30．编写一个Shape类，具有属性：周长和面积； 定义其子类三角形和矩形，分别具有求周长的方法。 定义主类E，在其main方法中创建三角形和矩形类的对象， 并赋给Shape类的对象a、b，使用对象a、b来测试其特性。

package zuoye8;

public abstract class Shape {

	private double zhouchang ;
	private double mianji ;

	public Shape(double zhouchang, double mianji) {
		super();
		this.zhouchang = zhouchang;
		this.mianji = mianji;

	}

	public Shape() {
		super();
	}
	//抽象一个周长方法
	 public  double zhouchang ()
	 {
		return zhouchang;}

}

　　

package zuoye8;

public class Juxing extends Shape {

	private double c;
	private double d;

	public Juxing( double c, double d) {
		super();
		this.c = c;
		this.d = d;

	}

	@Override
	public double zhouchang() {
		double zhouchang ;
		zhouchang =(c+d)*2;
		return zhouchang ;
	}

}

　　

package zuoye8;

public class Sanjaio extends Shape {
	//三角形三边属性
	private double a;
	private double b;
	private double c;

	public Sanjaio( double a, double b, double c) {
		super();
		this.a = a;
		this.b = b;
		this.c = c;
	}
	public Sanjaio() {

	}

	//周长

	 public void
	 zhouchang (double a,double b,double c)
	 {
		 if(a+b>c&&Math.abs(a-b)<c)//绝对值，保证是正整数
			{
			 double zhouchang ;
			 System.out.println(zhouchang =a+b+c);

			}
			else
			{
				System.out.println("这不是三角形");

			}

	 }

}

　　

package zuoye8;

public class E {

	public static void main(String[] args) {

		Juxing j=new Juxing(3,4);
		System.out.println("长=4，宽=3的矩形周长="+j.zhouchang());

		Sanjaio s=new Sanjaio();
		System.out.println( "边分别是3,4,5的三边形周长是：");
		s.zhouchang(3, 4, 5);

		s.zhouchang(3,1,5);

	}

}