的mouseMoved給空例外指針 - 徹底難倒了（更斯坦福CS106疼痛）

Question

我有這個練習完美的工作：

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; 
} 

但是，當我在一個較大的項目中使用完全相同的代碼，我的日食投空指針異常，甚至徹底崩潰的日食，我打我的桌子，說壞事，我不希望我的家人聽到。我的eclipse副本是否損壞？可能會發生什麼？

這是我較大的硬碰撞項目。

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

你在遮蔽Paddle變量：在類中聲明它，但在run方法中重新聲明它。類中保存的Paddle變量不會被初始化，因此它保持爲空。解決方法是不要在run方法中重新聲明這個變量。

例如，在你工作的代碼，您有：

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

，並在代碼不起作用：

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(); 
} 

是上述初始化槳變量只在run方法中可見，否則在你的類中沒有其他地方。具有相同名稱的類變量保留爲空。

解決方法：不要重新申報槳：

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(); 
}