Count Vowels in Java: Solution, Explanation & Practice

Count vowels in a string

Problem summary

Write a program that counts all vowels (a, e, i, o, u) in a string.

Starter code

public class Main {
    public static void main(String[] args) {
        String text = "Hello World"; // Test case 1
        
        // Count vowels (case insensitive)
        // Print: Vowel count: <result>
    }
}

Expected output and test cases

e, o, o in Hello World
```
Vowel count: 3
```
All vowels once
```
Vowel count: 5
```
No vowels
```
Vowel count: 0
```

Hints

Check each character against vowels
Use toLowerCase() for case insensitive
Can use indexOf or contains on vowel string

Validated solution

Reveal Java solution

public class Main {
    static int vowelCount(String text) {
        int count = 0;
        for (char ch : text.toLowerCase().toCharArray()) {
            if ("aeiou".indexOf(ch) >= 0) count++;
        }
        return count;
    }
    public static void main(String[] args) {
        System.out.println("Vowel count: " + vowelCount("Hello World"));
    }
}

How to approach the problem

Normalize case once, then test every character against the five-vowel set. This keeps the rule visible and avoids a long chain of repeated comparisons.

Approach

Choose whether y counts as a vowel; this exercise does not.
Normalize case.
Increment only for a recognized vowel.

Time and space complexity

Time: O(n). Space: O(n) for toLowerCase; O(1) extra with per-character case checks.

Edge cases to test

The empty string has zero vowels.
Locale-specific case rules matter for general text processing.

Common mistakes

Counting only lowercase letters.
Using contains on a one-character String without considering unnecessary allocations.

Follow-up challenge

Return a frequency map for each vowel rather than one total.

Related Strings exercises

Practice all Strings exercises · Run this idea in the Java compiler