beginnerString ProblemsJava

Count the Vowels and Consonants in a String

Count how many characters of a string are vowels and how many are consonants.

Quick Answer

Walk through the string one character at a time. Lowercase each letter, and if it is a, e, i, o or u add it to the vowel count; if it is any other letter add it to the consonant count. Ignore spaces, digits and punctuation. One pass gives both totals in O(n) time.

Problem Statement

Given a string, count how many of its characters are vowels (a, e, i, o, u) and how many are consonants. The check should be case-insensitive, so A and a both count as vowels.

Ignore any character that is not an English letter, such as spaces, digits and punctuation. Report the two counts.

Input: A single string s.

Output: The vowel count and the consonant count.

Examples

Example 1
Input:  "hello"
Output: Vowels: 2, Consonants: 3

e and o are vowels; h, l, l are consonants.

Example 2
Input:  "Java Programming"
Output: Vowels: 5, Consonants: 10

Vowels a, a, o, a, i; the space is skipped and ten letters remain as consonants.

Constraints

  • 0 <= s.length <= 10^5
  • Only English letters a-z and A-Z are counted

Think Before You Code

Reveal the questions to ask yourself first
  • How do you test whether a single character is a vowel?
  • How do you make the check work for both uppercase and lowercase letters?
  • What should happen to spaces, digits and punctuation?
  • Do you need two separate counters or can you derive one from the other?

Hints

Open them one at a time — try after each before revealing the next.

Hint 1
Lowercase each character first so you only compare against a, e, i, o, u.
Hint 2
Confirm the character is actually a letter (between 'a' and 'z') before deciding vowel vs consonant.
Hint 3
If it is a letter and one of the five vowels, increment vowels; otherwise increment consonants.

Approach

Reveal the step-by-step approach

Make one pass over the string, classifying each character.

  1. Start vowels and consonants at 0.
  2. For each character, convert it to lowercase.
  3. If it is not in the range a–z, skip it (not a letter).
  4. If it equals a, e, i, o or u, increment vowels.
  5. Otherwise it is a consonant, so increment consonants.
  6. After the loop, report both counts.

Lowercasing first collapses the upper- and lowercase cases into one check, and the letter-range test keeps spaces and punctuation out of the totals.

Dry Run

Walk through the example step by step

Counting in "hello":

char | lower | letter? | vowel? | vowels | consonants
-----+-------+---------+--------+--------+-----------
h    | h     | yes     | no     | 0      | 1
e    | e     | yes     | yes    | 1      | 1
l    | l     | yes     | no     | 1      | 2
l    | l     | yes     | no     | 1      | 3
o    | o     | yes     | yes    | 2      | 3
final: Vowels: 2, Consonants: 3

Solution

Reveal the full Java solution
public class VowelConsonantCounter {
    public static String count(String s) {
        int vowels = 0;
        int consonants = 0;
        for (int i = 0; i < s.length(); i++) {
            char c = Character.toLowerCase(s.charAt(i));
            if (c < 'a' || c > 'z') {
                continue; // skip spaces, digits, punctuation
            }
            if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {
                vowels++;
            } else {
                consonants++;
            }
        }
        return "Vowels: " + vowels + ", Consonants: " + consonants;
    }

    public static void main(String[] args) {
        System.out.println(count("hello"));            // Vowels: 2, Consonants: 3
        System.out.println(count("Java Programming")); // Vowels: 5, Consonants: 10
    }
}

Converting each character to lowercase means the vowel test only lists the five lowercase vowels while still catching uppercase input. The range check c < 'a' || c > 'z' filters out everything that is not a letter, so spaces and symbols never inflate the consonant count. Everything happens in a single pass.

Time: O(n) where n is the string lengthSpace: O(1)

Common Mistakes

  • Counting spaces or punctuation as consonants by skipping the letter check.
  • Only checking lowercase vowels and missing uppercase A, E, I, O, U.
  • Treating 'y' as a vowel without deciding on that rule first.

Edge Cases to Test

  • An empty string yields Vowels: 0, Consonants: 0.
  • A string of only spaces or digits yields both counts at 0.
  • A string of only vowels like "aeiou" gives 5 vowels and 0 consonants.

Interview Follow-Ups

  • How would you also count digits and spaces separately in the same pass?
  • How would you return the results as a map instead of a formatted string?
  • How would you extend the check to treat 'y' as a vowel?

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