构造函数的问题，它将使用中心坐标、高（X 轴）和宽度（y 轴）创建矩形

Question

所以我需要由 2 个类组成的代码，已经形成的方法点和方法矩形，其中包括用于创建矩形本身的构造函数，矩形本身应该由这个测试形成

@Test
    public void testRectangle2() {
        Rectangle rect = new Rectangle(20, 30, 20, 20);
        assertAll(
                () -> assertEquals(10, rect.getTopLeft().getX()),
                () -> assertEquals(20, rect.getTopLeft().getY()),
                () -> assertEquals(30, rect.getBottomRight().getX()),
                () -> assertEquals(40, rect.getBottomRight().getY()),
                () -> assertEquals(20, rect.getWidth(), 20),
                () -> assertEquals(20, rect.getHeight(), 20)
        );
    }

类点工作正常，我只是为了清楚起见而添加它

public class Point {
    private int x, y;
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
    public Point() {
        this(0, 0);
    }
    public int getX() {
        return x;
    }
    public void setX(int x) {
        this.x = x;
    }
    public int getY() {
        return y;
    }
    public void setY(int y) {
        this.y = y;
    }
    public void moveTo(int newX, int newY) {
        x = newX;
        y = newY;
    }
    public void moveRel(int dx, int dy) {
        x += dx;
        y += dy;
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + x;
        result = prime * result + y;
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Point other = (Point) obj;
        if (x != other.x)
            return false;
        if (y != other.y)
            return false;
        return true;
    }
}

这是 Rectangle 本身的类，它包括构造函数和附加方法。

public class Rectangle {
    public int width = 0;
    public int height = 0;
    public Point center;
    public int xCenter;
    public int yCenter;

    public Point getTopLeft() {
        Point point = new Point(center.getX(), center.getY());
        point.moveRel(- width / 2, - height / 2);
        return point;
    }

    public Point getBottomRight() {
        Point point = new Point(center.getX(), center.getY());
        point.moveRel(width / 2,  height / 2);
        return point;
    }

    public int getWidth() {

        return width;
    }


    public int getHeight() {

        return height;
    }
    public Point getCenter() {
       Point center = new Point(xCenter, yCenter);
       return center;
    }
    public Rectangle(int xCenter, int yCenter, int width, int height) {
        this.xCenter=xCenter;
        this.yCenter=yCenter;
        this.width=width;
        this.height=height;
    }
}

问题在于它自己的构造函数，它应该从测试中获取值，然后形成矩形，但它没有这样做。

Answer 1

这段代码有几个问题。

构造函数的问题是没有创建中心点，然后测试失败，getTopLeft/getBottomRight 中的NullPointerException 先调用，因为这些方法不使用getCenter() 方法，而是直接引用center 字段。

所以，只要在上述方法中使用上述getter并删除冗余字段center就足够了。

class Rectangle {
    public int width = 0;
    public int height = 0;
    public int xCenter;
    public int yCenter;

    public Point getTopLeft() {
        Point point = getCenter();
        point.moveRel(- width / 2, - height / 2);
        return point;
    }

    public Point getBottomRight() {
        Point point = getCenter();
        point.moveRel(width / 2,  height / 2);
        return point;
    }

    public int getWidth() {

        return width;
    }


    public int getHeight() {

        return height;
    }
    public Point getCenter() {
       Point center = new Point(xCenter, yCenter);
       return center;
    }
    public Rectangle(int xCenter, int yCenter, int width, int height) {
        this.xCenter=xCenter;
        this.yCenter=yCenter;
        this.width=width;
        this.height=height;
    }
}

但是，这种设计存在一定的缺陷，Rectangle 字段是公开的，可以从外部更改，从而破坏了封装原则。此外，虽然可能会重复使用，但也会创建冗余点。

Rectangle 类的改进版本如下：

• “不可变”字段
• 延迟初始化topLeft和rightBottom点

public class Rectangle {
    private final int width;
    private final int height;
    private final int xCenter;
    private final int yCenter;
    
    private Point topLeft;
    private Point bottomRight;
    
    public Rectangle(int xCenter, int yCenter, int width, int height) {
        this.xCenter = xCenter;
        this.yCenter = yCenter;
        this.width = width;
        this.height = height;
    }

    public Point getTopLeft() {
        if (null == topLeft) {
            topLeft = getCenter();
            topLeft.moveRel(- width / 2, - height / 2);
        }
        return topLeft;
    }

    public Point getBottomRight() {
        if (null == bottomRight) {
            bottomRight = getCenter();
            bottomRight.moveRel(width / 2,  height / 2);    
        }
        return bottomRight;
    }

    public int getWidth() {
        return width;
    }

    public int getHeight() {
        return height;
    }
    
    public Point getCenter() {
       return new Point(xCenter, yCenter);
    }
}