©2010 Felleisen, Proulx, et. al.

5  Abstracting with Function Objects


In this lab you will learn how to abstract over the functional behavior.

5.1  Abstracting with Function Objects

Download the files in Lab5.zip. The folder contains the files ImageFile.java, ISelectImageFile.java, SmallImageFile.java, IListImageFile.java, MTListImageFile.java, ConsListImageFile.java, and ExamplesImageFile.java.

Starting with partially defined classes and examples will give you the opportunity to focus on the new material and eliminate typing in what you already know. However, make sure you understand how the class is defined, what does the data represent, and how the examples were constructed.

Create a new Project Lab5-su10 and import into it all of the given files. Also import tester.jar from the previous lab.

We will now practice the use of function objects. The only purpose for defining the class SmallImageFile is to implement one method that determines whether the given ImageFile object has the desired property (a predicate method). An instance of this class can then be used as an argument to a method that deals with ImageFiles.

  1. Start with defining in the ExamplesImageFile class the missing tests for the class SmallImageFile.

  2. Design the method allSmallerThan40000 that determines whether all items in a list are smaller that 40000 pixels. The method should take an instance of the class SmallImageFile as an argument.

  3. We now want to determine whether the name in the given ImageFile object is shorter than 4. Design the class NameShorterThan4 that implements the ISelectImageFile interface with an appropriate predicate method.

    Make sure in the class ExamplesImageFile you define an instance of this class and test the method.

  4. Design the method allNamesShorterThan4 that determines whether all items in a list have a name that is shorter than 4 characters. The method should take an instance of the class NameShorterThan4 as an argument.

  5. Design the method allSuchImageFile that that determines whether all items in a list satisfy the predicate defined by the select method of a given instance of the type ISelectImageFile. In the
    ExamplesImageFile class test this method by abstracting over the method allSmallerThan40000 and the method

  6. Design the class GivenKind that implements the
    ISelectImageFile interface with a method that produces true for all ImageFiles that are of the given kind. The desired kind is given as a parameter to the constructor, and so is specified when a new instance of the class GivenKind is created.

    Hint: Add a field to represent the desired kind to the class

  7. In the ExamplesImageFile class use the method allSuch and the class GivenKind to determine whether all files in a list are jpg files. This should be written as a test case for the method

    Do it again, but now ask about the giff files.

  8. If you have some time left, design the method filterImageFile that produces a list of all ImageFiles that satisfy the
    ISelectImageFile predicate. Test it with as many of your predicates as you can.

  9. Follow the same steps as above to design the method
    anySuchImageFile that that determines whether there is an item a list that satisfies the predicate defined by the select method of a given instance of the type ISelectImageFile.

  10. Finish the work at home and save it in your portfolio.

    Food for thought: Think how this program would be different if we have instead worked with lists of Books, or lists of Shapes.

5.2  Understanding Equality

Note: This material is covered in pages 321 - 330 in the textbook. Read it carefully.

  1. Create a Java Project and add following files to it’s source directory.

    • Account.java

    • Checking.java

    • Savings.java

    • Credit.java

    • Bank.java

    • ExamplesBankAccts.java

    We now want to define a method that will determine whether an account is the same as the given account. We may need such method to find the desired account in a list of accounts.

    Of course, now that we have the abstract class it would be easy to compare just account number and the name on the account. But, maybe, we want to make sure that the customer’s data match the data we have on file exactly - including the balances, the interest rates, and the minimum balances - as applicable.

    The design of the method same is similar to the technique described in the textbook. The relevant classes and examples that were handed out in the class can be found in the file Coffee.java. You may want to look at the code there as you work through this problem.

  2. Begin by designing the method same for the abstract class Account.

  3. Make examples that compare all kinds of accounts - both of the same kind and of the different kinds. For the accounts of the same kind you need both the expected true answer and the expected false answer. Comparing any checking account with another savings account must produce false.

  4. Now that you have sufficient examples, follow with the design of the same method in one of the concrete account classes (for example the Checking class). Write the template and think of what data and methods are available to us.

  5. You will need a helper method that determines whether the given account is a Checking account. So, design the method isChecking that determines whether this account is a checking account. You need to design this method for the whole class hierarchy - the abstract class Account and all subclasses. Do the same to define the methods isSavings and isCredit.

  6. We are not done. This helps with the first part of the same method. We need another helper method that tells Java that our account is of the specific type. Here is the method header and purpose for the checking account case:

      // produce a checking account from this account
      Checking toChecking();

    In the class Checking the body will be just

      // produce a checking account from this account
      Checking toChecking(){
        return this; }

    Of course, we cannot convert other accounts into checking account, and so the method should throw a RuntimeException with the appropriate message. We need the same kind of method for every class that extends the Account class.

  7. Finally, we can define the body of the same method in the class Checking:

    // produce a checking account from this account
    boolean same(Account that){
      if (that.isChecking()){
        return that.toChecking().sameChecking(this);
      } else {
        return false;

    That means, we still need the method sameChecking but this only needs to be defined within the Checking class and can be defined with a private visibility.

    Finish this - with appropriate test cases.

  8. Finish designing the same method for the other two account classes.

Alternative approaches - bad and good

Note 1 - Incorrect alternative:

The method above can be written with two Java language features, the instanceof operator and casting as follows:

// produce a checking account from this account
boolean same(Account that){
  if (that instanceof Checking){
    return ((Checking)that).sameChecking(this);
  } else {
    return false;

However, this version is problematic and not safe.

If the class PremiumChecking extends Checking, then any object constructed with a PremiumChecking constructor will be an instance of Checking and the trouble that can result is illustrated in the example TestSame.java. You can make a simple project and run the examples, but we include the output from the tester for illustration.

Note 2 - A correct alternative:

In the lecture we have introduced another version that also works correctly. It requires us to add a new method for each class that implements the common interface.

Lecture Notes for the lecture on equality for unions of classes show this technique for the classes that represent geometric shapes (IShape, Circle, Rect, and Combo).

Here the methods were:

  // is this shape the same as the given shape?
  boolean sameShape(IShape that);
  // is this shape the same as the given circle?
  boolean sameCircle(Circle that);
  // is this shape the same as the given rectangle?
  boolean sameRect(Rect that);
  // is this shape the same as the given circle?
  boolean sameCombo(Combo that);

Last modified: Monday, May 24th, 2010 4:27:51pm