.  定义矩形类Rectangle，含长、宽属性，静态属性ID；  定义两个构造方法，一个默认，一个能初始化长宽属性；  定义求面积、求周长方法，能返回计算结果；  定义长宽属性访问方法和设置方法（称为JavaBean）；  定义toString()方法，返回对象信息；  定义静态方法，获取静态属性ID；  定义静态初始化块，设置ID属性；  主类main方法中定义不同类对象，应用所有方法。

好的，以下是矩形类Rectangle的代码实现：

public class Rectangle {
    private double length;
    private double width;
    private static int ID;

    public Rectangle() {
        this.length = 0;
        this.width = 0;
    }

    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;
    }

    public double getArea() {
        return length * width;
    }

    public double getPerimeter() {
        return 2 * (length + width);
    }

    public static int getID() {
        return ID;
    }

    static {
        ID = 1000;
    }

    @Override
    public String toString() {
        return "Rectangle [length=" + length + ", width=" + width + "]";
    }
}

在主类中可以应用所有方法：

public class Main {
    public static void main(String[] args) {
        Rectangle r1 = new Rectangle();
        Rectangle r2 = new Rectangle(3, 4);

        r1.setLength(5);
        r1.setWidth(6);

        System.out.println("Rectangle r1's area is: " + r1.getArea());
        System.out.println("Rectangle r1's perimeter is: " + r1.getPerimeter());
        System.out.println("Rectangle r1's ID is: " + Rectangle.getID());
        System.out.println("Rectangle r1's info is: " + r1.toString());

        System.out.println("Rectangle r2's area is: " + r2.getArea());
        System.out.println("Rectangle r2's perimeter is: " + r2.getPerimeter());
        System.out.println("Rectangle r2's ID is: " + Rectangle.getID());
        System.out.println("Rectangle r2's info is: " + r2.toString());
    }
}

输出结果如下：

Rectangle r1's area is: 30.0
Rectangle r1's perimeter is: 22.0
Rectangle r1's ID is: 1000
Rectangle r1's info is: Rectangle [length=5.0, width=6.0]
Rectangle r2's area is: 12.0
Rectangle r2's perimeter is: 14.0
Rectangle r2's ID is: 1000
Rectangle r2's info is: Rectangle [length=3.0, width=4.0]