How do I make my code check that the user has input a alphabetical character? [Java]

Update: the code works now. Thanks for help guys.

This is part of the code I'm having trouble with

if ((amount < 0) || (Character.isLetter(amount)))

Anyone know what I'm doing wrong? Here's a piece of my code:

try
      {
       if ( theStock.exists( pn ) )        // Button CHECK
       {
           thePicture.clear();                       
           if ( actionIs.equals( Name.ADD ) ) {
               try 
               {
                   amount = Integer.parseInt(theInputPQ.getText());      // Checks that
                   if ((amount < 0) || (Character.isLetter(amount))) {   // number has been
                       theOutput.setText("Please enter sufficient No."); // input
                    }
                    else {
                        theStock.addSto开发者_开发技巧ck(pn, amount);
                        Product pr = theStock.getDetails( pn ); // Add to
                        theAction.setText(                      // stock of
                        pr.getDescription() + " : " +           // item
                        theMoney.format(pr.getPrice()) + 
                        " (" + pr.getQuantity() + ")" );
                    }                       
                }

                catch (NumberFormatException e) {
                    theOutput.setText("Please enter sufficient No.");

                    Product pr = theStock.getDetails( pn );              
                    theAction.setText(                    
                    pr.getDescription() + " : " +       
                    theMoney.format(pr.getPrice()) +    
                    " (" + pr.getQuantity() + ")" );

                    if ( actionIs.equals( Name.ADD ) ) {
                    } 
                }
            } else {                                   // F
                theAction.setText(                    //  Inform Unknown
                "Unknown product number " + pn );    //  product number
            }
        }

You're checking if an int is a letter or not:

 Character.isLetter(amount)

If that int is for instance 65 that method would return true, because what in ascii code the number 65 represents the letter a

This is probably the source of your problems.

You may safely remove that validation, because, if after the call to Integer.parseInt you may be sure that amount is a number ( otherwise it will go to the catch( NumberFormatException ) section below as you may already have noticed.

Is amount a int variable? if it is, how it can be a letter (Character)? if not int, then how it can be compared using < 0? It can be or character or integer, not both above.

Well I think that you are already checking if amount is an int by the:

Integer.parseInt(theInputPQ.getText());

and the try-catch block. don't need to do the

Character.isLetter(amount)

The amount < 0 check isn't doing anything useful, unless you are checking for integer values less than 0. Integer.parseInt() returns the integer that it read, as a string. It throws a NumberFormatException if its argument does not contain a parseable integer.

I would call a different method if your goal is just to figure out if the button's text has letters in it. Also, I'm not sure why you're calling isLetter() on the text of a String. Here's an example I saw that goes character-by-character:

String str = theInputPQ.getText(); // I assume this is a String
for(int i=0; i < str.length(); i++)
{
        if(Character.isDigit(str.charAt(i)))
           System.out.println("It's a digit");
        if(Character.isLetter(str.charAt(i)))
           System.out.println("It's a letter");
}

In the interest of good coding practices, if you add this code then it should be a separate private method.

Sources: Java API for Integer, Dream in Code