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Java: Roller program - debug - variables not updating from JTextField and NumberFormatException thrown regardless of valid information.

Answer 1

+1 A:

You initialize this member variable, but you never add this field to the UI:

JTextField inputSides = new JTextField(" ");

Here you read from this text field that still contains the String " ", which is not an integer.

getSides = inputSides.getText();
sidesToRoll = Integer.parseInt(getSides);

When you build your UI in the constructor, you declare another, local variable with the same name, pointing to a new object:

JTextField inputSides = new JTextField("6");

This is the object that appears in the UI. The local variable inputSides hides the member variable of the same name.

As an aside, this is not the right way to compare strings:

arg == "ROLL"

This only works because arg is pointing to the same, interned object instance. The correct way to compare these values would be:

"ROLL".equals(arg)

EDIT:

The trick you are missing is in thinking about the objects you are creating and which ones your reference variables are pointing at.

Consider this simplified version of your code:

public class ObjectReferences {
  private final JLabel label = new JLabel("I am FOO"); // label 1

  public ObjectReferences() {
    JLabel label = new JLabel("I am BAR"); // label 2

    JButton button = new JButton("Click me");
    button.addActionListener(new ActionListener() {
      @Override public void actionPerformed(ActionEvent e) {
        onclick();
      }
    });

    // will display "I am BAR"
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(600, 400);
    frame.setLayout(new FlowLayout());
    frame.add(label); // label 2
    frame.add(button);
    frame.setVisible(true);
  }

  public void onclick() {
    // will display "I am FOO"
    JOptionPane.showMessageDialog(null, label.getText()); // label 1
  }

  public static void main(String[] args) {
    new ObjectReferences();
  }
}

Two label objects are created, but only one is added to the frame. label 1 is referenced by the member variable and this is the one visible to the onclick() method. Within the constructor, label 2 is referenced by a local variable called label that hides the member variable called label.

Your intent is to write something like this instead:

public class ObjectReferences {
  private final JLabel label = new JLabel("I am FOO");

  public ObjectReferences() {
    JButton button = new JButton("Click me");
    button.addActionListener(new ActionListener() {
      @Override public void actionPerformed(ActionEvent e) {
        onclick();
      }
    });

    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(600, 400);
    frame.setLayout(new FlowLayout());
    frame.add(label);
    frame.add(button);
    frame.setVisible(true);
  }

  public void onclick() {
    JOptionPane.showMessageDialog(null, label.getText());
  }

  public static void main(String[] args) {
    new ObjectReferences();
  }
}

McDowell 2009-12-14 10:15:07

I appreciate the swift response. I was able to switch the arg == "ROLL" with "ROLL".equals(arg), as well as the Exit and Help/About buttons. I've also cleaned up the code above to reflect that change, along with removing unnecessary code. Still haven't figured it out, though.

Sean Schnoor 2009-12-14 12:40:53

I edited my answer, so hopefully the problem is clearer. _Also, note that you haven't initialized your `roll` member, so you'll get a NullPointerException when you try to use it._

McDowell 2009-12-14 16:44:45

This is starting to make sense. I'll keep at it. :)

Sean Schnoor 2009-12-14 20:26:33

Thank you! Sorry for closing this so late... I was unable to log in O_o

Sean Schnoor 2009-12-20 18:53:50

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Java: Roller program - debug - variables not updating from JTextField and NumberFormatException thrown regardless of valid information.

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