Language elements: Interfaces

class Horse {
  ...
}

class Unicorn extends Horse {
  ...
}

Element of pure design. 

interface Calculator {
  int add(int n, int m);
}

So we put together this example:

interface Calculator {
  int add(int n, int m);
}

class One implements Calculator {
  public int add(int n, int m) {
    return n + m; 
  }
  public static void main(String[] args) {
    Calculator a = new One(); 
    System.out.println(a.add(5, 3)); 
  }
}

class Two implements Calculator {
  public int add(int n, int m) {
    if (n == 0) 
      return m;
    else 
      return this.add(n-1, m+1); 
  } 
  public static void main(String[] args) {
    Calculator a = new Two(); 
    System.out.println(a.add(5, 3)); 
  }
}

Then we have this example:

import java.util.*;

class Circle extends Shape {
  String area() {
    return "As a circle I'd multiply pi with the square of ...";   
  }   
}

class Rectangle extends Shape {
  String area() {
    return "As a rectangle I'd multiply the length of ...";   
  } 
}

class Triangle extends Shape {
  String area() {
    return "As a triangle I'd apply Heron's formula and ...";   
  } 
}

class Shape {
  String area() {
    return "As a shape it would wrong for me to say I can do this..."; 
  } 
} 

class One {
  public static void main(String[] args) {
    int[] nums; 
    nums = new int[3]; 
    nums[0] = -1;
    nums[1] = 6;
    nums[nums.length - 1] = 12;
    System.out.println (Arrays.toString(nums));

    Circle    a = new Circle(); 
    Rectangle b = new Rectangle(); 
    Triangle  c = new Triangle(); 

    Shape[] stuff; 
    stuff = new Shape[3];    
    stuff[0] = a;
    stuff[1] = b;
    stuff[2] = c;
    for (int i = 0; i < stuff.length; i++)
      System.out.println(stuff[i].area());
  } 
}

Which could be modified as follows: 

import java.util.*;

class Circle implements Shape {
  public String area() {
    return "As a circle I'd multiply pi with the square of ...";   
  }   
}

class Rectangle implements Shape {
  public String area() {
    return "As a rectangle I'd multiply the length of ...";   
  } 
}

class Triangle implements Shape {
  public String area() {
    return "As a triangle I'd apply Heron's formula and ...";   
  } 
}

interface Shape {
  String area();
} 

class One {
  public static void main(String[] args) {
    int[] nums; 
    nums = new int[3]; 
    nums[0] = -1;
    nums[1] = 6;
    nums[nums.length - 1] = 12;
    System.out.println (Arrays.toString(nums));

    Circle    a = new Circle(); 
    Rectangle b = new Rectangle(); 
    Triangle  c = new Triangle(); 

    Shape[] stuff; 
    stuff = new Shape[3];    
    stuff[0] = a;
    stuff[1] = b;
    stuff[2] = c;
    for (int i = 0; i < stuff.length; i++)
      System.out.println(stuff[i].area());
  } 
}

We could also use abstract classes for a compromise between
these two alternatives. Next time: ArrayLists, GUIs and Tomcat.