当我从另一个类更新时，为什么“javax.swing.JProgressBar”在旧值之上显示新值？

Question

Problem:

ClassA
    static JProgressBar progressBar;
ClassB
    ClassA.progressBar.setValue(0);
    ...
    ClassA.progressBar.setValue(10);
    ...
    ClassA.progressBar.setValue(20);

Progress bar displays 0, then 10 on top of the 0, then 20 on top of the 10.

Code:

public class ClassA
{
    static JProgressBar progressBar;
    public ClassA()
    {
        ...
        progressBar = new JProgressBar(0, 100);
        progressBar.setStringPainted(true);
        ...
    }
    public static void main(String[] args)
    {
        ClassA cA = new ClassA();
        cA.something();
    }
    public void something()
    {
        ...
        panel.add(progressBar, BorderLayout.CENTER);
        ...
    }
    ...
}
public class ClassB
{
    public void something()
    {
        ...
        ClassA.progressBar.setValue(0);
        ClassA.progressBar.update(ClassA.progressBar.getGraphics());
        ...
    }
    ...
}

I am also adding the progress bar to the panel. As it was mentioned above, it displays the progress bar and the percentage but it show each value on top of another.

EDIT: The problem is very similar to this: Java Swing revalidate() vs repaint() . However, instead of

the old content is still actually visible (though obscured by the the new content)

being problem with JPanel, mine is problem with JProgressBar. I tried ClassA.progressBar.revalidate(); and ClassA.progressBar.repaint(); but nothing works except ClassA.progressBar.update(ClassA.progressBar.getGraphics()); , which, again, as it was mentioned above, displays new values on top of old values.

Answer 1

You don't need this line:

ClassA.progressBar.update(ClassA.progressBar.getGraphics());

Calling setValue() will update the progress bar UI.

Otherwise there is nothing wrong with the code you posted here. The issue must lie elsewhere. Here's a SSCCE based on your code, which works as expected with no paint issues.

public class ClassA {
    static JProgressBar progressBar;
    private ClassB classb;
    private JPanel panel;


    public ClassA() {
        progressBar = new JProgressBar(0, 100);
        progressBar.setStringPainted(true);

        classb = new ClassB();

        JButton button = new JButton("test");
        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
                classb.something();
            }
        });

        panel = new JPanel();
        panel.add(button);
        panel.add(progressBar, BorderLayout.CENTER);
        JFrame frame = new JFrame();
        frame.add(panel);
        frame.pack();
        frame.setVisible(true);
    }


    public static void main(String[] args) {
        new ClassA();
    }
}

public class ClassB
{
    private int index;

    public void something()
    {
        ClassA.progressBar.setValue(++index);
        //ClassA.progressBar.update(ClassA.progressBar.getGraphics());
    }
}

Answer 2

Arthur, your code snippet doesn't show how ClassB.something gets called. Most of Swing is not thread safe and unless the javadoc explicitly states otherwise, Swing ui components need to be modified from the Event Dispatch Thread. Usually if a Swing component is thread-safe it explicitly calls this out in the javadoc. I took a look at the JProgressBar javadoc and saw no disclaimer, actually it has a warning that Swing is not threadsafe.
If Swing components are accessed from other threads it can cause applications to behave strangely. I've seen components render strangely and incompletely because of improper threading. If ClassA is going to construct and modify swing components in the constructor, the constructor should get executed from the Event Dispatch Thread.

public class ClassA 
{
    public static void main(String[] args) 
    {
        SwingUtilities.invokeLater(new ClassA());
    }
}