fboeller

interface Person { 
  public String getMiddleName(); 
}

fboeller

interface Shape {
  double perimeter();
  double area();
}

fboeller

class Rectangle implements Shape {
  double x, y;
  
  public double perimeter() {
    return (x + y) * 2;
  }
  
  public double area() {
    return x * y;
  }

fboeller

class Circle implements Shape {
  double radius;
  
  public double perimeter() {
    return 2 * PI * radius;
  }
  
  public double area() {
    return PI * radius * radius;
  }

fboeller

class Parallelogram implements Shape { }

fboeller

class Parallelogram implements Shape {
  
  public double perimeter() {
    return 0.;
  }
  
  public double area() {
    return 0.;
  }
}

fboeller

class Parallelogram implements Shape {
  double base, height, side;
  
  public double perimeter() {
    return 2 * (base + side);
  }
  
  public double area() {
    return base * height;
  }

fboeller

interface Shape {
  double perimeter();
  double area();
  double diameter();
}

fboeller

class Rectangle implements Shape {
  double x, y;
  
  /* ... */
  
  public double diameter() {
    return Math.sqrt(x * x + y * y);
  }
}

fboeller

class Circle implements Shape {
  double radius;
  
  /* ... */
  
  public double diameter() {
    return 2 * radius;
  }
}