How to repaint Java Applet

Question

I have the following code that is a simple Applet that converts Celsius to Fahrenheit.

If a correct number is entered and the button is clicked, the temperature is converted and displayed no issues. However, if the text box contains an invalid entry a message needs to show up on a label on the error pannel but this doesn't happen unless I resize the applet. So the question is "How to repaint!!!"

import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class TemperatureConverter extends Applet{

private JLabel lblCel=new JLabel("Celsius Temperature   :");
private JLabel lblFar=new JLabel("Fahrenheit value      :");
private JLabel lblResult=new JLabel("");
private JLabel lblError=new JLabel("");
private JTextField txtFahr;
private JPanel celsPanel;
private JPanel farPanel;
private JPanel errorPanel;
private JButton btnEnter = new JButton("Convert");
public double temp = 0.0;

public void init(){



    celsPanel = new JPanel(new GridLayout(1,3,2,2));
    celsPanel.add(lblCel);
    txtFahr = new JTextField(50);
    celsPanel.add(txtFahr);
    btnEnter.addActionListener(new ActionListener() {       
        @Override
        public void actionPerformed(ActionEvent arg0) {
            try{
                temp = Double.parseDouble(txtFahr.getText());
                double X = (temp - 32)* 0.69;
                lblResult.setText(String.valueOf(X));
                lblError.setText(" ");
            }
            catch(NumberFormatException ex){
                lblError.setText("Invalid Celsius Value");
            }
        }
    });
    celsPanel.add(btnEnter);

    farPanel = new JPanel(new GridLayout(1,2,2,2));
    farPanel.add(lblFar);
    lblResult.setSize(100, 30);
    farPanel.add(lblResult);

    errorPanel = new JPanel();
    lblError.setSize(100, 30);
    errorPanel.add(lblError);

    setLayout(new GridLayout(3,2,2,2));
    add(celsPanel);
    add(farPanel);
    add(errorPanel);
    setSize(550,200);

}

}

Can anyone figure this out? i was thinking of a label size issue, but then I'm not sure about that.

Answer 1

try this :

    btnEnter.addActionListener(new ActionListener() {       
        @Override
        public void actionPerformed(ActionEvent arg0) {
            try{
                temp = Double.parseDouble(txtFahr.getText());
                double X = (temp - 32)* 0.69;
                lblResult.setText(String.valueOf(X));
                lblError.setText(" ");
                this.repaint();
            }
            catch(NumberFormatException ex){
                lblError.setText("Invalid Celsius Value");
            }
        }
    });

Answer 2

1. extend JApplet , make height of applet bigger in tag on html page
2. cannot use grid layout and set size, if you want to use own size then setLayout Manager as null for applet and then setBounds
3. call invalidate on the component to force repaint lblError.invalidate() if 1 and 2 does not work

About null layout manager & set Bounds : this works on all OS (have tested with Apple, unix, windows and linux) though the textbooks say use a layout manager in reality all are pretty goof with pixels in sizes up to 1000x800 in my tests over 12 years. samples image xy ; scrabble rack ; info capture

Reference read about invalidate, repaint and ignore repaint in javadoc