java-count-chars

Posted on: 2019-10-08 2019-11-19
Tags: Java String

Count Occurrences of a Char in a String

1. Overview

There are many ways to count the number of occurrences of a char in a String in Java.

In this quick article, we’ll focus on a few examples of how to count characters, first, with the core Java library and then with other libraries and frameworks such as Spring and Guava.

2. Using Core Java Lib

==== 2.1. Imperative Approach

Some developers may prefer to use core Java. There are many ways for counting the number of occurrences of a char in a String.

Let’s start with a simple/naive approach:

String someString = "elephant";
char someChar = 'e';
int count = 0;

for (int i = 0; i < someString.length(); i++) {
    if (someString.charAt(i) == someChar) {
        count++;
    }
}
assertEquals(2, count);

Not surprisingly this will work, but – also not surprisingly – there are better ways to do this.

2.2. Using Recursion

A less obvious, but still interesting solution is to use recursion:

private static int countOccurences(
  String someString, char searchedChar, int index) {
    if (index >= someString.length()) {
        return 0;
    }

    int count = someString.charAt(index) == searchedChar ? 1 : 0;
    return count + countOccurences(
      someString, searchedChar, index + 1);
}

We can invoke this recursive method in the following way: useRecursionToCountChars(“elephant”, ‘e’, 0)

2.4. Using Regular Expressions

Another way would be to use regular expressions:

Pattern pattern = Pattern.compile("[^e]*e");
Matcher matcher = pattern.matcher("elephant");
int count = 0;
while (matcher.find()) {
    count++;
}

assertEquals(2, count);

Just note that this solution is technically correct, but sub-optimal, as it’s overkill to use the very powerful regular expressions to solve such a simple problem as finding the number of occurrences of a character in a string.

2.5. Using Java 8 Features

New features available in Java 8 can be very helpful here.

Let’s use streams and lambdas to implement the count:

String someString = "elephant";
long count = someString.chars().filter(ch -> ch == 'e').count();
assertEquals(2, count);

long count2 = someString.codePoints().filter(ch -> ch == 'e').count();
assertEquals(2, count2);

So, this is clearly a cleaner and more readable solution using the core library.

3. Using External Libraries

Let’s now look at a few solutions that make use of utilities from external libraries.

3.1. Using StringUtils

In general, it is always better to use an existing solution instead of inventing our own. The commons.lang.StringUtils class provides us with the countMatches() method, which can be used for counting chars or even sub-strings in given String.

First, we need to include the appropriate dependency:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.5</version>
</dependency>

We can find the latest version on Maven Central.

Let’s now use countMatches() to count the number of ‘e’ characters in the “elephant” String literal:

int count = StringUtils.countMatches("elephant", "e");
assertEquals(2, count);

3.2. Using Guava

Guava can also be helpful in counting chars. We need to define the dependency:

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>21.0</version>
</dependency>

We can find the latest version on Maven Central.

Let’s see how Guava can quickly help us to count chars:

int count = CharMatcher.is('e').countIn("elephant");
assertEquals(2, count);

3.3. Using Spring

Naturally adding the Spring Framework into your project just to count chars doesn’t make sense. However, if we already have it in our project, we just need to use the countOccurencesOf() method:

int count = StringUtils.countOccurrencesOf("elephant", "e");
assertEquals(2, count);

4. Conclusion

In this quick tutorial, we focused on various ways to count chars in the String. Some of them were designed purely in Java; some required additional libraries.

Our recommendation is to use already existing utilities from StringUtils, Guava or Spring. However, if one prefers to use only plain Java, this article offers some possibilities to accomplish just that well with Java 8.

The complete source code for these examples is available in this GitHub project.

getdocs

3043