（1）定义矩形类Rectangle，
* - 声明实例变量长和宽，全部私有化private，
* - 提供相应的get/set方法，如果set方法的参数值<=0，则提示矩形的长和宽必须是正数
* - 声明public double area()，返回矩形面积
* - 声明public double perimeter()：返回矩形的周长
* - 声明public String getInfo()：返回矩形的长、宽、面积、周长信息
（2）测试类的main中创建一个可以装3个矩形对象的数组，
* - 并调用set方法为对象的属性赋值，依次长是8,7,6，宽是2,3,4
* - 遍历输出矩形对象数组信息
* - 按照矩形对象的length属性值从小到大排序后，遍历输出矩形对象数组
* - 按照矩形对象的面积从小到大排序后，遍历输出矩形对象数组
public class Rectangle {private double length;private double height;public double getLength() {return length;}public void setLength(double length) {if (length <= 0) {System.out.println("矩形的长必须是正数！");return;}this.length = length;}public double getHeight() {return height;}public void setHeight(double height) {if (height <= 0) {System.out.println("矩形的宽必须是正数！");return;}this.height = height;}//矩形面积//可以不传递参数//用的是属性public double area() {return length * height;}//矩形周长public double perimeter() {return (length + height) * 2;}//矩形信息public String getInfo() {return "矩形的长是:" + length + "m," + "宽是:" + height + "m," + "面积是:" + area() + ",m" + "周长是:" + perimeter() + ",m";}
}
public class TestRectangle {public static void main(String[] args) {Rectangle[] rec = new Rectangle[3];//1.创建矩形数组rec，矩形数组中没有矩形对象，所以出现空指针异常//NullPointerException,解决方式如下：//2.创建矩形数组中的矩形对象：for (int i = 0; i < rec.length; i++) {rec[i] = new Rectangle();}rec[0].setLength(8);rec[0].setHeight(2);rec[1].setLength(7);rec[1].setHeight(3);rec[2].setLength(6);rec[2].setHeight(4);//2.遍历输出矩形对象数组信息for (int i = 0; i < rec.length; i++) {System.out.println(rec[i].getInfo());}//按照矩形对象的length属性值从小到大排序后，遍历输出矩形对象数组//3.按照矩形对象的length属性值从小到大排序后，遍历输出矩形对象数组//冒泡排序for (int i = 0; i < rec.length - 1; i++) {double rel = 0;for (int j = 0; j < rec.length - i - 1; j++) {if (rec[j].getLength() > rec[j + 1].getLength()) {rel = rec[j].getLength();rec[j].setLength(rec[j + 1].getLength());rec[j + 1].setLength(rel);}}}for (int i = 0; i < rec.length; i++) {System.out.println(rec[i].getInfo());}//4.按照矩形对象的面积从小到大排序后，遍历输出矩形对象数组for (int i = 0; i < rec.length - 1; i++) {for (int j = 0; j < rec.length - i - 1; j++) {if (rec[j].area() > rec[j + 1].area()) {Rectangle rel1 = rec[j];rec[j] = rec[j + 1];rec[j + 1] = rel1;}}}for (int i = 0; i < rec.length; i++) {System.out.println(rec[i].getInfo());}}
}
