Question

I need a listener that will constantly check if a static boolean value has been changed so that I can repaint a component on a frame. Can someone please help me I really don’t know much about listeners and haven’t worked with them much? Help will be greatly appreciated.

edit(more clarity): I have two separate classes in which on class is the “main frame” the second class is an extension of JLabel and implements MouseListner for a “clickable photo”. The “main frame” creates instances of the photo and when the photo is clicked the “main frame” is supposed to paint on the panel a description of the photo. This is “main frame”

MenuBar menuBar;
static AnnotationVisual a;
Picture pic;
Picture pic2;

GalleryScreen(int rows, int columns){
    this.setBorder(BorderFactory.createEmptyBorder(500,500,0,0));
    pic = new Picture("pic1", "Z:/My Documents/Downloads/Ball.jpg", new Coordinate(0,0));
    pic2 = new Picture("pic2", "Z:/My Documents/Downloads/hoop.jpg" , new Coordinate(1,0));

    this.add(pic);
    this.add(pic2);
    a = new AnnotationVisual();
}

public void paintComponent(Graphics g){
    super.paintComponent(g);
    if(a.shouldAnnotate()){
        FontMetrics size= g.getFontMetrics(); 
        if(getWidth()>=(a.dispX()+size.stringWidth(a.annotationText()))){
            g.setColor(Color.white);
            g.fillRect(a.dispX()-3,a.dispY()-12,size.stringWidth(a.annotationText())+5,15);
            g.setColor(Color.black);
            g.drawRect(a.dispX()-3,a.dispY()-12,size.stringWidth(a.annotationText())+5,15);
            g.drawString(a.annotationText(), a.dispX(), a.dispY());
        }else{
            String sub="";
            int letters=0;
            g.setColor(Color.white);
            g.fillRect(a.dispX()-3,a.dispY()-12,getWidth(),15);
            g.setColor(Color.black);

            for(int i=0;i<a.annotationText().length();i++){
                if(a.dispX()+letters+16<=getWidth()){
                    sub+=a.annotationText().substring(i,i+1);
                    letters=size.stringWidth(sub);
                }else{
                    sub=sub+"...";
                    i=a.annotationText().length();
                }
            }
            g.drawRect(a.dispX()-3,a.dispY()-12,size.stringWidth(sub)+3,15);
            g.drawString(sub,a.dispX(),a.dispY());
        }
    }
}

public static AnnotationVisual getA()
{
    return a;
}

This is “clickable photo”

import java.awt.Dimension;
import java.awt.Toolkit;
import java.awt.event.*;
import javax.swing.*;

public class Picture extends JLabel implements MouseListener
{
    String myAnnotation;
    String filePath;
    Coordinate imageCoord;
    private boolean wasDoubleClick;
    private Timer timer;
    EditAnnotation newEdit; 
    AnnotationVisual newVisual;

    public Picture(String annotation, String filePath, Coordinate coord)
    {
        super(new ImageIcon(filePath));
        this.addMouseListener(this);
        myAnnotation=annotation;
        this.filePath = filePath;
        imageCoord = coord;
        newEdit = new EditAnnotation(annotation);
        newVisual = new AnnotationVisual();
    }
    public Picture(String filePath)
    {
        super(new ImageIcon(filePath));
        this.addMouseListener(this);
        this.filePath = filePath;
        newEdit  = new EditAnnotation();
        newVisual = new AnnotationVisual();
    }
    public String getAnnotation()
    {
        return myAnnotation;
    }
    public AnnotationVisual getAnnotationVisual()
    {
        return newVisual;
    }
    public void setAnnotation(String annotation)
    {
        myAnnotation=annotation;
    }
    public Coordinate getCoordinate()
    {
        return imageCoord;
    }
    public void setCoordinate(Coordinate coord)
    {
        imageCoord = coord;
    }
    public Dimension getSize()
    {
        return new Dimension(super.getIcon().getIconWidth(), super.getIcon().getIconHeight());
    }
    public void mouseClicked(MouseEvent e)
    {
        final int scrLocX = (int)e.getLocationOnScreen().getX();
        final int scrLocY = (int)e.getLocationOnScreen().getY();

        if (e.getClickCount() == 2) 
        {
            wasDoubleClick = true; 
        } 
        else if(e.getClickCount() == 1)
        {
            Integer timerinterval = (Integer) Toolkit.getDefaultToolkit().getDesktopProperty("awt.multiClickInterval"); 

            timer = new Timer(timerinterval.intValue(), new ActionListener() 
            {
                public void actionPerformed(ActionEvent evt) 
                {
                    if (wasDoubleClick) 
                    {
                        GalleryScreen.getA().deleteAnnotation();
                        myAnnotation = newEdit.getAnnotation();
                        newEdit.show(myAnnotation);
                        wasDoubleClick = false;
                    } 
                    else 
                    {
                        GalleryScreen.getA().deleteAnnotation();
                        GalleryScreen.getA().showAnnotation(scrLocX, scrLocY , myAnnotation);
                    } 
                }
            });
            timer.setRepeats(false);
            timer.start();
        }
    }
    public void mouseEntered(MouseEvent e)
    {

    }
    public void mouseExited(MouseEvent e)
    {

    }
    public void mousePressed(MouseEvent e)
    {

    }
    public void mouseReleased(MouseEvent e)
    {

    }
}

AnnotationVisual is the thing that supposed to pop up when single clicked

Answer 1

You don’t set a listener on a field in Java, you set it on a property. While properties (according to the JavaBeans spec) can be fields, they’re usually done as pairs of methods (one getter, one setter; the latter being not needed for read-only fields) as that lets you hook extra logic in to be called when the property is accessed. Such as firing a listener callback to say that the value has changed. (You could use a thread to monitor for that sort of thing, but that’s really nasty and error-prone. Wasteful too.)

One thing to be aware of though: you don’t know what thread the value will have been modified from. Take care when invoking back into Swing…