How to count the occurrences of an element in array – Java?

Write a program to count the occurrences of an element in an array is one of the basic question which will be asked in a Java Interview. After the introduction of Collection framework performing this operation is a pretty easy task. Let see how we can count the occurrence using HashMap.

Java Program to count the occurrences of an element in an array

package com.javainterviewpoint;

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

public class CountOccurence
{
    public static void main(String[] args)
    {
        //Input Array
        int[] input = new int[]{ 1,4,5,2,3,5,1,6,4,7,1,3,6,8,2,5};

        //countMap holds the count details of each element
        Map<Integer, Integer> countMap = new HashMap<Integer, Integer>();
        for (int i = 0; i < input.length; i++)
        {
            int key = input[i];
            if (countMap.containsKey(key))
            {
                int count = countMap.get(key);
                count++;
                countMap.put(key, count);
            } else
            {
                countMap.put(key, 1);
            }
        }
        
        //Printing the Element and its occurrence in the array
        for(Entry<Integer,Integer> val : countMap.entrySet())
        {
            System.out.println(val.getKey() + " occurs " + val.getValue() + " time(s)");
        }
    }
}

Output :

Count the occurrences of an element in an array in Java

We will be performing the below steps to count the occurrence.

• As a first step we will be creating a HashMap “countMap” to hold the element (Key) and the count as the value.
• For each of the element in the input array, check if it is present in the countMap, using containsKey() method.

 if (countMap.containsKey(key))

• If the element is present in the countMap, then increment the count flag by 1 and put the element as the Key and the count as the value to our countMap

count++;
countMap.put(key, count);

• If the element is not present then add the element as the key and 1 as the value

countMap.put(key, 1);

• Finally print the countMap, which will be containing the elements and its occurrences.