Identify Duplicated Items In Java List and count

The Below example show you how to count the total number of duplicated entries in a List, using Collections.frequency and Map




CountDuplicatedList.java

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<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);

System.out.println("\nSorted Map");
Map<String, Integer> treeMap = new TreeMap<String, Integer>(map);
printMap(treeMap);

  }

  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());
}

  }


}

Output:

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

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

Key : d Value : 1