Key : d Value : 1
Key : b Value : 2
Key : c Value : 2
Key : a Value : 4

Good luck!