import javax.swing.*;
    
public class Question4
{
public static void main (String [] args) throws IOException
    {
    boolean test;
    System.out.println("Enter a character");
    test = Character.isLetter (letter);
    char letter = JOptionPane.showInputDialog.charAt(0);
    if ( Character.toUpperCase (letter) >= 'A' && Character.toUpperCase (letter) <= 'Z')
    {
    System.out.println( letter + " Is a letter");
    }
    else //[u] i don't know how to put the isDigit here?[/u]
    }
    System.out.println( letter + " Is a digit");
    }
       {
       //i have to create a white space here but don't know how to do that?

}
       //i could either put the isdigit is the else but i wanted to maybe put it in a separate method.
    public static boolean isDigit ( int Letter )
    
    {
    if ( letter = isDigit )
    {
    System.out.println ( letter + "Is a digit");
    }

/* Please note that I haven't tried to remove all the problems. I just tried to add the logic for things you asked for.
    so it might be possible that I missed something which is wrong and you didn't mentioned here.  [maybe like taking inputs etc. ] */

import javax.swing.*;
import java.lang.Math;
    
public class Question4
{
public static void main (String [] args) throws IOException
    {
      int digit;
    System.out.println("Enter a character");
    char letter = JOptionPane.showInputDialog.charAt(0);
    if ( Character.toUpperCase (letter) >= 'A' && Character.toUpperCase (letter) <= 'Z')
    {
    System.out.println( letter + " Is a letter");
    }
    else if(letter == ' ')  // You can compare it directly as it's a character datatype
    {
      System.out.println( letter + " Is a Whitespace");
    }
    else if(Character.isdigit(letter))   // I don't think you need to create any new function for this.
    {
      System.out.println( letter + " Is a digit");
      digit = Integer.parseInt(Character.toString(letter));
      System.out.println( "Square Root of " + letter + " is " + Math.sqrt(digit));
     }
   }
}