jtextfield - java.lang.NumberFormatException: For input string: ""

Question

When running this code:

JTextField ansTxt;
...
ansTxt = new JTextField(5);
String aString = ansTxt.getText();
int aInt = Integer.parseInt(aString);

Why do I get this error?

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: ""

UPDATE:

JTextField ansTxt;
ansTxt = new JTextField(5);

ansTxt.addKeyListener(new KeyAdapter() {
   public void keyReleased(KeyEvent e) {
    ansTxt = (JTextField) e.getSource();
    String aString = ansTxt.getText().trim();
    int aInt = Integer.parseInt(aString);
   }
}

Answer 1

The integer argument to the JTextField constructor is actually the width in number of columns. From the docs:

public JTextField(int columns)

Constructs a new empty TextField with the specified number of columns. A default model is created and the initial string is set to null.

By constructing it with

ansTxt = new JTextField(5);

you'll basically get an empty text-field (slightly wider than if you constructed it using no-argument constructor). If you want it to contain the string "5" you should write

ansTxt = new JTextField("5");

Update: IIRC, you'll get one event for keyDown, one for keyTyped, and one for keyUp. Presumably the text-field has not yet been updated on the keyDown event. Either way I suggest that you encapsulate the Integer.parseInt in a

try { ... } catch (NumberFormatException e) { ... }

block since the user may very well write something else than an integer. -->