Count characters in a piece of text as a one-liner using Java 8 Streaming API

ByTomas Zezula

A WordCount app is arguably the most commonly used example of the MapReduce principle – see Hadoop or Spark tutorials. I wanted to do a similar thing in plain old Java as “a one-liner” (kind of). My objective was to explore the power and expressiveness of the Streaming API and Lambda expressions.

Here is what I wanted to achieve:

Input: "Hello world!"
Result: 
' ': 1
'!': 1
'd': 1
'e': 1
'h': 1
'l': 3
'o': 2
'r': 1
'w': 1

Essentially, the program takes the input text, makes all letters lowercase, counts occurrences of each and every character and prints a sorted array of unique characters along with their counts.

Without further ado, here is how I went about solving the problem:

import java.util.List;
import java.util.stream.Collectors;

public class CharCounter {
     public List<Count> count(String text) {
     return text
              .toLowerCase()
              .chars()
              .distinct()
              .mapToObj(i -> new Count((char) i,
                text
                  .toLowerCase()
                  .chars()
                  .filter(j -> j == i).count()))
              .sorted()
              .collect(Collectors.toList());
  }

  // Java (still!) lacks Tuples, so I created my own.
  static class Count implements Comparable<Count> {
    char key;     // a character
    long count;   // the character count
    Count(char key, long count) {..}
    @Override String toString() {..} // pretty print
    @Override int compareTo(Count o) {
       return this.key - o.key;
    }
  }
  public static void main(String[] args) {
    new CharCounter()
     .count("Hello World!")
     .stream()  // Yeah, it's a list and thus 'streamable'
     .forEach(System.out::println);
  }
}

Hope it makes sense. You might argue that I could have used something like a Map.Entry instead of a custom Count object. However, I find it much cleaner that way. The complete example is available on GitHub.

Thanks for reading and definitely let me know your thoughts on how this could be done differently.

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