2007 Felleisen, Proulx, et. al.

8  Designing Tests for State Change Abstracting with Function Objects Introducing Type Parameters

Goals

In the first part of this lab you will learn how to correctly design tests for the methods that change the state of an object.

In the second part of the lab you will learn to abstract over the functional behavior.

In the third part you will get the first introduction to the abstraction over the type of data using type parameters.

8.1  Designing Tests for State Change

For this part download the files in Lab8Sp2007-s1.zip. The folder contains the files ImageFile.java, ISame.java, ISelect.java, SmallImageFile.java, AList.java, MTList.java, ConsList.java, and Examples.java, as well as TestHarness.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 Lab8Part1 and import into it all of the given files. Also import jpt.jar from the previous lab.

• Design the method crop that changes the dimensions of an ImageFile object to the given width and height. The Examples class contains comments on what needs to be done to design the tests. Follow the outline given by the comments to design the needed tests.

• Design the method changeName that allows us to change the name field of an ImageFile object. Design the tests.

8.2  Abstracting with Function Objects

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. An instance of this class can then be used as an argument to a method that deals with ImageFiles.

1. In the Examples class design the 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. Design the class NameShorterThan4 that implements the ISelect interface with a method that determines whether the name in the given ImageFile object is shorter than 4.

   Make sure in the class Examples 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 allSuch that that determines whether all items in a list satisfy the predicate defined by the select method of a given instance of the type ISelect. In the Examples class test this method by abstracting over the method allSmallerThan40000 and the method allNamesShorterThan4.

6. For the first etude, at home, follow the same steps as above to design the method anySuch 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 ISelect.

8.3  Abstracting over the Datatype: Generics

For this part download the files in Lab8Sp2007-s2.zip. The folder contains the files ImageFile.java, ISame.java, ISelect.java, SmallImageFile.java, AList.java, MTList.java, ConsList.java, and Examples.java, as well as TestHarness.java.

Create a new Project Lab8Part2 and import into it all the given files. Again, use jpt.jar from the previous lab.

Look at the files. The interface ISelect now includes a type parameter T:

public interface ISelect<T> {
  /* Return true if this Object of the type T should be selected */
  public boolean select(T o);
}

That means that the implementing class can decide what type of data should be used as the argument to the select method. This greatly simplifies the class SmallImageFile:

/* Select image files smaller than 40000 */
public class SmallImageFile implements ISelect<ImageFile> {
  
  /* Return true if the size of the given ImageFile is smaller than 40000 */
  public boolean select(ImageFile o) {
    return o.height * o.width < 40000;
  }
}

Similarly, the ISame interface also provides a type parameter for its argument:

interface ISame<T>{
  // is this object the same as the given one?  
  boolean same(T that); 
} 

The implementation of the same method in the class ImageFile is then greatly simplified:

  // is this imagefile the same as the given one?
  public boolean same(ImageFile that){
    return this.name.equals(that.name) &&
           this.width == that.width &&
           this.height == that.height &&
           this.kind.equals(that.kind);
  }

Look at the class definition for the class ImageFile to see the use of the type parameter there.

Moreover, the classes that represent a list of arbitrary items can now specify the type of items that can be included in the list construction. However, because the class implements a parametrized interface ISame, the use of type parameters is quite complicated. For now, you can just use the implementation and do not have to follow every detail of the use of the type parametrs there.

Re-do all of the problems from the previous part, but using the type parameters

1. In the Examples class design the tests for the class SmallImageFile, just as you did before.

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. Design the class NameShorterThan4 that implements the ISelect<ImageFile> interface with a method that determines whether the name in the given ImageFile object is shorter than 4.

   Make sure in the class Examples 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 allSuch<T> that that determines whether all items in a list (of items of the type T) satisfy the predicate defined by the select method of a given instance of the type ISelect<T>. In the Examples class test this method by abstracting over the method allSmallerThan40000 and the method allNamesShorterThan4.

6. For the second etude, at home, follow the same steps as above to design the method anySuch 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 ISelect.