Class 04: Polymorphism (2)

Held: Friday, 3 September 2004

Summary: Today we continue our investigation of the candy problem, which builds upon our understanding of polymorphism. Along the way, we'll learn some cool Java tricks and at least one important design pattern.

Related Pages:

• EBoard.
• The candy package.
• Using Packages in Java.

Overview:

• Using Java packages.
• The Candy Machine Problem, Revisited.
• Solution one: Java Introspection.
• Solution two: An Important Design Pattern.
• Improving solution two: Anonymous Inner Classes.

Bigger Machines, Revisited

• Remember, we're trying to write a candy machine that can build the same kind of candy machine again and again.
• We want to tell it how to build that machine!

Using Java Introspection

• We can take advantage of Java's introspection methods and classes.
• Pass the appropriate kind of machine (e.g., GumballMachine).
• Get the class of that machine with getClass.
• Get the constructor of that class with getConstructor.
• Call the constructor of that class.
• Moral: We need to learn Java well.

Factory

• Unfortunately, that solution is overly complicated and won't handle machines that have non-null constructors (like a doubled chicklet machine).
• We can recognize this as an instance of the Abstract Factory problem and build a CandyMachineFactory.
• Moral: We need to learn design patterns.
• Side note: This was so obviously a factory problem that when Daren was talking about it with me, he said, So, what did you do with the factory problem?

Anonymous Inner Classes

• Factories are nice, but they don't completely solve the doubled chicklet machine problem (except by forcing us to create a DoubleChickletMachine class).
• Java comes to the rescue with the cool anonymous inner class technique.
• Such classes are anonymous: You can create them without naming them.
• Such classes are inner: You can create them within other classes (in the middle of code).
• Normal mechanism:

  new Constructor() {
    NewAndOverridenMethods
  } 
  

• For example,

  PrintWriter pen = new PrintWriter(System.out, true) {
    public void println(String whatever) {
      super.println("*** " + whatever + " ***");
    }
  };
  

• For our particular case, something like

    CandyMachineTester cmt = new CandyMachineTester(
                              new BiggerCandyMachine(
  			      new CandyMachineFactory() {
  			        public CandyMachine build()
  				{
  				  return new DoubledCandyMachine(
  				           new ChickletMachine());
  				}
  			      }));