[java] Java 2D Graphics, Threads and Classes

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Exception in thread "Thread-4" java.lang.NullPointerException at MainDr.run(MainDr.java:38) at java.lang.Thread.run(Thread.java:619)

import java.awt.*;import java.applet.*;public class MainDr extends Applet implements Runnable{	Thread drawThread = new Thread();	boolean running = false;	Animate draw;	int tempVal = 0;		public void init()	{		System.out.println("Starting right now.");		Animate draw = new Animate();		draw.init();		//setBackground(Color.darkGray);		drawThread = new Thread(this);		running = true;		//drawThread.start();		drawThread.start();		System.out.println("done.");	}		public void destroy()	{		running = false;		drawThread = null;		}		public void run()	{		Thread tempThread = Thread.currentThread();				while (drawThread == tempThread)		{			System.out.println ("in DrawThread");			draw.repaint(); // boom error <-----------------****----						try{				Thread.sleep(500);			}			catch (InterruptedException e) {				System.out.println( "whoops.");			}		}		// something else			}}

Animate draw = new Animate();

draw = new Animate();

import java.awt.*;import java.applet.*;public class MainDr extends Applet implements Runnable{	Thread drawThread = new Thread();	boolean running = false;	Animate draw;	int tempVal = 0;		public void init()	{		System.out.println("Starting right now.");		draw = new Animate();		draw.init();		//setBackground(Color.darkGray);		drawThread = new Thread(this);		running = true;		//drawThread.start();		drawThread.start();		System.out.println("done.");	}		public void start(){		Animate draw = new Animate();		draw.init();				if (drawThread == null){			drawThread = new Thread(this);			drawThread.start();		}	}		public void destroy()	{		running = false;		drawThread = null;		}		public void run()	{		Thread tempThread = Thread.currentThread();				while (drawThread == tempThread)		{			System.out.println ("in DrawThread");			//draw.repaint();			Graphics screen = getGraphics();			draw.setScreen(screen);			try{				Thread.sleep(500);			}			catch (InterruptedException e) {				System.out.println( "whoops.");			}		}		// something else			}}

import java.awt.*;import java.applet.*;import java.io.*;import java.net.*;public class Animate extends Applet {	Image offscreen;	Graphics buffGraphs;	Image yellowPic;	URL crtPath;	int x =60, y=60;	Graphics g;		public Animate()	{		//offscreen = createImage(200, 200);		//buffGraphs = offscreen.getGraphics();		//yellowPic = getImage(crtPath, "yellow.png");	}		public void init()	{		g = getGraphics();	}		public void setScreen(Graphics scrn)	{		g = scrn;	}		public void paint(Graphics g)	{		//g = getGraphics();		//g = screen;		if (g == null)		{			g = getGraphics();		}		//g.drawImage(yellowPic, 20,20,(this));		 g.setColor(Color.red);		 x++;		 y++;		 		 // This will draw a rectangle (xco,yco,xwidth,height);		 g.drawRect(5,5,20,20); 		 g.drawRect(x, y, 20, 20);		// screen = g;		//buffGraphs = 		//g.drawRect(20, 20, 50, 50);	}	}

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Original post by Instigator When I try to run the applet I can see that the thread is getting executed every 500 milliseconds (what I need). However, I don't see the rectangles appear on the applet. Thus, the 'draw.repaint()' method is getting called yet there's no drawing done to the screen.

// MainDR.javaimport java.applet.Applet;public class MainDR extends Applet implements Runnable {	// running should be declared volatile since it will be used as a flag	// by more than one thread.	private volatile boolean running = false;		private Thread drawThread;	private Animate draw;		public void init() {		System.out.println("Initializing Applet");				draw = new Animate();	// create a new Animate Canvas				// make sure the canvas is the proper size, otherwise it won't		// be visible (the Canvas class is funny like that)		draw.setSize(getSize());					add(draw);				// add the Canvas to the Applet				// This next line belongs in the run method, since the thread isn't really running yet		// running = true;  				System.out.println("Init complete.");	}			public void start() {		System.out.println("Starting Applet");				drawThread = new Thread(this);		drawThread.start();				System.out.println("Start complete");	}		public void stop() {		System.out.println("Stopping Applet");				running = false;		// signal the draw thread to exit				// since the draw thread might be blocked in a sleep call right now,		// go ahead and interrupt it so that it can wake up		drawThread.interrupt();				// wait for the draw thread to exit		try {			drawThread.join();		} catch(InterruptedException ie) {}				drawThread = null;				System.out.println("Applet stopped");	}		// Don't need this -- no resources to deallocate, but put it here anyway	// for demonstration purposes.	 public void destroy() {		 System.out.println("Destroying Applet");	 }		public void run() {		System.out.println("Draw thread has been entered");				running = true;		// now the draw thread is running				while(running) {						System.out.println("Top of draw thread loop");						draw.repaint();		// redraw the canvas						try {				Thread.sleep(500);			} catch(InterruptedException ie) {				// This isn't a 'whoops'. Threads are interrupted all of the				// time, and in this case we want it to be interrupted if it				// is sleeping when the app exits.			}						System.out.println("Bottom of draw thread loop");		}				System.out.println("Draw thread is exiting");	}		}// Animate.javaimport java.awt.Canvas;import java.awt.Color;import java.awt.Graphics;public class Animate extends Canvas {	private int x, y;		public Animate() {		setBackground(Color.darkGray);				x = y = 60;	}		public void paint(Graphics g) {		g.setColor(Color.red);				g.drawRect(5, 5, 20, 20);				x++;		y++;		g.drawRect(x, y, 20, 20);	}	}

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Original post by capn_midnight You really want to be doing rendering on the main application thread. While Sun has done some work for Java 6 and is doing more to make cross-thread rendering more stable for Java 7, it isn't very good yet, and I don't have much confidence for the future.

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Now, in most other GUI systems, doing ALL of your rendering in a single, arbitrary thread would be okay, even if it weren't the main application thread. However, because everything is fundamentally built on AWT or Swing, you aren't in control of all of the rendering. Since AWT and Swing do their own bit of this-and-that, you're stuck to using the main application thread to keep everything in one thread.

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I don't trust tutorials for anything and I don't trust the Sun documentation on Best Practices anymore.