基于中心坐标构造Rectangle类的构造器问题排查及修复

Let's break down why your current Rectangle class isn't passing the test and fix it up step by step.

First, understand the test expectations #

The test creates a Rectangle(20, 30, 20, 20) and expects:

• Top-left point: (10, 20) → calculated by centerX - width/2 and centerY - height/2
• Bottom-right point: (30, 40) → calculated by centerX + width/2 and centerY + height/2
• Width and height to both equal 20

What's wrong with your existing Rectangle code? #

• The center member variable is never initialized in the constructor. When getTopLeft() or getBottomRight() tries to call center.getX(), it throws a NullPointerException.
• You have redundant fields: xCenter/yCenter and center store the same coordinate data, leading to unused variables and potential inconsistency.

Fix Option 1: Simplify with the center field (Recommended) #

Remove the redundant xCenter/yCenter fields and properly initialize the center Point in the constructor:

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

    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() {
        // Return a copy to prevent external code from modifying the internal center
        return new Point(center.getX(), center.getY());
    }

    public Rectangle(int xCenter, int yCenter, int width, int height) {
        this.center = new Point(xCenter, yCenter); // Initialize the center Point
        this.width = width;
        this.height = height;
    }
}

Fix Option 2: Keep xCenter/yCenter and remove unused center field #

If you prefer to keep separate coordinate fields, update the getTopLeft() and getBottomRight() methods to use them instead of the uninitialized center:

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

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

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

    public int getWidth() {
        return width;
    }

    public int getHeight() {
        return height;
    }

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

    public Rectangle(int xCenter, int yCenter, int width, int height) {
        this.xCenter = xCenter;
        this.yCenter = yCenter;
        this.width = width;
        this.height = height;
    }
}

Why these fixes work #

Both options correctly calculate the required points using the center coordinates and dimensions passed to the constructor. When you create new Rectangle(20, 30, 20, 20):

• Top-left: 20 - 20/2 = 10 (x), 30 - 20/2 = 20 (y)
• Bottom-right: 20 + 20/2 = 30 (x), 30 + 20/2 = 40 (y)
• Width and height remain 20, matching all test assertions.

内容的提问来源于stack exchange，提问作者user14041112