change listener for variable in other class

问题

EDITED: below is my try of SSCCE for this example to give u an opportunity to compile this on your own. I need to: inform class PanelNumber2 that object Generator g in class PanelNumber1 have been changed.

I tried with ChangeListner and ActionPerformed however on command:

listener.stateChanged(new ChangeEvent(this));

i have got nullpointer exception. correct me if u have got any solution of this problem. or any better way to do this

import java.awt.BorderLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;


public class MainFrame extends JFrame{
private PanelNumber1 p1;
private PanelNumber2 p2;

public MainFrame(){
    super("Frame");

    JPanel panel = new JPanel(new BorderLayout());
    this.getContentPane().add(panel);

    p1 = new PanelNumber1();
    Generator g = p1.getGenerator();
    p2 = new PanelNumber2(g); 


    panel.add(p1,BorderLayout.CENTER);
    panel.add(p2,BorderLayout.EAST);



    this.setExtendedState(JFrame.MAXIMIZED_BOTH);

    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        

    this.setVisible(true);
    this.setResizable(true);
}
public static void main(String[] args) {
    // TODO Auto-generated method stub
    new MainFrame();
}
}

class PanelNumber2 extends JPanel implements ChangeListener{    
public PanelNumber2(Generator g){
    super();
    setLayout(new GridLayout(7,1));

    g.addListener(this);   
}
@Override
public void stateChanged(ChangeEvent e) {
    // TODO Auto-generated method stub
    this.repaint();
}
}

class PanelNumber1 extends JPanel{

Generator g;
BufferedImage mapa;
File imageFile;

public PanelNumber1(){
    super();

    g = new Generator();
    g.setPanel(this);

    imageFile = new File("C:\\Users\\Katie\\Documents\\Eclipse\\Samolot\\src\\Pics\\img_ww1.png");

    try {
        mapa = ImageIO.read(imageFile);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

@Override
public void paintComponent(Graphics g) {
    Graphics2D g2d = (Graphics2D) g;
    g2d.drawImage(mapa, 0, 0, getWidth(), getHeight(), null); 
}
public Generator getGenerator(){
    return g;
}
}

class Generator{
ChangeListener listener;
ArrayList<String> s;
JPanel p;
Timer generatorTimer;


public Generator(){
    this.s = new ArrayList<String>();
      generatorTimer = new Timer(40, new ActionListener() {
          @Override
          public void actionPerformed(ActionEvent e) {
                  addItem();     
                  p.repaint();
          }
        });
        generatorTimer.start();
}
private void addItem(){
    s.add(new String("string"));
    listener.stateChanged(new ChangeEvent(this));
}
public void setPanel(JPanel p){
    this.p = p;
}

public ArrayList<String> getArray(){
    return s;
}   
public void addListener(ChangeListener list){
    this.listener = list;
}
}

Need help guys :(

回答1:

SOLUTION!

Oh my God, after few hours figuiring out whats wrong i got it! :P

everything what i need to do more was adding a condition in timer

      public void actionPerformed(ActionEvent e) {
              if(p != null){  //becouse after creating Generator object im doing setPanel(this) a short while after :) 
              addItem();     
              p.repaint();
            }
      }

now it works perfectly. Thanks going for MadProgrammer for trying to help me and for YMY - he find that mistake. topic can be closed

来源：https://stackoverflow.com/questions/20309896/change-listener-for-variable-in-other-class