编写一个抽象的图形类 Shape,里面有方法 getArea()计算面积以及方法 displayArea()显示面积，
编写子类矩形类和三角型类分别实现两个方法
package magic;
abstract class Shape {
double a, b;
abstract double getArea();
abstract void displayArea();
Shape(double a, double b) {
this.a = a;
this.b = b;
}
}
class Rectangle extends Shape {
Rectangle(double a, double b) {
super(a, b);
}
double getArea() {
return a * b;
};
void displayArea() {
double area = getArea();
System.out.println("矩形面积为:" + area);
}
}
class Triangle extends Shape {
Triangle(double a, double b) {
super(a, b);
}
double getArea() {
return a * b / 2;
};
void displayArea() {
double area = getArea();
System.out.println("三角形面积为:" + area);
}
