How the simples GUI countdown is supposed to work?
- by Roman
I am trying to write the simples GUI countdown. I found in Internet some code but it is already too fancy for me. I am trying to keep it as simple as possible. So, I just want to have a window saying "You have 10 second left". The number of second should decrease every second from 10 to 0. I wrote a code. And I think I am close to the working solution. But I still missing something. Could you pleas help me to find out what is wrong? Here is my code:
import javax.swing.*;
public class Countdown {
static JLabel label;
// Method which defines the appearance of the window.
private static void showGUI() {
JFrame frame = new JFrame("Simple Countdown");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JLabel label = new JLabel("Some Text");
frame.add(label);
frame.pack();
frame.setVisible(true);
}
// Define a new thread in which the countdown is counting down.
static Thread counter = new Thread() {
public void run() {
for (int i=10; i>0; i=i-1) {
updateGUI(i,label);
try {Thread.sleep(1000);} catch(InterruptedException e) {};
}
}
};
// A method which updates GUI (sets a new value of JLabel).
private static void updateGUI(final int i, final JLabel label) {
SwingUtilities.invokeLater(new Runnable(i,label) {
public Runnable(int i, JLabel label) {
this.i = i;
this.label = label;
}
public void run() {
label.setText("You have " + i + " seconds.");
}
});
}
// The main method (entry point).
public static void main(String[] args) {
javax.swing.SwingUtilities.invokeLater(new Runnable() {
public void run() {
showGUI();
//counter.start();
}
});
//counter.start();
}
}
And I have several concrete question about this code:
Where should I place the counter.start();? (In my code I put it on 2 places. Which one is correct?)
Why compiler complains about the constructor for Runnable? It says that I have an invalid method declaration and I need to specify the returned type.
ADDED:
I made the suggested corrections. And then I execute the code and get:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Worker.run(Worker.java:12)
In the Worker.java in the line 12 I have: label.setText("You have " + i + " seconds.");.
