Word Frequency Counter in Java: Solution, Explanation & Practice

Problem summary

Write a program that counts how many times each word appears in a sentence.

Starter code

import java.util.*;

public class Main {
    public static void main(String[] args) {
        String sentence = "the quick brown fox jumps over the lazy dog the"; // Test case 1
        
        // Count word frequencies
        // Print each word and count (alphabetical order)
    }
}

Expected output and test cases

• Word frequencies

  brown: 1
  dog: 1
  fox: 1
  jumps: 1
  lazy: 1
  over: 1
  quick: 1
  the: 3

• Simple frequencies

  a: 3
  b: 2
  c: 1

• Same word repeated

  hello: 5

Hints

1. Use HashMap<String, Integer>
2. getOrDefault() for counting
3. Sort keys for output

Validated solution

import java.util.Map;
import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        String[] words = "the quick brown fox jumps over the lazy dog the".split(" ");
        Map<String, Integer> counts = new TreeMap<>();
        for (String word : words) counts.put(word, counts.getOrDefault(word, 0) + 1);
        for (Map.Entry<String, Integer> entry : counts.entrySet()) System.out.println(entry.getKey() + ": " + entry.getValue());
    }
}

How to approach the problem

A map connects each word to its count. TreeMap keeps keys in alphabetical order, which makes the shown output stable; HashMap would be fine if ordering did not matter.

Approach

1. Split or tokenize the sentence according to the exercise’s word rule.
2. Increment getOrDefault for each word.
3. Choose an ordered map only when ordered output is required.

Time and space complexity

Time: O(n log k) with TreeMap. Space: O(k).

Edge cases to test

• Case normalization changes whether The and the are one key.
• Punctuation may need removal before counting.

Common mistakes

• Using a list and repeatedly scanning it for every word.
• Assuming HashMap iteration is alphabetical.

Follow-up challenge

Count words from a streaming reader without retaining the full text.

Related Collections exercises

• Practice Queue Operations in Java
• Practice Stack Operations in Java
• Practice Remove Duplicates from List in Java

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