replaceAll("\\s","")

\w = Anything that is a word character

\W = Anything that isn't a word character (including punctuation etc)

\s = Anything that is a space character (including space, tab characters etc)

\S = Anything that isn't a space character (including both letters and numbers, as well as punctuation etc)

(Edit: As pointed out, you need to escape the backslash if you want \s to reach the regex engine, resulting in \\s.)

Why not use a regex for this?

a = a.replaceAll("\\s","");

In the context of a regex, \s will remove anything that is a space character (including space, tab characters etc). You need to escape the backslash in Java so the regex turns into \\s. Also, since Strings are immutable it is important that you assign the return value of the regex to a.

Try this code,

public class Main {
    public static void main(String[] args) throws Exception {
        String s = " Test example    hello string    replace  enjoy   hh ";
        System.out.println("Original String             : "+s);
        s = s.replace(" ", "");
        System.out.println("Final String Without Spaces : "+s);
    }
}

Output :

Original String             :  Test example    hello string    replace  enjoy   hh                                                                         
Final String Without Spaces : Testexamplehellostringreplaceenjoyhh 

Another way by using char array :

public class Main {
    public static void main(String[] args) throws Exception {
        String s = " Test example    hello string    replace  enjoy   hh ";
        System.out.println("Original String             : "+s);
        String ss = removeWhitespace(s);
        System.out.println("Final String Without Spaces : "+ss);
      
    } 
    
    public static String removeWhitespace(String s) {
        char[] charArray = s.toCharArray();
        String gtg = "";
        
        for(int i =0; i<charArray.length; i++){                
            if ((charArray[i] != ' ') && (charArray[i] != '\t') &&(charArray[i] != '\n')) {
                gtg = gtg + charArray[i];
            }
        }    
        return gtg;
    }
}

Output :

Original String             :  Test example    hello string    replace  enjoy   hh                                                                         
Final String Without Spaces : Testexamplehellostringreplaceenjoyhh