现有类：圆 Circle 、正方形 Square 、长方形 Rectangle 等他们有共同父类 Shape 。编码实现：1求实现：圆、正方形、长方形面积的 getAera 方法

public abstract class Shape { public abstract double getArea(); }

public class Circle extends Shape { private double radius;

public Circle(double radius) {
    this.radius = radius;
}

public double getRadius() {
    return radius;
}

public void setRadius(double radius) {
    this.radius = radius;
}

@Override
public double getArea() {
    return Math.PI * radius * radius;
}

}

public class Square extends Shape { private double side;

public Square(double side) {
    this.side = side;
}

public double getSide() {
    return side;
}

public void setSide(double side) {
    this.side = side;
}

@Override
public double getArea() {
    return side * side;
}

}

public class Rectangle extends Shape { private double length; private double width;

public Rectangle(double length, double width) {
    this.length = length;
    this.width = width;
}

public double getLength() {
    return length;
}

public void setLength(double length) {
    this.length = length;
}

public double getWidth() {
    return width;
}

public void setWidth(double width) {
    this.width = width;
}

@Override
public double getArea() {
    return length * width;
}