问题补充：

补充：class Trangle{double sideA,sideB,sideC,area,length;boolean boo;public Trangle(double a,double b,double c){sideA=a;sideB=b;sideC=c;if(a+b>c && a+c>b && c+b>a){boo=true;}else{boo=false;}}double getLength(){length=sideA+sideB+sideC;return length;}public double getArea(){if(boo){double p=(sideA+sideB+sideC)/2.0;area=Math.sqrt(p*(p-sideA)*(p-sideB)*(p-sideC));return area;}else{System.out.println("不是一个三角形，不能计算面积");return 0;}}public void setABC(double a,double b,double c){sideA=a;sideB=b;sideC=c;if(a+b>c && a+c>b && c+b>a){boo=true;}else{boo=false;}}}class Lader{double above,bottom,height,area;Lader(double a,double b,double h){above=a;bottom=b;height=h;}double getArea(){area=(above+bottom)*height/2.0;return area;}}class Circle{double radius,area;Circle(double r){radius=r;}double getArea(){area=Math.PI*radius*radius;return area;}double getLength(){returm=2.0*Math.PI*radius;}void setRadius(double newRadius){radius=newRadius;}double getRadius(){return radius;}}class AreaAndLength{public static void main(String args[]){double length,area;Circle circle=null;Trangle trangle;Lader lader;circle=new Circle(9);trangle=new Trangle(4,5,6);lader=new Lader(7,8,9);length=circle.getLength();System.out.println("圆的周长："+length);area=circle.getArea();System.out.println("圆的面积："+area);length=trangle.getLength();System.out.println("三角形的周长："+length);area=trangle.getArea();System.out.println("三角形的面积："+area);area=lader.getArea();System.out.println("梯形的面积："+area);trangle.setABC(12,34,1);area=trangle.getArea();System.out.println("三角形的面积："+area);length=trangle.getLength();System.out.println("三角形的周长："+length);}}