how to accept only numbers and whitespace in java

To accept only numbers and whitespace in a string in Java, you can use a regular expression to match the string against a pattern that allows only these characters.

Here's an example of how you could do this using the java.util.regex.Pattern and java.util.regex.Matcher classes:

import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class Main {
  public static void main(String[] args) {
    String input = "123 456 789";

    // Define a regular expression pattern that allows only numbers and whitespace
    String pattern = "^[0-9\\s]+$";

    // Compile the pattern and create a Matcher object
    Pattern p = Pattern.compile(pattern);
    Matcher m = p.matcher(input);

    // Check if the input string matches the pattern
    if (m.matches()) {
      System.out.println("Input is valid");
    } else {
      System.out.println("Input is invalid");
    }
  }
}

In this example, the matches method of the Matcher class is used to check if the input string matches the regular expression pattern. If the input string contains only numbers and whitespace, the matches method will return true, and the program will print "Input is valid". If the input string contains any other characters, the matches method will return false, and the program will print "Input is invalid".

You can customize the regular expression pattern to match the specific characters and formats that you want to allow. For example, you can use the \\d pattern to match only digits, or the \\s pattern to match only whitespace characters. You can also use character classes and other regular expression syntax to create more complex patterns.

Keep in mind that regular expressions can be difficult to understand and debug if you are not familiar with the syntax. It is usually a good idea to test your regular expression patterns with a variety of inputs to make sure that they are working securely.