1 定义—个点，包含X，y坐标。包含无参构造函数和有参构遥的数。 2 定义一条边，拥有起点、终点，拥有长魔属性（长度可计算） 3定义一个长方形，由四条边（或四个顶点）组成，拥有面积、周长属性（可计算）。判断其有效性。 4分别测试点对象、边对象和长方形对象。

1. 点类 (Point) 的定义通常是这样的：

public class Point {
    private double x;
    private double y;

    // 无参构造函数
    public Point() {
        this.x = 0.0;
        this.y = 0.0;
    }

    // 有参构造函数
    public Point(double x, double y) {
        this.x = x;
        this.y = y;
    }
    
    // getter and setter methods for x and y
    public double getX() { return x; }
    public void setX(double x) { this.x = x; }
    public double getY() { return y; }
    public void setY(double y) { this.y = y; }
}

2. 边类 (LineSegment)：

public class LineSegment {
    private Point start;
    private Point end;
    private double length;

    public LineSegment(Point start, Point end) {
        this.start = start;
        this.end = end;
        this.length = calculateLength();
    }

    // 计算两点之间的距离
    private double calculateLength() {
        return Math.sqrt(Math.pow(end.getX() - start.getX(), 2) + Math.pow(end.getY() - start.getY(), 2));
    }

    // getter methods
    public Point getStart() { return start; }
    public Point getEnd() { return end; }
    public double getLength() { return length; }
}

3. 长方形类 (Rectangle)：

public class Rectangle {
    private Point topLeft;
    private Point bottomRight;
    private double width;
    private double height;

    public Rectangle(Point topLeft, Point bottomRight) {
        this.topLeft = topLeft;
        this.bottomRight = bottomRight;
        this.width = bottomRight.getX() - topLeft.getX();
        this.height = bottomRight.getY() - topLeft.getY();
        checkValidity();
    }

    // 检查长宽是否为正数
    private void checkValidity() {
        if (width <= 0 || height <= 0) {
            throw new IllegalArgumentException("Invalid rectangle dimensions");
        }
    }

    // getters and setters for width and height
    // ... (类似 Point 类的方法)
}

// 判断有效性通常在构造函数内部完成，如上面所示

4. 测试示例：

public static void main(String[] args) {
    Point p1 = new Point(1, 2);
    Point p2 = new Point(4, 6);

    LineSegment line = new LineSegment(p1, p2);
    System.out.println("Line segment from (" + p1.getX() + ", " + p1.getY() + ") to (" + p2.getX() + ", " + p2.getY() + ") has length: " + line.getLength());

    Rectangle rect = new Rectangle(new Point(0, 0), new Point(5, 10));
    System.out.println("Rectangle area: " + rect.getArea());
    System.out.println("Rectangle perimeter: " + rect.getCircumference());
}