The following Java program removes all space characters from the given string. Please note that this program doesn't work if you want to remove all whitespaces (not just spaces!) from a string.

// Java Example Program to remove all spaces from a String
public class RemoveSpacesExample {

    public static void main(String[] args) {
        String input = "hello world";
        String output = removeSpaces(input);
        System.out.println(input);
        System.out.println(output);
    }

    // Remove all space characters
    private static String removeSpaces(String input) {
        return input.replaceAll(" ", "");
    }
}

Following Java program removes all whitespaces from the given string. In Java, whitespace includes space, tab, line feed, form feed, etc. The full list of whitespace characters are listed here.

// Java example program to remove all whitespaces from a string
public class RemoveAllWhiteSpaces {

    public static void main(String[] args) {
        String input = "hello world\texample\rwith\twhitespaces";
        String output = removeWhiteSpaces(input);
        System.out.println(input);
        System.out.println(output);
    }

    // Remove all whitespace characters (space, tab, return etc.)
    private static String removeWhiteSpaces(String input) {
        return input.replaceAll("\\s+", "");
    }
}

Finally here is a Java implementation that doesn't use replaceAll method. In this example we extract all characters to an array and then create a new string from the array ignoring whitespace characters.

// This example Java program removes all whitespaces in a string
public class RemoveWhiteSpaceAlternate {
    public static void main(String[] args) {
        String input = "hello world\texample\rwith\twhitespaces";
        String output = removeWhiteSpaces(input);
        System.out.println(input);
        System.out.println(output);
    }

    // Remove all whitespace characters (space, tab, return etc.)
    private static String removeWhiteSpaces(String input) {
        char[] characters = input.toCharArray();
        StringBuffer buffer = new StringBuffer();
        for(char c:characters) {
            if(!Character.isWhitespace(c)) {
                buffer.append(c);
            }
        }
        return buffer.toString();
    }
}