定义一个矩形 Rectangle 类和 三角形 Triangle 类，都继承 Shape 类，在这两个类中分别定义 一个名为 area 的方法，实现计算该形状面积的功能（Shape 类已经定义完成，可以在 Shape.java 文件查看相应代码）。

Rectangle 类实现：

public class Rectangle extends Shape {
    private double length;
    private double width;

    public Rectangle(double length, double width) {
        this.length = length;
        this.width = width;
    }

    public double getLength() {
        return length;
    }

    public void setLength(double length) {
        this.length = length;
    }

    public double getWidth() {
        return width;
    }

    public void setWidth(double width) {
        this.width = width;
    }

    @Override
    public double area() {
        return length * width;
    }
}

Triangle 类实现：

public class Triangle extends Shape {
    private double base;
    private double height;

    public Triangle(double base, double height) {
        this.base = base;
        this.height = height;
    }

    public double getBase() {
        return base;
    }

    public void setBase(double base) {
        this.base = base;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }

    @Override
    public double area() {
        return 0.5 * base * height;
    }
}