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

Question

Shortest solution I can think of:

if (string.trim().length() > 0) ...

This only checks for (non) white space. If you want to check for particular character classes, you need to use the mighty match() with a regexp such as:

if (string.matches(".*\\w.*")) ...

...which checks for at least one (ASCII) alphanumeric character.

I would use the Apache Commons Lang library. It has a class called StringUtils that is useful for all sorts of String operations. For checking if a String is not all whitespaces, you can use the following:

StringUtils.isBlank(<your string>)

Here is the reference: StringUtils.isBlank

Slightly shorter than what was mentioned by Carl Smotricz:

!string.trim().isEmpty();

StringUtils.isBlank(CharSequence)

https://commons.apache.org/proper/commons-lang/javadocs/api-release/org/apache/commons/lang3/StringUtils.html#isBlank-java.lang.CharSequence-

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.

if(target.matches("\\S")) 
    // then string contains at least one non-whitespace character

Note use of back-slash cap-S, meaning "non-whitespace char"

I'd wager this is the simplest (and perhaps the fastest?) solution.

This answer focusses more on the sidenote "i.e. has at least one alphanumeric character". Besides that, it doesn't add too much to the other (earlier) solution, except that it doesn't hurt you with NPE in case the String is null.

We want false if (1) s is null or (2) s is empty or (3) s only contains whitechars.

public static boolean containsNonWhitespaceChar(String s) {
  return !((s == null) || "".equals(s.trim()));
}