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1. Create a subdirectory called lab10 for all the files of this lab. Create and save a separate file for each of the following classes and interface.

Code Block

/**
 * Abstract list structure.
 */
public abstract class IntList {
    public abstract Object accept(IntListVisitor v);
    public ConsIntList cons(int n) {
        return new ConsIntList(n, this);
    }
}
 
/**
 * Concrete empty list structure containing nothing.
 */
public class EmptyIntList extends IntList {
    public static final EmptyIntList ONLY = new EmptyIntList();
    private EmptyIntList() {
    }
 
    public Object accept(IntListVisitor v) {
        return v.forEmptyIntList(this);
    }
}
 
/**
 * Concrete non-empty list structure containing an int, called first,
 * and a rest, which is a list structure.
 */
public class ConsIntList extends IntList {
    private int first;
    private IntList rest;
 
/* NOTE: Programmer must write constructor code and gettors code in full Java.
 */
    public ConsIntList(int f, IntList r) {
        first = f;
        rest = r;
    }
 
    public int first() {
        return first;
    }
   
    public IntList rest() {
        return rest;
    }
 
    public Object accept(IntListVisitor v) {
        return v.forConsIntList(this);
    }
}
 
/**
 * Abstract operation on IntList.
 */
public interface IntListVisitor {
    public Object forEmptyIntList(EmptyIntList host);
    public Object forConsIntList(ConsIntList host);
}

2. Add the declaration package funList ; to the top of IntList.java . Compile it using DrJava Tools/Compile Current document. You should get an error message saying that you are in the wrong package. Close the file for now.

You need to create a subdirectory called funList and move IntList.java into it. The full class name for IntList is now funlist.IntList.

Reopen the file in the funList subdirectory. Now compile again. You should get no error error messages saying it can't find class IntListVisitor and class ConsIntList this time. You will need to package all the other classes/intefaces and move them into appropriate subdirectories.

3. Add the package funList ; declaration to the top of EmptyIntList.java , ConstIntList.java , and IntListVisitor.java , and move them into the funList subdirectory. You should be able to compile each file individually. Try it.

Note: if you use the command window to compile with the command javac, you should always compile from your project's main directory. If you compile from within a package subdirectory, it doesn't find all the supporting definitions.

We can't run anything yet, because that's just a piece of the whole program.

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4. Create and save a JUnit test class called TestEmptyIntList . Do not make TestEmptyIntList.java part of the package. TestEmptyIntList.java does not have a package name, and is thus said to be in the no-name (or default) package. Save TestEmptyList.java in lab10 subdirectory (right above funlist).

Add code to test the accept method of EmptyIntList . What can we do here?

Also, remove the public access from the EmptyIntList class. By default, a class is "package-private", i.e., it is known within the package, but not from outside. If you try to compile TestEmptyIntList.java now, you will get an error message. Try it to see what happens.

You need to add the statement import funList.* ; to the top of TestEmptyIntList.java to indicate to the compiler that you are using all the public classes in that package. Try to compile it again. Is everything OK?

Now, remove the public access from the EmptyIntList class. By default, a class is "package-private", i.e., it is known within the package, but not from outside. Try to compile again. You should see a few error messages saying that you can't use EmptyIntList.java because it is not public. This is because the TestEmptyIntList class is not part of the funList package. One way to resolve this problem by making TestEmptyIntList part of the funList package. A class of a package can access all the classes (public or "package-private") in the package. However this is not a good solution in general because a client may be using many classes from different packages, but no class can be part of more than one package. For now, just make EmptyIntList.java public again, and recompile TestEmptyIntList.java . You should get no error. Try to run Test_List.java now by click the Test button in DrJava.

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2. In !DrJava, use the menu Project/New to create a DrJava project, save it as listVisitorProj ListVisProj inside of lab10 . A dialog window will popup asking for the Project Root, Build Directory and Working Directory, etc.

Set the project root directory lab10, the build directory to lab10/ bin and working directory to lab10/ src . Click OK and now we have an empty project file called {{ListVisProj.drjava }} saved in xml format. Take a look at the subdirectory lab10 to see what's there.

3. To add the whole funlist package to the project, use the File/Open Folder and select the folder src (and be sure the check the Open folder recursively checkbox). All the java files in funlist should be displayed. Save the project. Compile the project by clicking on the Compile Project button. Everything should compile. Check the subdirectory bin to see all the class files generated by the compiler.

Now let's write a list visitor to compute the length of a list and add it to the project.

4. Here is the list visitor code. It is not quite compilable on purpose.

Code Block

/**
 * Computes the length of the host list using a tail-recursive
 * helper visitor.
 */
public class GetLength implements IntListVisitor {
    static GetLength ONLY = new GetLength();
    private GetLength() {
    }
   
    /**
     * @return an Integer
     */
    Object forEmptyIntList(EmptyIntList host) {
        return 0;
    }
   
  /**
     * @return an Integer
     */
    Object forConsIntList(ConsIntList host) {
        return host.rest().accept(new GetLengthHelp(1));
    }
}
 
class GetLengthHelp implements IntListVisitor {
    int acc;
 
    // need constructor
   
    Object forEmptyIntList(EmptyIntList host) {
        return acc ;
    }

   Object forConsIntList(ConsIntList host) {
        return host.rest().accept(new GetLengthHelp(acc + 1));
    }
}

Save it in a subdirectory of funlist called visitor. The full class name for GetLength and GetLengthHelp are now funlist.visitor.GetLength and funlist.visitor.GetLengthHelp, respectively. Note that GetLength is public while GetLengthHelp is package private. Now try to compile it! You will see a few error messages. Fix the errors until everything compiles.

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