我的这项练习非常顺利:
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,当我在一个较大的项目中使用完全相同的代码时,我的日食会引发空指针异常,甚至会彻底破坏日食,我打着办公桌说不好的话,我不希望家人听到。我的日食副本是否已损坏?可能会发生什么?
这是我的失败较大的项目。
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;
}
最佳答案
您正在遮蔽Paddle变量:在类中声明它,但是在run方法中重新声明它。该类中保存的Paddle变量从不初始化,因此它保持为空。解决方法是不要在run方法中重新声明此变量。
例如,在您有效的代码中,您具有:
public void run() {
Paddle = new GRect(0,getHeight() - 30,100,20); // **** notice the difference?
add(Paddle);
addMouseListeners();
}
并在无效的代码中:
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();
}
上面初始化的Paddle变量仅在run方法中可见,而在您的类中其他任何地方都不可见。具有相同名称的类变量保持为空。
解决方案:不要重新声明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();
}
关于java - mouseMoved提供空异常指针-完全困惑(更多斯坦福CS106痛苦),我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/11604594/