Count occurrences of digit in string

This example will determine how to count the number of digits contained within a string using java, java 8 and Google guava. Each snippet below will check if a string is a number then increment a counter validating that there are 4 numbers using junit assertEquals.

Straight up Java

@Test
public void digits_in_string_guava_java() {

    String phrase = "Creating xml using java gives me heart burn, "
            + "hopefully java 8, java 9, java 10 address this";

    int numberOfDigits = 0;
    char[] checkForNumbers = phrase.toCharArray();
    for (char val : checkForNumbers) {
        if (Character.isDigit(val)) {
            numberOfDigits++;
        }
    }
    assertEquals(4, numberOfDigits);
}

Java 8

@Test
public void digits_in_string_guava_java8() {

    String phrase = "Creating xml using java gives me heart burn, "
            + "hopefully java 8, java 9, java 10 address this";

    assertEquals(4, phrase.chars().filter(Character::isDigit).count());
}

Google Guava

@Test
public void digits_in_string_guava() {

    String phrase = "Creating xml using java gives me heart burn, "
            + "hopefully java 8, java 9, java 10 address this";

    String numberOfDigits = CharMatcher.DIGIT.retainFrom(phrase);

    assertEquals(4, numberOfDigits.length());
}