Question

I have got this exercise working perfectly:

import acm.program.*;
import acm.graphics.*;
import java.awt.event.*;
public class ProgramHierarchy extends GraphicsProgram {
public void run() {
    Paddle = new GRect(0,getHeight() - 30,100,20);
    add(Paddle);
    addMouseListeners();
    }

public void mouseMoved(MouseEvent e) {
    if(e.getX() - Paddle.getWidth()>0) {
    Paddle.move(e.getX()-Paddle.getX()-Paddle.getWidth(), 0);
    }
}

private GRect Paddle;
}

But when I use the exact same code in a larger project, my eclipse throws null pointer exceptions and even hard-crashes eclipse altogether and I hit my desk and say bad things I would not like my family to hear. Is my copy of eclipse corrupt? What could be going on?

Here’s my larger hard-crashing project.

import acm.graphics.*;
import acm.program.*;
import acm.util.*;
import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class Breakout extends GraphicsProgram {

public static final int APPLICATION_WIDTH = 400;
public static final int APPLICATION_HEIGHT = 600;
private static final int WIDTH = APPLICATION_WIDTH;
private static final int HEIGHT = APPLICATION_HEIGHT;
private static final int PADDLE_WIDTH = 60;
private static final int PADDLE_HEIGHT = 10;
private static final int PADDLE_Y_OFFSET = 30;
private static final int NBRICKS_PER_ROW = 10;
private static final int NBRICK_ROWS = 10;
private static final int BRICK_SEP = 3;
private static final int BRICK_WIDTH =
  (WIDTH - (NBRICKS_PER_ROW - 1) * BRICK_SEP) / NBRICKS_PER_ROW;
private static final int BRICK_HEIGHT = 8;
private static final int BALL_RADIUS = 10;
private static final int BRICK_Y_OFFSET = 70;
private static final int NTURNS = 3;
private static final int DELAY = 30;



public void run() {
    setup();
    GRect Paddle = new GRect(0,HEIGHT - PADDLE_HEIGHT - PADDLE_Y_OFFSET,PADDLE_WIDTH,PADDLE_HEIGHT);
    add(Paddle);
    addMouseListeners();
    }







private void setup() {

    int xStart = (WIDTH - ((NBRICKS_PER_ROW * BRICK_WIDTH) + ((NBRICKS_PER_ROW -1) * BRICK_SEP)))/2;

    int y = BRICK_Y_OFFSET;

    for (int i = 0; i< NBRICK_ROWS; i++) {
        int x = xStart;

        for (int j = 0; j < NBRICKS_PER_ROW; j++) {
            GRect block = new GRect(x, y, BRICK_WIDTH, BRICK_HEIGHT);
            if (i<2) {
                block.setColor(Color.RED);
                block.setFilled(true);
                block.setFillColor(Color.red);
            }
            if (i==2||i==3) {
                block.setColor(Color.orange);
                block.setFilled(true);
                block.setFillColor(Color.orange);
            }
            if (i==4||i==5) {
                block.setColor(Color.yellow);
                block.setFilled(true);
                block.setFillColor(Color.yellow);
            }
            if (i==6||i==7) {
                block.setColor(Color.green);
                block.setFilled(true);
                block.setFillColor(Color.green);
            }
            if (i==8||i==9) {
                block.setColor(Color.cyan);
                block.setFilled(true);
                block.setFillColor(Color.cyan);
            }
            add (block);
            x=x+BRICK_WIDTH + BRICK_SEP;
        }
    y=y+BRICK_HEIGHT+BRICK_SEP;
    }
}


public void mouseMoved(MouseEvent PadX) {
    if(PadX.getX() - Paddle.getWidth()>0) {
    Paddle.move(PadX.getX()-Paddle.getX()-Paddle.getWidth(), 0);
    }
}


private GRect Paddle;
}

Answer 1

You’re shadowing the Paddle variable: declaring it in the class, but re-declaring it in the run method. The Paddle variable held in the class is never initialized and so it remains null. The solution is to not re-declare this variable in the run method.

For example, in the code of yours that works, you have:

public void run() {
  Paddle = new GRect(0,getHeight() - 30,100,20); // **** notice the difference?
  add(Paddle);
  addMouseListeners();
}

and in the code that doesn’t work:

public void run() {
  setup();

  // notice that you re-declare Paddle here!
  GRect Paddle = new GRect(0,HEIGHT - PADDLE_HEIGHT - PADDLE_Y_OFFSET,PADDLE_WIDTH,PADDLE_HEIGHT);
  add(Paddle);
  addMouseListeners();
}

The Paddle variable that is initialized above is only visible within the run method and no-where else in your class. The class variable with the same name remains null.

Solution: don’t re-declare Paddle:

public void run() {
  setup();

  // Now we don't re-declare Paddle 
  Paddle = new GRect(0,HEIGHT - PADDLE_HEIGHT - PADDLE_Y_OFFSET,PADDLE_WIDTH,PADDLE_HEIGHT);
  add(Paddle);
  addMouseListeners();
}