How to change the value of a progress bar through your Main - Nullpointerexception error

Question

I am trying to add a value and display it on a progress bar i have created on a jframe through my main (because i have methods left and right and i want that value to be from 1 of theses) but when i run it it gives me

Exception in thread "main" java.lang.NullPointerException 

i understand that i have to initialize the "var" so I wont be getting that error but i dont really know how. (i am kind of new to java)

This is my code (there are probably unnecessary stuff here but they were created automatically through the design)

import java.io.IOException;

public class UI extends javax.swing.JFrame {

/**
 * Creates new form UI
 */
public UI() {
    initComponents();
}


/**
 * This method is called from within the constructor to initialize the form.
 * WARNING: Do NOT modify this code. The content of this method is always
 * regenerated by the Form Editor.
 */
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

    p1hp = new javax.swing.JProgressBar();

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(layout.createSequentialGroup()
            .addContainerGap()
            .addComponent(p1hp, javax.swing.GroupLayout.PREFERRED_SIZE, 200, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addContainerGap(768, Short.MAX_VALUE))
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
            .addContainerGap(469, Short.MAX_VALUE)
            .addComponent(p1hp, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addGap(100, 100, 100))
    );

    pack();
}// </editor-fold>                        

/**
 * @param args the command line arguments
 * @throws java.io.IOException
 */
public static void main(String[] args) throws IOException {


    java.awt.EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            new UI().setVisible(true);
        }
    });
// THIS IS WHAT I AM TRYING TO DO
  p1hp.setStringPainted(true);
  p1hp.setValue(12);
  }

// Variables declaration - do not modify                     
public static javax.swing.JProgressBar p1hp;
// End of variables declaration   
}

aside that though, i tried creating a button, which i added thoses 2 lines and it worked fine
(the code for the button isnt here)

Note1: I manually maked the progressbar static bacause if not, I would get the error

non-static variable p1hp cannot be referenced from a static context

Note2: I have read this answer: What is a NullPointerException, and how do I fix it?
Still not sure how to fix it

Note3: I am using Netbeans 8.2

Answer 1

Do not put the JProgressBar static, instead, add a getter that would allow you to get it in your main. You will face other issue because of your design tho.

public JProgressBar getProgressBar()
{
    return p1hp;
}

And then in your main :

UI ui = new UI();
final JProgressBar progressBar = ui.getProgressBar();

To set values in your main use :

public static void main(String[] args) throws IOException {

    java.awt.EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            UI ui = new UI();
            ui.setVisible(true);
            ui.getProgressBar().setStringPainted(true);
            ui.getProgressBar().setValue(12);
        }
    });

Answer 2

I manually maked the progressbar static bacause if not, I would get the error

You can create your class object and then access using that object. This will prevent you from

non-static variable p1hp cannot be referenced from a static context

But it is not good for here because when you create new object from UI new JFrame will create.

Wrap using try-catch block. This will handle your exception.

try{
    //where null values occur
}catch(NullPointerException e){
  System.out.println("errr");//you dont want add this but it is good to display something.
}

UPDATE:

From looking at your code I think you are creating GUI's in netbeans. If it is you have to do as following,

Right click on JProgressBar in navigator or in JFrame. Next, goto custom code and then tick the static and change Access: to `public. And then you can do any changes.

Or:

In that window add your code for JProgressBar in initialization code

Now you donot even need to make it static.

Answer 3

You are initializing Progress bar p1hp variable in the UI constructor but UI constructor is being run inside invokeLater that does not guarantee that it will be executed right before the next line that calls p1hp.setStringPainted(true);

As a quick fix initialize p1hp right in the declaration like this:

public static javax.swing.JProgressBar p1hp = new javax.swing.JProgressBar();

UPDATE:

Here is a sample code that starts/stops progress. The code is not intended to be a complete solution but is a good start to get going with Swing and progress handling. I have not commented the code much but anyone is free to ask questions in form of comments right in the code:

Screen:

Progress Handler:

public interface ProgressHandler {

public void startProgress();

public void updateProgress(String message, int progress);

public void stopProgress();

}

Main View:

public class MainView {
private JComponent view = null;

private ProgressView progressView = null;
private LoadDataView loadDataView = null;

public MainView(){
    progressView = new ProgressView();
    loadDataView = new LoadDataView();
}

public JComponent getView(){
    if (view == null) view = createView();
    return view;
}

private JComponent createView(){
    JPanel result = new JPanel( new BorderLayout() );

    result.add( progressView.getView(), BorderLayout.NORTH );
    result.add( loadDataView.getView(), BorderLayout.CENTER );

    return result;
}

public static void main(String[] args) throws IOException {
    MainView mainView = new MainView();

    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
    frame.getContentPane().add( mainView.getView() );

    frame.setSize(400, 200);
    frame.setVisible(true);
}

}

Progress View

public class ProgressView {

private static ProgressHandlerImpl progressHandler = new ProgressHandlerImpl(); 

private JComponent view = null;

public static ProgressHandler getProgressHandler(){
    return progressHandler;
}

public JComponent getView(){
    if (view == null) view = createView();
    return view;
}

private JComponent createView(){
    JPanel result = new JPanel( new BorderLayout() );        
    result.add( progressHandler.getMessageLabel(), BorderLayout.NORTH );
    result.add( progressHandler.getProgressBar(), BorderLayout.CENTER );
    return result;
}

private static class ProgressHandlerImpl implements ProgressHandler{
    public JProgressBar progressBar = null;
    public JLabel messageLabel = null;

    public ProgressHandlerImpl(){
        progressBar = new JProgressBar();
        progressBar.setVisible(false);
        progressBar.setStringPainted(true);

        messageLabel = new JLabel();
        messageLabel.setVisible(false);
    }

    public JProgressBar getProgressBar(){
        return progressBar;
    }

    public JLabel getMessageLabel(){
        return messageLabel;
    }

    @Override
    public void startProgress(){
        java.awt.EventQueue.invokeLater( new Runnable(){ public void run(){
        progressBar.setValue(0);
        progressBar.setVisible(true); } } );            
    }

    @Override
    public void updateProgress(String message, int progress) {
        java.awt.EventQueue.invokeLater( new Runnable(){  public void run(){
        progressBar.setValue(progress); 

        if (message == null) return;

        if ( !messageLabel.isVisible() ) messageLabel.setVisible(true);

        messageLabel.setText(message);
        } } );  
    }

    @Override
    public void stopProgress(){
        java.awt.EventQueue.invokeLater( new Runnable(){  public void run(){
        progressBar.setValue(0);
        progressBar.setVisible(false); 
        messageLabel.setVisible(false);} } );  
    }       
}

}

Load View

public class LoadDataView {
private JComponent view = null;
private Thread thread = null;

public JComponent getView(){
    if (view == null) view = createView();
    return view;
}

private JComponent createView(){
    JPanel result = new JPanel();

    JButton startProgressButton = new JButton("start");
    startProgressButton.addActionListener( new ActionListener() { public void actionPerformed(ActionEvent e) { startProgress(); } } );        
    result.add(startProgressButton);

    JButton stopProgressButton = new JButton("stop");
    stopProgressButton.addActionListener( new ActionListener() { public void actionPerformed(ActionEvent e) { stopProgress(); } } );        
    result.add(stopProgressButton);

    return result;
}

private void startProgress(){
    if (thread != null) throw new IllegalStateException();

    thread = new Thread( new Runnable(){ public void run(){
        ProgressView.getProgressHandler().startProgress();

        for (int i = 0; i < 100; i++){
            ProgressView.getProgressHandler().updateProgress("Loading...", i);

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {                   
                ProgressView.getProgressHandler().updateProgress("Loading stopped!", i);
                //do some other logging
                break;
            }
        }

    } } );

    thread.start();
}

private void stopProgress(){
    if (thread == null) return;

    thread.interrupt(); 

    thread = null;
}

}