Odtnhj

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(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) "".equals(s.trim()));

If you are only checking for whitespace and don't care about null then you can use org.apache.commons.lang.StringUtils.isWhitespace(String str),

StringUtils.isWhitespace(String str);

(Checks if the String contains only whitespace.)

If you also want to check for null(including whitespace) then

StringUtils.isBlank(String str);

With JDK/11, now you can make use of the String.isBlank API to check if the given string is not all made up of

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()

The trim method should work great for you.

http://download.oracle.com/docs/cd/E17476_01/javase/1.4.2/docs/api/java/lang/String.html#trim()

Returns a copy of the string, with
leading and trailing whitespace
omitted. If this String object
represents an empty character
sequence, or the first and last
characters of character sequence
represented by this String object both
have codes greater than 'u0020' (the
space character), then a reference to
this String object is returned.

Otherwise, if there is no character
with a code greater than 'u0020' in
the string, then a new String object
representing an empty string is
created and returned.

Otherwise, let k be the index of the
first character in the string whose
code is greater than 'u0020', and let
m be the index of the last character
in the string whose code is greater
than 'u0020'. A new String object is
created, representing the substring of
this string that begins with the
character at index k and ends with the
character at index m-that is, the
result of this.substring(k, m+1).

This method may be used to trim
whitespace from the beginning and end
of a string; in fact, it trims all
ASCII control characters as well.

Returns: A copy of this string with
leading and trailing white space
removed, or this string if it has no
leading or trailing white space.leading or trailing white space.

You could trim and then compare to an empty string or possibly check the length for 0.

If you are using Java 11, 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.

Alternative:

boolean isWhiteSpaces( String s ) 
 return s != null && s.matches("\s+");
 

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