Lab 10 Java Package; DrJava Project Utility

Java Package

A Java package is a grouping of classes similar to the notion of a directory is a grouping of files. Packages are used to help avoid name clashes and to hide particular pieces of code from the clients. A package has a name, such as utility or java.lang.util. In general, a package name is a series of strings of alphanumeric characters (starting with an alphabetic character) and separated by periods. To make a java class part of a particular package, say funList, you must add the declaration package funList; to the very top of the class source file.

Also, you will need to put the file in the directory structure that mirrors the package name. For example, the java classes that belong to the package funList should be in a directory also named funList. If you don't do this, it will still compile, but it won't run correctly.

NOTE: DrJava language levels do not support packaging. We must use Full Java to work with packages.

Exercises:

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.

/**
 * 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.

Reopen the file in the funList subdirectory. Now compile again. You should get no error this time.

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.

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.
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.

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.

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.

DrJava Project

DrJava provides a utility called Project as a way to manage large Java programs with many files and many packages. We will illustrate the use of DrJava Project by writing a few visitors for the funlist package.

In general, a project has a bunch of java files (the source code) and class files. It is a good idea to separate the java files from the compiled class files by creating a subdirectory called bin for the class files and src for the java files.

1. Inside of lab10 subdirectory, create subdirectory called bin and a subdirectory called src. Move the funlist subdirectory inside of src.

2. In DrJava, use the menu Project/New to create a DrJava project, save it as listVisitorProj 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 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.

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.

/**
 * 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. Now try to compile it! You will see a few error messages. Fix the errors until everything compiles.

5. Write a JUnit test class called TestGetLength to test the GetLength visitor written in the above. Save this test class in a subdirectory of visitor called test. You will need to create a new folder when you try to save the test class.
1. Add appropriate packaging declaration so that it compiles.
2. Incrementally add test code and incrementally compile the test file.
3. Incrementally run the test code by clicking the Test Project button (or the menu Project/Test Project).

Be sure to save the project code periodically.

6. Write a visitor called ToString to compute a Scheme-like String representation of the list. (See ListString^? visitor done in lab 09). Package it in funlist.visitor. Write stub code only and make sure that everything compiles.
1. Now write a JUnit test class called TestToString to test ToString. Package it in the test subdirectory as done in the above step 5. Make sure it compiles.
2. Incrementally write ToString^? and incrementally test it until all the test code pass.