Java Program to Count the Occurrences of Each Character in String

In this post, we will discuss three ways to count the number of occurrences of a char in a String in Java.

All the three methods are generic, simply you can pass input string and input char as argument and method return character count.

1. Using simple for loop.
2. using Java 8 using Java 8 streams and lambdas(single line).
3. Using regular expressions

Java Program to Count the Occurrences of Each Character in String

import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Java Program to Count the Occurrences of Each Character in String
 * @author javaguides.net
 */

public class CountOccuranceOfCharInString {

    public static void main(String[] args) {
        int count = countMatches("javaguides", 'a');
        System.out.println("using for loop : " + count);

        System.out.println("using java 8 streams and lambdas : " +
            countOccurrencesOf("javaguides", 'a'));

        System.out.println("using regular expressions : " +
            countCharsUsingReg("javaguides", 'a'));
    }

    /**
     * <p>
     * Counts how many times the char appears in the given string.
     * </p>
     *
     * <p>
     * A {@code null} or empty ("") String input returns {@code 0}.
     * </p>
     * 
     * @param str
     *            the CharSequence to check, may be null
     * @param ch
     *            the char to count
     * @return the number of occurrences, 0 if the CharSequence is {@code null}
     * @since 3.4
     */
    private static int countMatches(final String str, final char ch) {
        if (str == null || str.length() == 0) {
            return 0;
        }
        int count = 0;
        // We could also call str.toCharArray() for faster look ups but that
        // would generate more garbage.
        for (int i = 0; i < str.length(); i++) {
            if (ch == str.charAt(i)) {
                count++;
            }
        }
        return count;
    }

    // Using Java 8 Features
    /**
     * Method to finding the number of occurrences of a character in a string
     * using java 8 streams and lambdas.
     * 
     * @param str
     * @param character
     * @return
     */
    private static long countOccurrencesOf(final String str, final char character) {
        return str.codePoints().filter(ch - > ch == character).count();
    }

    /**
     * Method to finding the number of occurrences of a character in a string
     * using regular expressions;
     * 
     * @param str
     * @param character
     * @return
     */
    private static long countCharsUsingReg(final String str, final char character) {
        Pattern pattern = Pattern.compile("[^" + character + "]*" + character + "");
        Matcher matcher = pattern.matcher(str);
        int count = 0;
        while (matcher.find()) {
            count++;
        }
        return count;
    }
}

Output:

using for loop : 2
using java 8 streams and lambdas : 2
using regular expressions : 2

Let's discuss above program and it's methods.

Using simple for loop

Counts how many times the char appears in the given string. handles null or empty ("") String input which returns 0.

private static int countMatches(final String str, final char ch) {
    if (str == null || str.length() == 0) {
         return 0;
    }
    int count = 0;
    // We could also call str.toCharArray() for faster look ups but that
    // would generate more garbage.
    for (int i = 0; i < str.length(); i++) {
         if (ch == str.charAt(i)) {
              count++;
         }
    }
    return count;
}

Using Java 8 using Java 8 streams and lambdas(single line)

Method to finding the number of occurrences of a character in a string using java 8 streams and lambdas.

private static long countOccurrencesOf(final String str, final char character) {
    return str.codePoints().filter(ch -> ch == character).count();
}

Using regular expressions

Method to finding the number of occurrences of a character in a string using regular expressions.

private static long countCharsUsingReg(final String str, final char character) {
    Pattern pattern = Pattern.compile("[^" + character + "]*" + character + "");
    Matcher matcher = pattern.matcher(str);
    int count = 0;
    while (matcher.find()) {
         count++;
    }
    return count;
}

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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