Remove all white spaces from String in Java

This is a common question in coding interviews for junior developers.

There are multiple ways we can remove all white spaces from String:

1. Using the built-in trim() method
This is only if we need to remove spaces from the beginning and end of the string.

public class Main {
    public static void main(String[] args) {
        String str = " Hello "; // white space at beginning and end of the string
        System.out.println("The length of the string with white spaces: " + str.length());
        str = removeWhiteSpaces(str);
        System.out.println("The length of the string after removing white spaces: " + str.length());
        System.out.println("Final string: " + str);
    }

    private static String removeWhiteSpaces(String str) {
        return str.trim();
    }
}

Output:
The length of the string with white spaces: 7
The length of the string after removing white spaces: 5
Final string: Hello

Note: When we use any of the methods of the String class, we need to remember that we get a completely new String object, so we need to assign the return value to a variable since String is an immutable object in Java.

2. Using the built-in replaceAll() method
This method accepts a regular expression and removes all white spaces contained in the String.

public class Main {
    public static void main(String[] args) {
        String str = " Hello again, this is a Dev Primer blog. "; // white space at beginning, middle, and end of the string
        System.out.println("The length of the string with white spaces: " + str.length());
        str = removeWhiteSpaces(str);
        System.out.println("The length of the string after removing white spaces: " + str.length());
        System.out.println("Final string: " + str);
    }

    private static String removeWhiteSpaces(String str) {
        // replace all white spaces (regex = "\\s") with empty string
        return str.replaceAll("\\s", "");
    }
}

Output:
The length of the string with white spaces: 41
The length of the string after removing white spaces: 32
Final string: Helloagain,thisisaDevPrimerblog.

3. Creating a new String object and checking for white space using Character.isWhitespace(char c) method.

public class Main {
    public static void main(String[] args) {
        String str = " Hello again, this is a Dev Primer blog. "; // white space at beginning, middle, and end of the string
        System.out.println("The length of the string with white spaces: " + str.length());
        str = removeWhiteSpaces(str);
        System.out.println("The length of the string after removing white spaces: " + str.length());
        System.out.println("Final string: " + str);
    }

    private static String removeWhiteSpaces(String str) {
        String withoutWhiteSpaces = "";

        for (int i = 0; i < str.length(); i++) {
            if (!Character.isWhitespace(str.charAt(i))) {
                withoutWhiteSpaces += str.charAt(i);
            }
        }

        return withoutWhiteSpaces;
    }
}

Output:
The length of the string with white spaces: 41
The length of the string after removing white spaces: 32
Final string: Helloagain,thisisaDevPrimerblog.

And that is how we can remove white spaces from String in Java.