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Canvas object is not displaying, but location is updating correctly in Java Applet

由 安稳与你 提交于 2019-12-07 12:19:14

问题


My basic bouncing ball Applet should have a ball moving around according to the values in loc variable, but nothing shows up. Printing out loc shows that the numbers/math behind moving it around and bouncing off boundaries is indeed working as it should, but nothing shows up. Looking at examples online I didn't see why my drawing/painting code wasn't working as intended. Here's what I have, issues are probably focused about halfway down where the Ball object is. I posted most of the program as it does compile/run if someone will run it.

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class Bounce2 extends Applet implements ActionListener, AdjustmentListener, Runnable
{
    //runtime variables
    boolean running = false;
    boolean kill = false;

    //buttons
    Button runbutton = new Button("Run"); 
    Button pausebutton = new Button("Pause");
    Button quitbutton = new Button("Quit");

    //text
    Label speedlabel = new Label("Speed");
    Label sizelabel = new Label("Size");

    //scrollbars
    private final int barHeight = 20, SLIDER_WIDTH = 10, MAXSPEED = 110, MINSPEED = 0, MAX_SIZE = 110, MIN_SIZE = 10;
    Scrollbar speedbar = new Scrollbar(Scrollbar.HORIZONTAL, MAXSPEED/2, SLIDER_WIDTH, MINSPEED, MAXSPEED);
    Scrollbar sizebar = new Scrollbar(Scrollbar.HORIZONTAL, MAX_SIZE/2, SLIDER_WIDTH, MIN_SIZE, MAX_SIZE);

    //drawn objs
    Ball ball;
    int size = 50;  
    private Graphics obj;
    Image offscreen = null;
    Point loc = new Point(100,100); //location of the ball
    private Thread ballThread;

    //boundaries
    int boundx = 640;
    int boundy = 400;

    //directions
    int dx = 1; //1 = left, -1 = right
    int dy = 1; //1 = up, -1 = down

    //speed
    int speed = speedbar.getValue();
    int delay;

    //initialize the applet and draw everything
    public void init()
    {
        double colWeight[] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1};//15 cols
        double rowWeight[] = {1,1,1,1,1,1,1,1,1,1}; //10 rows
        int colWidth[] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1};//15 cols
        int rowHeight[] = {1,1,1,1,1,1,1,1,1,1}; //10 rows
        GridBagConstraints c = new GridBagConstraints();
        GridBagLayout gbl = new GridBagLayout();
        gbl.rowHeights = rowHeight;
        gbl.rowWeights = rowWeight;
        gbl.columnWeights = colWeight;
        gbl.columnWidths = colWidth;
        c.anchor = GridBagConstraints.CENTER;

        setBounds(0,0,480,640);
        setLayout(new BorderLayout());
        Panel controlpanel = new Panel();
        controlpanel.setLayout(gbl);
        controlpanel.setSize(640,80);

        Panel drawingpanel = new Panel(null);
        drawingpanel.setSize(640,400);
        ball = new Ball();
        drawingpanel.add(ball);
        drawingpanel.setVisible(true);

        //speed scrollbar
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 1;
        c.gridy = 7;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.speedbar,c);

        //run button
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 2;
        c.gridheight = 1;
        c.gridx = 5;
        c.gridy = 7;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.runbutton,c);

        //pause button
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 2;
        c.gridheight = 1;
        c.gridx = 8;
        c.gridy = 7;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.pausebutton,c);

        //size scrollbar
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 11;
        c.gridy = 7;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.sizebar,c);

        //speed text label
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 1;
        c.gridy = 8;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.speedlabel,c);

        //size text label
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 11;
        c.gridy = 8;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.sizelabel,c);

        //quit button
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 6;
        c.gridy = 9;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.quitbutton,c);

        //add to the screen
        controlpanel.add(this.speedbar);
        controlpanel.add(this.runbutton);
        controlpanel.add(this.pausebutton);
        controlpanel.add(this.sizebar);
        controlpanel.add(this.speedlabel);
        controlpanel.add(this.sizelabel);
        controlpanel.add(this.quitbutton);

        //add listners
        speedbar.addAdjustmentListener(this);
        runbutton.addActionListener(this);
        pausebutton.addActionListener(this);
        sizebar.addAdjustmentListener(this);
        quitbutton.addActionListener(this);

        //add the panels
        add("South", controlpanel);
        add("Center", drawingpanel);

        //drawing paramaters
        obj = drawingpanel.getGraphics();
        loc = new Point(loc.x+dx, loc.y+dy);

        setVisible(true);
        validate();
    }

    public void start()
    {
        if (ballThread == null)
        {
            ballThread = new Thread(this);
            ballThread.start();
            repaint();
        }
    }

    public void run()
    {
        Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
        while (!kill)
        {
            if (running)
            {
                repaint();
            }
            try
            {
                Thread.sleep(delay);
            }
            catch(InterruptedException e){System.err.println("Interrupted.");}
        }
        stop();
    }

    //class to handle animations
    class Ball extends Canvas 
    {
        public void move()
        {
            //if it will hit the right or left, flip the x direction and set it 
            if (loc.x+size >= boundx || loc.x <= 0)
            { dx *= -1; }
            loc.x += dx;
            //if it will hit the top or bottom, flip the y direction and set it
            if (loc.y+size >= boundy || loc.y <= 0)
            { dy *= -1; }
            loc.y += dy;
            setBounds(dx,dy,size,size);
        }   

        public void paint(Graphics g)
        {
            update(g);
        }

        public void update(Graphics g)
        {
            Graphics buffer;
            if (offscreen == null);
            {
                offscreen = createImage(boundx, boundy);
                buffer = offscreen.getGraphics();
            }

            buffer.setColor(getBackground());
            buffer.fillRect(loc.x,loc.y,boundx, boundy);
            //update loc
            move();

            //draw
            buffer.setColor(Color.black);
            buffer.drawOval(loc.x, loc.y, size, size);
            buffer.fillOval(loc.x, loc.y, size, size);

            //draw rectangles out of vector 
            super.paint(g);
            g.drawImage(offscreen, loc.x, loc.y, null);
        }
    }

    public void actionPerformed(ActionEvent e)
    {
        Object source = e.getSource();
        if (source == this.runbutton)
        {
            running = true;
        }
        else if (source == this.pausebutton)
        {
            running = false;
        }
        else if (source == this.quitbutton)
        {
            //kill processes
            kill = true;
            //remove listeners
            stop();
        }
    }

    public void adjustmentValueChanged(AdjustmentEvent e)
    {
        Object source = e.getSource();
        //set the new size. 
        if (source == sizebar)
        {
            //check for clipping
            int newsize = sizebar.getValue();

            // x
            if (loc.x+newsize >= boundx)
            { 
                newsize = boundx - loc.x - 1;
                sizebar.setValue(newsize);
            }

            // y
            if (loc.y+newsize >= boundy + 100)
            {
                newsize = boundy+100 - loc.y - 1;
                sizebar.setValue(newsize);
            }
            size = newsize;
        }
        if (source == speedbar)
        {
            speed = speedbar.getValue();
            delay = MAXSPEED - speedbar.getValue();
        }
    }

    public void stop()
    {
        this.speedbar.removeAdjustmentListener(this);
        this.runbutton.removeActionListener(this);
        this.pausebutton.removeActionListener(this);
        this.sizebar.removeAdjustmentListener(this);
        this.quitbutton.removeActionListener(this);
        Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
    }
}

回答1:


This is modified version of your code that is running. 

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class Bounce2 extends Applet implements ActionListener, AdjustmentListener, Runnable
{
  //runtime variables
  boolean running = false;
  boolean kill = false;

  //buttons
  Button runbutton = new Button("Run");
  Button pausebutton = new Button("Pause");
  Button quitbutton = new Button("Quit");

  //text
  Label speedlabel = new Label("Speed");
  Label sizelabel = new Label("Size");

  Panel drawingpanel;

  //scrollbars
  private final int barHeight = 20, SLIDER_WIDTH = 10, MAXSPEED = 110, MINSPEED = 0, MAX_SIZE = 110, MIN_SIZE = 10;
  Scrollbar speedbar = new Scrollbar(Scrollbar.HORIZONTAL, MAXSPEED/2, SLIDER_WIDTH, MINSPEED, MAXSPEED);
  Scrollbar sizebar = new Scrollbar(Scrollbar.HORIZONTAL, MAX_SIZE/2, SLIDER_WIDTH, MIN_SIZE, MAX_SIZE);

  //drawn objs
  Ball ball;
  int size = 50;
  private Graphics obj;
  Image offscreen = null;
  Point loc = new Point(100,100); //location of the ball
  private Thread ballThread;

  //boundaries
  int boundx = 640;
  int boundy = 400;

  //directions
  int dx = 1; //1 = left, -1 = right
  int dy = 1; //1 = up, -1 = down

  //speed
  int speed = speedbar.getValue();
  int delay = 3000;

  //initialize the applet and draw everything
  public void init()
  {
    double colWeight[] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1};//15 cols
    double rowWeight[] = {1,1,1,1,1,1,1,1,1,1}; //10 rows
    int colWidth[] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1};//15 cols
    int rowHeight[] = {1,1,1,1,1,1,1,1,1,1}; //10 rows
    GridBagConstraints c = new GridBagConstraints();
    GridBagLayout gbl = new GridBagLayout();
    gbl.rowHeights = rowHeight;
    gbl.rowWeights = rowWeight;
    gbl.columnWeights = colWeight;
    gbl.columnWidths = colWidth;
    c.anchor = GridBagConstraints.CENTER;

    setBounds(0,0,480,640);
    setLayout(new BorderLayout());
    Panel controlpanel = new Panel();
    controlpanel.setLayout(gbl);
    controlpanel.setSize(640,80);

    drawingpanel = new Panel();
    drawingpanel.setSize(640,400);
    ball = new Ball();
    drawingpanel.setLayout(new BorderLayout());
    drawingpanel.add("Center",ball);

    //speed scrollbar
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 1;
    c.gridy = 7;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.speedbar,c);

    //run button
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 2;
    c.gridheight = 1;
    c.gridx = 5;
    c.gridy = 7;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.runbutton,c);

    //pause button
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 2;
    c.gridheight = 1;
    c.gridx = 8;
    c.gridy = 7;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.pausebutton,c);

    //size scrollbar
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 11;
    c.gridy = 7;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.sizebar,c);

    //speed text label
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 1;
    c.gridy = 8;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.speedlabel,c);

    //size text label
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 11;
    c.gridy = 8;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.sizelabel,c);

    //quit button
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 6;
    c.gridy = 9;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.quitbutton,c);

    //add to the screen
    controlpanel.add(this.speedbar);
    controlpanel.add(this.runbutton);
    controlpanel.add(this.pausebutton);
    controlpanel.add(this.sizebar);
    controlpanel.add(this.speedlabel);
    controlpanel.add(this.sizelabel);
    controlpanel.add(this.quitbutton);

    //add listners
    speedbar.addAdjustmentListener(this);
    runbutton.addActionListener(this);
    pausebutton.addActionListener(this);
    sizebar.addAdjustmentListener(this);
    quitbutton.addActionListener(this);

    //add the panels
    add("South", controlpanel);
    add("Center", drawingpanel);

    //drawing paramaters
    loc = new Point(loc.x+dx, loc.y+dy);
  }

  public void start()
  {
    if (ballThread == null)
    {
      ballThread = new Thread(this);
      ballThread.start();
    }
  }

  public void run()
  {
    Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
    while (!kill)
    {
      if (running)
      {
        ball.invalidate();
        ball.repaint();
      }
      try
      {
        Thread.sleep(delay);
      }
      catch(InterruptedException e){System.err.println("Interrupted.");}
    }
    stop();
  }

  public void paint(Graphics g){
    super.paint(g);

  }

  //class to handle animations
  class Ball extends Canvas
  {
    public void move()
    {
      //if it will hit the right or left, flip the x direction and set it
      if (loc.x+size >= boundx || loc.x <= 0)
      { dx *= -1; }
      loc.x += dx;
      //if it will hit the top or bottom, flip the y direction and set it
      if (loc.y+size >= boundy || loc.y <= 0)
      { dy *= -1; }
      loc.y += dy;
    }

    public void update(Graphics g)
    {
      super.update(g);
      Graphics buffer;
      if (offscreen == null)
      {
        offscreen = createImage(getWidth(), getHeight());
      }
      buffer = offscreen.getGraphics();
      mypaint(buffer);
      g.drawImage(offscreen, 0, 0, this);

    }

    public void mypaint(Graphics g)
    {
      g.setColor(getBackground());
      g.fillRect(loc.x,loc.y,boundx, boundy);

      //update loc
      move();

      //draw
      g.setColor(Color.BLACK);
      g.drawOval(loc.x, loc.y, size, size);
      g.fillOval(loc.x, loc.y, size, size);

      //draw rectangles out of vector
    }
  }

  public void actionPerformed(ActionEvent e)
  {
    Object source = e.getSource();
    if (source == this.runbutton)
    {
      running = true;
    }
    else if (source == this.pausebutton)
    {
      running = false;
    }
    else if (source == this.quitbutton)
    {
      //kill processes
      kill = true;
      //remove listeners
      stop();
    }
  }

  public void adjustmentValueChanged(AdjustmentEvent e)
  {
    Object source = e.getSource();
    //set the new size.
    if (source == sizebar)
    {
      //check for clipping
      int newsize = sizebar.getValue();

      // x
      if (loc.x+newsize >= boundx)
      {
        newsize = boundx - loc.x - 1;
        sizebar.setValue(newsize);
      }

      // y
      if (loc.y+newsize >= boundy + 100)
      {
        newsize = boundy+100 - loc.y - 1;
        sizebar.setValue(newsize);
      }
      size = newsize;
    }
    if (source == speedbar)
    {
      speed = speedbar.getValue();
      delay = MAXSPEED - speedbar.getValue();
    }
  }

  public void stop()
  {
    this.speedbar.removeAdjustmentListener(this);
    this.runbutton.removeActionListener(this);
    this.pausebutton.removeActionListener(this);
    this.sizebar.removeAdjustmentListener(this);
    this.quitbutton.removeActionListener(this);
    Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
  }
}



回答2:


I did the following changes in your code and could get it working fine -

First I set the initial location of ball to 

Point loc = new Point(10,10);

After initializing ball = new Ball(); added -

drawingpanel.setLayout(new BorderLayout());
            drawingpanel.add("Center",ball);

In the run() method repaint the ball -

if(running){
ball.repaint(); 
}

In the move() method remove/comment the line -

//setBounds(dx,dy,size,size);

In the update(Graphics g) method remove/comment the lines -

//buffer.setColor(Color.black);
//buffer.fillRect(loc.x,loc.y,boundx, boundy);

The ; after if (offscreen == null); doesnt make any sense, so I commented it -

 //if (offscreen == null);

Bingo, everything was working fine ! Let me know if you are able to make it, else I will post the updated code.




回答3:


Add a layout manager to your drawingpanel:

drawingpanel.setLayout(new BorderLayout());
drawingpanel.add("Center",ball);

Then, call ball.repaint(); in the run() method to get into your custom update method.

Finally, remove the setBounds(dx,dy,size,size); statement from your move() method.

The final code would be this:

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class Bounce2 extends Applet implements ActionListener, AdjustmentListener, Runnable
{
//runtime variables
boolean running = false;
boolean kill = false;

//buttons
Button runbutton = new Button("Run"); 
Button pausebutton = new Button("Pause");
Button quitbutton = new Button("Quit");

//text
Label speedlabel = new Label("Speed");
Label sizelabel = new Label("Size");

//scrollbars
private final int barHeight = 20, SLIDER_WIDTH = 10, MAXSPEED = 110, MINSPEED = 0, MAX_SIZE = 110, MIN_SIZE = 10;
Scrollbar speedbar = new Scrollbar(Scrollbar.HORIZONTAL, MAXSPEED/2, SLIDER_WIDTH, MINSPEED, MAXSPEED);
Scrollbar sizebar = new Scrollbar(Scrollbar.HORIZONTAL, MAX_SIZE/2, SLIDER_WIDTH, MIN_SIZE, MAX_SIZE);

//drawn objs
Ball ball;
int size = 50;  
private Graphics obj;
Image offscreen = null;
Point loc = new Point(100,100); //location of the ball
private Thread ballThread;

//boundaries
int boundx = 640;
int boundy = 400;

//directions
int dx = 1; //1 = left, -1 = right
int dy = 1; //1 = up, -1 = down

//speed
int speed = speedbar.getValue();
int delay;

//initialize the applet and draw everything
public void init()
{
    double colWeight[] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1};//15 cols
    double rowWeight[] = {1,1,1,1,1,1,1,1,1,1}; //10 rows
    int colWidth[] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1};//15 cols
    int rowHeight[] = {1,1,1,1,1,1,1,1,1,1}; //10 rows
    GridBagConstraints c = new GridBagConstraints();
    GridBagLayout gbl = new GridBagLayout();
    gbl.rowHeights = rowHeight;
    gbl.rowWeights = rowWeight;
    gbl.columnWeights = colWeight;
    gbl.columnWidths = colWidth;
    c.anchor = GridBagConstraints.CENTER;

    setBounds(0,0,480,640);
    setLayout(new BorderLayout());
    Panel controlpanel = new Panel();
    controlpanel.setLayout(gbl);
    controlpanel.setSize(640,80);

    Panel drawingpanel = new Panel(null);
    drawingpanel.setSize(640,400);
    ball = new Ball();
    drawingpanel.setLayout(new BorderLayout());
    drawingpanel.add("Center",ball);
    drawingpanel.setVisible(true);

    //speed scrollbar
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 1;
    c.gridy = 7;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.speedbar,c);

    //run button
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 2;
    c.gridheight = 1;
    c.gridx = 5;
    c.gridy = 7;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.runbutton,c);

    //pause button
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 2;
    c.gridheight = 1;
    c.gridx = 8;
    c.gridy = 7;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.pausebutton,c);

    //size scrollbar
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 11;
    c.gridy = 7;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.sizebar,c);

    //speed text label
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 1;
    c.gridy = 8;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.speedlabel,c);

    //size text label
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 11;
    c.gridy = 8;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.sizelabel,c);

    //quit button
    c.weightx = 1;
    c.weighty = 1;
    c.gridwidth = 3;
    c.gridheight = 1;
    c.gridx = 6;
    c.gridy = 9;
    c.fill= GridBagConstraints.HORIZONTAL;
    gbl.setConstraints(this.quitbutton,c);

    //add to the screen
    controlpanel.add(this.speedbar);
    controlpanel.add(this.runbutton);
    controlpanel.add(this.pausebutton);
    controlpanel.add(this.sizebar);
    controlpanel.add(this.speedlabel);
    controlpanel.add(this.sizelabel);
    controlpanel.add(this.quitbutton);

    //add listners
    speedbar.addAdjustmentListener(this);
    runbutton.addActionListener(this);
    pausebutton.addActionListener(this);
    sizebar.addAdjustmentListener(this);
    quitbutton.addActionListener(this);

    //add the panels
    add("South", controlpanel);
    add("Center", drawingpanel);

    //drawing paramaters
    obj = drawingpanel.getGraphics();
    loc = new Point(loc.x+dx, loc.y+dy);

    setVisible(true);
    validate();
}

public void start()
{
    if (ballThread == null)
    {
        ballThread = new Thread(this);
        ballThread.start();
        repaint();
    }
}

public void run()
{
    Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
    while (!kill)
    {
        if (running)
        {
            repaint();
            ball.repaint();
        }
        try
        {
            Thread.sleep(delay);
        }
        catch(InterruptedException e){System.err.println("Interrupted.");}
    }
    stop();
}

//class to handle animations
class Ball extends Canvas 
{
    public void move()
    {
        //if it will hit the right or left, flip the x direction and set it 
        if (loc.x+size >= boundx || loc.x <= 0)
        { dx *= -1; }
        loc.x += dx;
        //if it will hit the top or bottom, flip the y direction and set it
        if (loc.y+size >= boundy || loc.y <= 0)
        { dy *= -1; }
        loc.y += dy;
//            setBounds(dx,dy,size,size);
    }   

    public void paint(Graphics g)
    {
        update(g);
    }

    public void update(Graphics g)
    {
        Graphics buffer;
        if (offscreen == null);
        {
            offscreen = createImage(boundx, boundy);
            buffer = offscreen.getGraphics();
        }

        buffer.setColor(getBackground());
        buffer.fillRect(loc.x,loc.y,boundx, boundy);
        //update loc
        move();

        //draw
        buffer.setColor(Color.black);
        buffer.drawOval(loc.x, loc.y, size, size);
        buffer.fillOval(loc.x, loc.y, size, size);

        //draw rectangles out of vector 
        super.paint(g);
        g.drawImage(offscreen, loc.x, loc.y, null);
    }
}

public void actionPerformed(ActionEvent e)
{
    Object source = e.getSource();
    if (source == this.runbutton)
    {
        running = true;
    }
    else if (source == this.pausebutton)
    {
        running = false;
    }
    else if (source == this.quitbutton)
    {
        //kill processes
        kill = true;
        //remove listeners
        stop();
    }
}

public void adjustmentValueChanged(AdjustmentEvent e)
{
    Object source = e.getSource();
    //set the new size. 
    if (source == sizebar)
    {
        //check for clipping
        int newsize = sizebar.getValue();

        // x
        if (loc.x+newsize >= boundx)
        { 
            newsize = boundx - loc.x - 1;
            sizebar.setValue(newsize);
        }

        // y
        if (loc.y+newsize >= boundy + 100)
        {
            newsize = boundy+100 - loc.y - 1;
            sizebar.setValue(newsize);
        }
        size = newsize;
    }
    if (source == speedbar)
    {
        speed = speedbar.getValue();
        delay = MAXSPEED - speedbar.getValue();
    }
}

public void stop()
{
    this.speedbar.removeAdjustmentListener(this);
    this.runbutton.removeActionListener(this);
    this.pausebutton.removeActionListener(this);
    this.sizebar.removeAdjustmentListener(this);
    this.quitbutton.removeActionListener(this);
    Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
}
}


来源：https://stackoverflow.com/questions/13164162/canvas-object-is-not-displaying-but-location-is-updating-correctly-in-java-appl

标签
java
applet
awt
paint
java-canvas
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