Problems Making a variable known in the whole script

I am trying to make a simple stop watch, i'm just going to post my code here first, then explain my problem in the buttom:

Code:

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package gui;

import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JTextField;

public class hovedpanel {

        private JFrame frame;
        private JTextField textField;
        protected long startTime;

        /**
        * Launch the application.
        */
        public static void main(String[] args) {
                EventQueue.invokeLater(new Runnable() {
                        public void run() {
                                try {
                                        hovedpanel window = new hovedpanel();
                                        window.frame.setVisible(true);
                                } catch (Exception e) {
                                        e.printStackTrace();
                                }
                        }
                });
        }

        /**
        * Create the application.
        */
        public hovedpanel() {
                initialize();
        }

        /**
        * Initialize the contents of the frame.
        */
        private void initialize() {
                frame = new JFrame();
                frame.setBounds(100, 100, 395, 154);
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.getContentPane().setLayout(null);
               
                JButton btnNewButton = new JButton("Start");
                btnNewButton.addActionListener(new ActionListener() {
                        public void actionPerformed(ActionEvent e) {
                                //What happens when start button is pressed:
                                long startTime = System.nanoTime();
                                System.out.println(startTime);
                        }
                });
                btnNewButton.setToolTipText("Start time");
                btnNewButton.setBounds(66, 97, 91, 23);
                frame.getContentPane().add(btnNewButton);
               
                JButton btnNewButton_1 = new JButton("Stop");
                btnNewButton_1.setToolTipText("Stop time");
                btnNewButton_1.addActionListener(new ActionListener() {
                        public void actionPerformed(ActionEvent e) {
                                //What happens when stop button is pressed:
                                Long endTime = System.nanoTime();
                                System.out.println("time: " + (float)(endTime - startTime) / 1E9);
                        }
                });
                btnNewButton_1.setBounds(230, 97, 91, 23);
                frame.getContentPane().add(btnNewButton_1);
               
                textField = new JTextField();
                textField.setEditable(false);
                textField.setBounds(151, 11, 86, 20);
                frame.getContentPane().add(textField);
                textField.setColumns(10);
        }
}

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I am pretty sure my problem happens at line 67 where I endTime - startTime, but the varibale "startTime" is unknown/equals 0. because it's in a different action-listener?
But I am in no way sure.
To create the gui I used window-builder.

If anyone can explain me what I am doing wrong, you would save my day!

On line 53 you declare startTime again by using "long" in front of it. That makes it a local variable to the inner class. Delete the "long" part and it will assign it to the class variable.

Quote:

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Originally Posted by SurfMan

On line 53 you declare startTime again by using "long" in front of it. That makes it a local variable to the inner class. Delete the "long" part and it will assign it to the class variable.

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Thanks for the help! It works now, crazy that such a little thing can really make a that big of a difference in programming.

Quote:

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Originally Posted by Joaim

Thanks for the help! It works now, crazy that such a little thing can really make a that big of a difference in programming.

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It's not a little thing, it's an important concept: the scope (visibility) of a variable is the block where it's declared. You can declare a variable at class level (which then usually are called fields). But the "int i" in for( int i=0; i<9; i+ ) {} limits the scope of the "i" variable to the for-loop itself, not outside of it.

I see, I got it to the previous to work. But now I have a new problem, I would like my stopwatch to update real-time. The only thing I see that would be suitable, is a while loop, looping all the time updating the Seconds, but it doesn't really work: Here's my new code:

Code:

---

package gui;
 
import java.awt.EventQueue;
 
import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JTextField;

 
public class hovedpanel {
 
    private JFrame frame;
    private JTextField textField;
    protected long startTime;
    private JButton btnReset;
    public boolean turn = false;
 
    /**
    * Launch the application.
    */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    hovedpanel window = new hovedpanel();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
 
    /**
    * Create the application.
    */
    public hovedpanel() {
        initialize();
    }
 
    /**
    * Initialize the contents of the frame.
    */
    private void initialize() {
        frame = new JFrame();
        frame.setBounds(100, 100, 395, 154);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(null);
       
        JButton btnNewButton = new JButton("Start");
        btnNewButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                //What happens when start button is pressed:
                startTime = System.nanoTime();
                turn = true;
                while (turn = true){
                        Long endTime = System.nanoTime();
              textField.setText((float)(endTime - startTime) / 1E9 + " sekunder er der gået");
            }
            }
        });
        btnNewButton.setToolTipText("Start timer");
        btnNewButton.setBounds(66, 97, 91, 23);
        frame.getContentPane().add(btnNewButton);
       
        JButton btnNewButton_1 = new JButton("Stop");
        btnNewButton_1.setToolTipText("Stop timer");
        btnNewButton_1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                //What happens when stop button is pressed:
                turn = false;
            }
        });
        btnNewButton_1.setBounds(230, 97, 91, 23);
        frame.getContentPane().add(btnNewButton_1);
       
        textField = new JTextField();
        textField.setText("Tryk start for at starte stopuret");
        textField.setEditable(false);
        textField.setBounds(10, 11, 367, 20);
        frame.getContentPane().add(textField);
        textField.setColumns(10);
       
        btnReset = new JButton("Reset");
        btnReset.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent e) {
                        textField.setText("Tryk start for at starte stopuret");
                        turn = false;
                }
        });
        btnReset.setBounds(149, 55, 91, 23);
        frame.getContentPane().add(btnReset);
            }
    }

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Checking for equality uses == and no =, in line 58. But then again this problem doesn't need a loop. Remove the turn variable, it's not needed. What you really want to use here is a Swing Timer (How to Use Swing Timers (The Java™ Tutorials > Creating a GUI With JFC/Swing > Using Other Swing Features)). It's made for what you want. A basic Timer looks like this, with your textField used:

Code:

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        Timer t = new Timer(100, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                textField.setText((float)(Systen.nanoTime() - startTime) / 1E9 + " sekunder er der gået");
            }
        });

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The 100 means fire actionPerformed every 100 ms. Use btnNewButton to set the startTime and start the Timer, use btnNewButton_1 to stop the Timer.

Also, rename the btnNewButton_1 to stopButton or btnStop or anything more sensible. That will help tremendously when reading your own code.

Wow thanks! I will try it out, and let you know here later.