Check if a String Contains a Number Value in Java

1. Overview

In this short article, we’re going to highlight how to check if a string contains a number in Java.

First, we’ll kick things off by considering solutions using JDK. Then, we’re going to illustrate how to achieve the same objective using external libraries such as Guava and Apache Commons Lang.

2. Introduction to the Problem

Our main goal here is to check if a string contains a number or not. So, the basic idea is to create methods that return true if the given string contains a number, and false otherwise.

Please note that we’ll work under the assumption that our strings are non-null and non-empty so as to avoid repeating boilerplate code for null and empty checks.

3. Using the String Class

The String class comes with a set of handy methods that we can use to check whether a specific string contains a number.

So, let’s take a closer look at each of these methods.

3.1. String#matches Method

The matches() method is introduced typically to test a string against a regex. As the name implies, it simply checks whether or not a string matches a given regular expression.

In short, all we need to do is define the right regex that denotes “contains a number”:

static boolean checkUsingMatchesMethod(String input) {
    return input.matches(".*\\d.*");
}

In general, we used “.*\\d.*” as regex to denote that the string contains at least one digit. Alternatively, we can replace “\\d” with “[0-9]” as they describe the same thing (digit/number).

Now, let’s create a test case to unit test our method:

public class StrContainsNumberUtilsUnitTest {

    private static final String INPUT_WITH_NUMBERS = "We hope 2024 will be great";
    private static final String INPUT_WITHOUT_NUMBERS = "Hello world";

    @Test
    public void givenInputString_whenUsingMatchesMethod_ThenCheck() {
        assertTrue(StrContainsNumberUtils.checkUsingMatchesMethod(INPUT_WITH_NUMBERS));
        assertFalse(StrContainsNumberUtils.checkUsingMatchesMethod(INPUT_WITHOUT_NUMBERS));
        assertFalse(StrContainsNumberUtils.checkUsingMatchesMethod(""));
        assertFalse(StrContainsNumberUtils.checkUsingMatchesMethod(null));
    }

}

3.2. String#replaceAll Method

We can also use the replaceAll() method to address our central question. It returns a string replacing all the characters matching the specified regex.

The basic idea here is to use the regex that matches a digit to replace all the numbers in a particular string:

static boolean checkUsingReplaceAllMethod(String input) {
    String result = input.replaceAll("\\d", "");
    return result.length() != input.length();
 }

Here, we replaced all the numbers with a space. As a result, the input contains a number if the length of the returned string is different from the original one.

Please notice that the matches() method provides a more concise and direct manner to address our central question compared to replaceAll().

Now, let’s add a test case to confirm that everything works as expected:

@Test
public void givenInputString_whenUsingReplaceAllMethod_ThenCheck() {
    assertTrue(StrContainsNumberUtils.checkUsingReplaceAllMethod(INPUT_WITH_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingReplaceAllMethod(INPUT_WITHOUT_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingReplaceAllMethod(""));
    assertFalse(StrContainsNumberUtils.checkUsingReplaceAllMethod(null));
}

4. Using the Pattern Class

Similarly, we can use the Pattern class to fulfill the same goal. This class offers a convenient way to compile a regex and match it against a given string.

So, let’s reuse the same regex we already defined in the previous examples:

static boolean checkUsingPatternClass(String input) {
    return Pattern.compile(".*\\d.*")
      .matcher(input)
      .matches();
}

In a nutshell, the compile() method lets us create a pattern from the “.*\\d.*” regex. The matcher() method creates a Matcher object that can match our input against the compiled “contains a digit” regex.

As the name indicates, the matches() method does the job of checking if our input contains a digit.

Lastly, let’s add another test case for our method:

@Test
public void givenInputString_whenUsingPatternClass_ThenCheck() {
    assertTrue(StrContainsNumberUtils.checkUsingPatternClass(INPUT_WITH_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingPatternClass(INPUT_WITHOUT_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingPatternClass(""));
    assertFalse(StrContainsNumberUtils.checkUsingPatternClass(null));
}

5. Using the Character#isDigit Method

Another way to check if a string contains a number is to use the Character.isDigit() method, which determines whether a particular character is a digit or not.

We can loop through the string and call isDigit() to check if the current character denotes a number:

static boolean checkUsingIsDigitMethod(String input) {
   for (char c : input.toCharArray()) {
      if (Character.isDigit(c)) {
         return true;
      }
   }

   return false;
}

Now, we’ll create a new test case:

@Test
public void givenInputString_whenUsingIsDigitMethod_ThenCheck() {
    assertTrue(StrContainsNumberUtils.checkUsingIsDigitMethod(INPUT_WITH_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingIsDigitMethod(INPUT_WITHOUT_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingIsDigitMethod(""));
    assertFalse(StrContainsNumberUtils.checkUsingIsDigitMethod(null));
}

6. Using Stream API

The Stream API provides a convenient way to check if a given string contains a number. Java 8 introduced a new method chars() in the String class. This method returns an IntStream holding the code values of the characters in the given string.

So, let’s see it in action:

static boolean checkUsingStreamApi(String input) {
    return input.chars()
      .anyMatch(Character::isDigit);
}

As shown above, we used the anyMatch() method to check whether any of the characters represents a digit.

As always, let’s add another test case to confirm our new method:

@Test
public void givenInputString_whenUsingStreamApi_ThenCheck() {
    assertTrue(StrContainsNumberUtils.checkUsingStreamApi(INPUT_WITH_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingStreamApi(INPUT_WITHOUT_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingStreamApi(""));
    assertFalse(StrContainsNumberUtils.checkUsingStreamApi(null));
}

7. Using Apache Commons Lang

Moreover, we can use the Apache Commons Lang library to grapple with the key question. However, we need first to add its dependency to our pom.xml:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.14.0</version>
</dependency>

This library provides the StringUtils class to achieve string operations in a null-safe manner. The class comes with the getDigits() method, which extracts numbers from a specific string.

So, let’s see it in practice:

static boolean checkUsingApacheCommonsLang(String input) {
    String result = StringUtils.getDigits(input);
    return result != null && !result.isEmpty();
}

As depicted above, getDigits() returns a new string containing only digits. So, all we need to do is check if our returned string is not empty to make sure that our input contains digits.

In the usual manner, let’s create a test case:

@Test
public void givenInputString_whenUsingApacheCommonsLang_ThenCheck() {
    assertTrue(StrContainsNumberUtils.checkUsingApacheCommonsLang(INPUT_WITH_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingApacheCommonsLang(INPUT_WITHOUT_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingApacheCommonsLang(""));
    assertFalse(StrContainsNumberUtils.checkUsingApacheCommonsLang(null));
}

8. Using Guava

The Guava library is another option to consider if we want to check if a given string contains digits. Let’s begin by adding its dependency to our pom.xml:

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>31.0.1-jre</version>
</dependency>

Next, let’s see a quick example using the Guava library:

static boolean checkUsingGuava(String input) {
    String result = CharMatcher.forPredicate(Character::isDigit)
      .retainFrom(input);
    return !result.isEmpty();
}

Guava offers the CharMatcher() class to manipulate characters. Here, we used the forPredicate() method to filter only numbers and return them using the retainfrom() method.

Finally, we’ll add a new test case for our method:

@Test
public void givenInputString_whenUsingGuava_ThenCheck() {
    assertTrue(StrContainsNumberUtils.checkUsingGuava(INPUT_WITH_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingGuava(INPUT_WITHOUT_NUMBERS));
    assertFalse(StrContainsNumberUtils.checkUsingGuava(""));
    assertFalse(StrContainsNumberUtils.checkUsingGuava(null));
}

9. Conclusion

In this tutorial, we explored a variety of ways to check if a string contains numbers in Java.

First, we looked at some ways to achieve this using the JDK. Then, we showcased how to tackle our core challenge using a couple of third-party libraries.