Frequency of occurrence of Strings/Words in a list.

I had a problem where I have to keep a track of the frequency of occurrence of a given string in a record (row). I was thinking of doing this using a Map (Probably the best candidate here) where the key is the string and value is the frequency of occurrence.

But there was a catch here, what should we do on the first occurrence of the string as it won't be present in the Map. Below is the solution I came up with:

import java.util.HashMap;

public class MyHashMap {
       private HashMap<String, Integer> myHashMap;

       public MyHashMap(final HashMap<String, Integer> myHashMap) {
           this.myHashMap = myHashMap;
       }

       public void put(String key) {
           Integer integer = myHashMap.get(key);

           if (integer == null) {
                myHashMap.put(key, new Integer(1));
           } else {
                myHashMap.put(key, ++integer);
          }
        }

        @Override
        public String toString() {
            return "MyHashMap [myHashMap=" + myHashMap + "]";
         }

        public static void main(String[] args) {
             MyHashMap m = new MyHashMap(new HashMap<>());

             m.put("Steve");
             m.put("Steve");
             m.put("Simon");

             System.out.println(m);
       }
}

Output:

        MyHashMap [myHashMap={Simon=1, Steve=2}]

Above implementation works fine however you can see we had to do first occurrence handling in put method from our side which I wanted to avoid so I was looking at some other solution. No problem Java 8 Stream API is here and does this thing very smoothly in one liner.

Following is the Java 8 code snippet :

import java.util.ArrayList;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

import java.util.stream.Stream;

public class StreamsDemo {

public static void main(String[] args) {

List<String> list = new ArrayList<>();

list.add("Steve");

list.add("Steve");

list.add("Frank");

list.add("Tom");

list.add("Steve");

Stream<String> stream = list.stream();

Map<String, Long> collect = stream.collect(Collectors.groupingBy(e -> e, Collectors.counting()));

System.out.println(collect);

}

}

Output:

{Frank=1, Tom=1, Steve=3}

Above implementation which uses Java 8 Stream API does the trick as if we are executing a aggregate function COUNT(*) with GROUP BY. Cool !!