定义一个名为Triangle的三角形类，使其继承Shape抽象类，覆盖Shape类中的抽象方法

方法如下：
public class Test {
public static void main(String[] args) {
Circle circle = new Circle();
circle.setd(4);
System.out.println("圆的面积是" + circle.doublearea());
}

}
//抽象类
public abstract class Shape {
abstract double doublearea();
}
//圆类
public class Circle extends Shape {
public double r;

double doublearea() {
return 3.14 * r * r;
}

void setd(double R) {
r = R;
}
}

创建各个类（三角形）
Test2.java
package zh;
public class Test2 {
public static void main(String[] args) {
Triangle t=new Triangle();
t.a=3;
t.b=4;
t.c=5;
System.out.println("周长:"+t.perimeter());
System.out.println("面积:"+t.area());
}
}
Shape.java
package zh;
public abstract class Shape {
abstract double perimeter();
abstract double area();
}
Circle.java
package zh;
public class Triangle extends Shape {
double a,b,c,p,s;
@Override
double perimeter() {
p=(a+b+c)/2;
return p;
}

@Override
double area() {
s=Math.sqrt(p*(p-a)*(p-b)*(p-c));
return s;
}

}