How do I make a pop up in Swing GUI if wrong data is entered in a text field.

I try to make the input field accept only numbers preceded by a mathematical operator (+ - * /). for example: +7. if I enter just 7, it should pop up a window saying the same. I am using netbeans. Here is the Keypressed event code for you.
Please help me.
private void txtNumberKeyPressed(java.awt.event.KeyEvent evt) {                                     
        // KeyPressed handling code here:
        String badChar = "`!@#$[]\";%^&)~(_][{}|';:?|><,";
        char c = evt.getKeyChar();
        int keyCode = evt.getKeyCode();
        if(badChar.indexOf(c)>=0 || c <'0' && c>'9' || c >='a' && c <='z'
                || c >= 'A' && c <= 'Z' && keyCode != KeyEvent.VK_BACK_SPACE &&
                keyCode != KeyEvent.VK_ENTER)
        {
            evt.consume();
        }
        if(keyCode ==KeyEvent.VK_ENTER)
        {
            dataDisplay.append(txtNumber.getText()+ '\n');
            //now clear the textbox ready for next entry
            txtNumber.setText(null);
        }
    }

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chandanchoubeyAsked:
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basav_comCommented:
chandanchoubeyAuthor Commented:
Thanks for help basav_com. I tried using JOptionPane. It is giving me pop up. thats good. but it pops up even if I enter digits. I want pop should only appear when I enter text or not digits. please take a look of my code. What am I doing wrong?
Thanks for your help.
    private void txtNumberKeyPressed(java.awt.event.KeyEvent evt) {                                     
        // KeyPressed handling code here:
        String badChar = "`!@#$[]\";%^&)~(_][{}|';:?|><,";
        char c = evt.getKeyChar();
        int keyCode = evt.getKeyCode();
        if(badChar.indexOf(c)>=0 || c <'0' && c>'9' || c >='a' && c <='z'
                || c >= 'A' && c <= 'Z' && keyCode != KeyEvent.VK_BACK_SPACE &&
                keyCode != KeyEvent.VK_ENTER)
        {
            evt.consume();
        }
 
String goodChar = "1234567890+-/*";
        char d = evt.getKeyChar();
        if(keyCode ==KeyEvent.VK_ENTER)
            if(goodChar.indexOf(d) >=0 && goodChar.indexOf(d) <=14)
        {
                dataDisplay.append(txtNumber.getText()+ '\n');
 
            }
            else
            {
            JOptionPane.showMessageDialog(null, "wrong key");
            }
           
            //now clear the textbox ready for next entry
            txtNumber.setText(null);

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Thomas4019Commented:
This code should not be put in a keyPressed. You should have a JButton with an action listener. Then call get Text to get the whole string they input. Here is a simple example.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
 
class Example extends JFrame implements ActionListener
{
	JTextField input;
 
	Example()
	{
		setSize(320,240);
		setDefaultCloseOperation(EXIT_ON_CLOSE);
 
		input = new JTextField();
		add(input,BorderLayout.CENTER);
 
		JButton submit = new JButton("Submit");
		submit.addActionListener(this);
		add(submit,BorderLayout.SOUTH);
 
		setVisible(true);
	}
 
	public void actionPerformed(ActionEvent evt)
	{
		if(!input.getText().matches("^[\\+\\-\\*\\/].*"))
		{
			JOptionPane.showMessageDialog(null,"Bad Input");
		}
 
		//CODE USING INPUT HERE
	}
 
	public static void main(String args[])
	{
		new Example();
	}
}

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chandanchoubeyAuthor Commented:
Thanks Thomas4019. I really appreciate it. But I dont want a JButton. I want JOptionPane to appear when user presses "enter key" rather than clicking on Submit button
Thomas4019Commented:
very simple, simply add the actionlistener on the JTextField and remove the JButton, here is the new source code.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
 
class Example extends JFrame implements ActionListener
{
	JTextField input;
 
	Example()
	{
		setSize(320,240);
		setDefaultCloseOperation(EXIT_ON_CLOSE);
 
		input = new JTextField();
		input.addActionListener(this);
		add(input,BorderLayout.CENTER);
 
		setVisible(true);
	}
 
	public void actionPerformed(ActionEvent evt)
	{
		if(!input.getText().matches("^[\\+\\-\\*\\/].*"))
		{
			JOptionPane.showMessageDialog(null,"Bad Input");
		}
 
		//CODE USING INPUT HERE
	}
 
	public static void main(String args[])
	{
		new Example();
	}
}

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