This can be done using KeyEvent in the following way... written comments above the newly added code

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.JOptionPane;

class GUIExample1 extends JFrame implements ActionListener
{
JLabel numberLabel;
JTextField numberTextField;
JButton displayButton;
public GUIExample1()
{
Container pane = getContentPane();
pane.setLayout(new FlowLayout());
numberLabel = new JLabel("Enter a number:");
numberTextField = new JTextField(4);

//calling the keyEvent
numberTextField.addKeyListener(new KeyAdapter(){
public void keyTyped(KeyEvent evt){
numberTextField_KeyTyped(evt);
}
});

displayButton = new JButton("Enter");
pane.add(numberLabel);
pane.add(numberTextField);
pane.add(displayButton);
displayButton.addActionListener(this);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setTitle("GUI Example");
pack();
show();
}
public void actionPerformed(ActionEvent event)
{
Object source = event.getSource();
if (source == displayButton)
{
double result = Integer.parseInt(numberTextField.getText());
JOptionPane.showMessageDialog(null, "you entered " + (int)(result), "Hello", JOptionPane.PLAIN_MESSAGE);
System.exit(0);
}
}


//newly added code
//KeyEvent function
public void numberTextField_KeyTyped(KeyEvent evt)
{
try{
char dotChar=evt.getKeyChar();
int dot=(int)dotChar;
char c = evt.getKeyChar(); //character object to hold the typed character
if (!(Character.isDigit(c)))
{
getToolkit().beep();
evt.consume();
JOptionPane.showMessageDialog(this,"characters are not allowed");
numberTextField.requestFocus(true);

}
}
catch(Exception e){
System.out.println("---error in validation---"+e);
}
}//end of the method

public static void main(String[] args)
{
new GUIExample1();
}
}

This will help you out..

Brilliant! The keyEvent works, but the modified code extends the text field to type numbers in to more than 4. I'll study the code, but what will be the reason for this? (The question is for the answer above).

I'm the one who began this question. I'm new to the programming world and like it alot. I was wondering where can I go to learn more about java and C++.

Hi
You need not worry of where to go and learn about java..Jus search on google and try to find the books on java. Then select which you can be comfortable with and be sure that all the topics are covered..Then start practising the programs.Practice will definitely make you perfect


Thanks
pinky

Hi
You need not worry of where to go and learn about java..Jus search on google and try to find the books on java. Then select which you can be comfortable with and be sure that all the topics are covered..Then start practising the programs.Practice will definitely make you perfect


Thanks
pinky