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 ofIntList.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 ofEmptyIntList.java
,ConstIntList.java
, andIntListVisitor.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 makeTestEmptyIntList.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 oflab10
. 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 foldersrc
(and be sure the check the Open folder recursively checkbox). All the java files infunlist
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 theGetLength
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 thetest
class.- Add appropriate packaging declaration so that it compiles.
- Incrementally add test code and incrementally compile the test file.
- 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 infunlist.visitor
. Write stub code only and make sure that everything compiles.- Now write a JUnit test class called
TestToString
to testToString
. Package it in thetest
subdirectory as done in the above step 5. Make sure it compiles. - Incrementally write ToString and incrementally test it until all the test code pass.
- Now write a JUnit test class called
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