How do I check that a Java String is not all whitespaces?

If you are using Java 11 or more recent, the new isBlank string method will come in handy:

!s.isBlank();

If you are using Java 8, 9 or 10, you could build a simple stream to check that a string is not whitespace only:

!s.chars().allMatch(Character::isWhitespace));

In addition to not requiring any third-party libraries such as Apache Commons Lang, these solutions have the advantage of handling any white space character, and not just plain ' ' spaces as would a trim-based solution suggested in many other answers. You can refer to the Javadocs for an exhaustive list of all supported white space types. Note that empty strings are also covered in both cases.

Just an performance comparement on openjdk 13, Windows 10. For each of theese texts:

"abcd"
"    "
" \r\n\t"
" ab "
" \n\n\r\t   \n\r\t\t\t   \r\n\r\n\r\t \t\t\t\r\n\n"
"lorem ipsum dolor sit amet  consectetur adipisici elit"
"1234657891234567891324569871234567891326987132654798"

executed one of following tests:

// trim + empty
input.trim().isEmpty()

// simple match
input.matches("\\S")

// match with precompiled pattern
final Pattern PATTERN = Pattern.compile("\\S");
PATTERN.matcher(input).matches()

// java 11's isBlank
input.isBlank()

each 10.000.000 times.

The results:

METHOD    min   max   note
trim:      18   313   much slower if text not trimmed
match:   1799  2010   
pattern:  571   662   
isBlank:   60   338   faster the earlier hits the first non-whitespace character

Quite surprisingly the trim+empty is the fastest. Even if it needs to construct the trimmed text. Still faster then simple for-loop looking for one single non-whitespaced character...

EDIT: The longer text, the more numbers differs. Trim of long text takes longer time than just simple loop. However, the regexs are still the slowest solution.

With Java-11+, you can make use of the String.isBlank API to check if the given string is not all made up of whitespace -

String str1 = "    ";
System.out.println(str1.isBlank()); // made up of all whitespaces, prints true

String str2 = "    a";
System.out.println(str2.isBlank()); // prints false

The javadoc for the same is :

/**
 * Returns {@code true} if the string is empty or contains only
 * {@link Character#isWhitespace(int) white space} codepoints,
 * otherwise {@code false}.
 *
 * @return {@code true} if the string is empty or contains only
 *         {@link Character#isWhitespace(int) white space} codepoints,
 *         otherwise {@code false}
 *
 * @since 11
 */
public boolean isBlank()

trim() and other mentioned regular expression do not work for all types of whitespaces

i.e: Unicode Character 'LINE SEPARATOR' http://www.fileformat.info/info/unicode/char/2028/index.htm

Java functions Character.isWhitespace() covers all situations.

That is why already mentioned solution StringUtils.isWhitespace( String ) /or StringUtils.isBlank(String) should be used.

StringUtils.isEmptyOrWhitespaceOnly(<your string>)

will check : - is it null - is it only space - is it empty string ""

https://www.programcreek.com/java-api-examples/?class=com.mysql.jdbc.StringUtils&method=isEmptyOrWhitespaceOnly

While personally I would be preferring !str.isBlank(), as others already suggested (or str -> !str.isBlank() as a Predicate), a more modern and efficient version of the str.trim() approach mentioned above, would be using str.strip() - considering nulls as "whitespace":

if (str != null && str.strip().length() > 0) {...}

For example as Predicate, for use with streams, e. g. in a unit test:

@Test
public void anyNonEmptyStrippedTest() {
    String[] strings = null;
    Predicate<String> isNonEmptyStripped = str -> str != null && str.strip().length() > 0;
    assertTrue(Optional.ofNullable(strings).map(arr -> Stream.of(arr).noneMatch(isNonEmptyStripped)).orElse(true));
    strings = new String[] { null, "", " ", "\\n", "\\t", "\\r" };
    assertTrue(Optional.ofNullable(strings).map(arr -> Stream.of(arr).anyMatch(isNonEmptyStripped)).orElse(true));
    strings = new String[] { null, "", " ", "\\n", "\\t", "\\r", "test" };
}

public static boolean isStringBlank(final CharSequence cs) {
        int strLen;
        if (cs == null || (strLen = cs.length()) == 0) {
            return true;
        }
        for (int i = 0; i < strLen; i++) {
            if (!Character.isWhitespace(cs.charAt(i))) {
                return false;
            }
        }
        return true;
    }