Java：简单但不寻常的NullPointerException

Question

我坚持通常非常简单的事情。 调用这个简单类的构造函数时，我得到一个NullPointerException：

import java.awt.geom.*;

public class Brick extends ColorShape {
        private int xPos = 0;
        private int yPos = 0;
        private int width = 0;
        private int height = 0;
        private Rectangle2D.Double shape;

        // constructor
        public Brick(int x, int y, int w, int h) {
            super(new Rectangle2D.Double(x, y, w, h));

            //set brick x, y, width, and height
            xPos = x;
            yPos = y;
            width = w;
            height = h;

            // update shape
            shape.setRect((double)xPos, (double)yPos, (double)width, (double)height);
        }

        public int getX() {
            return xPos;
        }

        public Rectangle2D.Double getShape() {
            return shape;
        }
    }

它以这种方式调用：

for (int i = 0; i < numCols; i++) {
            for (int j = 0; j < numRows; j++) {

                // initialize bricks[i][j]
                bricks[i][j].setLocation((double)(i*brickWidth), (double)(j*brickHeight));
                bricks[i][j].setSize((double)brickWidth, (double)brickHeight);
                //bricks[i][j] = new Brick(i*brickWidth, j*brickHeight, brickWidth, brickHeight);
                //bricks[i][j] = new Brick(0, 0, 0, 0);
            }
        }

无论我尝试什么，我总是会尝试初始化该类的NullPointerException。

编辑：

Tristan的建议以及将嵌套for循环更改为下面的代码修复了它

// create new bricks and store them in bricks array
        for (int i = 0; i < numCols; i++) {
            for (int j = 0; j < numRows; j++) {


                // initialize bricks[i][j]
                //bricks[i][j].setLocation((double)(i*brickWidth), (double)(j*brickHeight));
                //bricks[i][j].setSize((double)brickWidth, (double)brickHeight);
                bricks[i][j] = new Brick(i*brickWidth, j*brickHeight, brickWidth, brickHeight);
                //bricks[i][j] = new Brick(0, 0, 0, 0);
            }
        }

Answer 1

我认为你不小心将形状重新定义为未初始化的领域。 您调用setRect on的形状尚未按您认为的方式初始化。

如果您尝试访问的父类中有一个形状，只需将其修饰符设置为protected并删除您发布的类中的私有形状声明。

/* REMOVE THIS */
private Rectangle2D.Double shape; // uninitialized!

// constructor
public Brick(int x, int y, int w, int h) {
    super(new Rectangle2D.Double(x, y, w, h));

    //set brick x, y, width, and height
    xPos = x;
    yPos = y;
    width = w;
    height = h;
    // update shape
    // This now references a protected instance variable inherited from the parent.
    shape.setRect((double)xPos, (double)yPos, (double)width, (double)height);
}

然而，通过这个构造函数，它似乎相当偏离。 如果父类本身具有形状，为什么需要设置矩形的方式与在父类中设置它的方式不同？

例如，此代码似乎在逻辑上是等效的。

// Call the parents constructor to set the shape of this brick..
public Brick(int x, int y, int w, int h) {
    super(new Rectangle2D.Double(x, y, w, h));
}

Answer 2

正如Tristan所提到的，初始砖构造函数的问题是已声明形状但未实例化。

也就是说，实例化形状很简单：

public Brick(int x, int y, int w, int h) {
    super(new Rectangle2D.Double(x, y, w, h));

    //set brick x, y, width, and height
    xPos = x;
    yPos = y;
    width = w;
    height = h;
    // update shape
    // This now references a protected instance variable inherited from the parent.
    shape = (Rectangle2D.Double)super.shape;
    shape.setRect(xPos, yPos, width, height);
}