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Week 16

The most important tasks to attempt this week are:

  1. The non-daggered questions on Generics in worksheet 3
  2. Task 2 on the visitor pattern

Task 1: Worksheet

For this week, please download Worksheet 3

These worksheets are not credit bearing. You are encouraged to work in your with your partner. We find that students who complete these worksheets typically get higher marks in the summative coursework at the end.

Solutions will be released "later".

Task 2: Visiting Houses

Past students often found the visitor pattern to be the trickiest thing to understand and use in their summative coursework, so this is an opportunity to get it all figured out in advance. It can be confusing, because it introduces two levels of indirection with double dynamic dispatch, so it is well worth using Intellij's debugger to step through the code and make sure you understand what's happening.

Understanding the visitor pattern

Though not its only application, we will primarily use the visitor pattern to look past the declared type we have for an object and determine its concrete type. To use House unknownHouse = new BrickHouse(); as an example, House is the declared type, ie. the type of the unknownHouse reference, and BrickHouse is the concrete type of the actual object unknownHouse refers to.

To introduce the different components of the visitor pattern by way of analogy: Imagine you're looking for a house to rent and you see a listing for "a House", with few specific details except an address. How can you find out what kind of House it is if the website doesn't tell you? One way is to visit the House yourself. You call to ask the House if you can go as a Visitor and they accept. Once accepted as a Visitor, you visit the House. Now you can see exactly what kind of House it is (be it a StrawHouse, a StickHouse, or a BrickHouse) and you can return from your visit with different information depending on which particular type of House it is.

For example, let's say you want to rate the sturdiness of the House out of 10, based on what particular kind of House it is. Here's how you could represent yourself as a Visitor in code:

public class StudentVisitor implements House.VisitorInt {
  public Integer visit(StrawHouse house) { return 1; }
  public Integer visit(StickHouse house) { return 5; }
  public Integer visit(BrickHouse house) { return 10; }
}

And here's how you could visit the house:

House unknownHouse = new BrickHouse();
StudentVisitor rian = new StudentVisitor();
Integer sturdinessRating = unknownHouse.accept(rian);
System.out.println(sturdinessRating); // Prints 10

Tasks

Download the skeleton code here

  1. Complete the ChartedSurveyor class which implements House.VisitorInt. It should return an integer representing the estimated rent per calendar month for a house. Which specific values should be returned is not important, but the BrickHouse should cost more than the StickHouse, which should in turn cost more than the StrawHouse. Use this ChartedSurveyor visitor to implement public static Integer estimateRentPCM(House house) in the OnceUponATime class

  2. Implement public static Integer estimateHeatingBillPCM(House house), which instead estimates the heating bill (you're free to make your own estimations). You should do this by using the visitor pattern with a class that implements the more generic House.Visitor<T> instead of House.VisitorInt. Implement the class in the following ways and reflect on the advantages/disadvantages of each:

    1. Define the class in its own separate file
    2. Define the class within the OnceUponATime class, ie. define it as a named inner class
    3. Implement the visitor as an anonymous inner class

  3. Implement public static String letMeComeIn(House house), which returns a string telling the story of what happens when the Big Bad Wolf visits the house. The exact string text is left up to your own imagination, but your implementation must:

    • Include the name of the LittlePig occupying the house in the returned string
    • Include whether the Big Bad Wolf manages to blow the house down
    • For the brick house, the story should be different depending on whether the pot is boiling or not
    • If the Big Bad Wolf can get to the LittlePig, he should eat them (setting their hp to 0)