Laboratory: Java from the Command Line

The command javac is used to compile Java files.

a. Use javac to compile Sorter.java. What kind of file is produced?

b. Remove Sorter.class Use javac to compile ISort.java. Do you get anything in addition to ISort.class? What are the implications?

c. You can run a Java .class file with the java command and the name of a class that contains a main method. Type the following command (without the dollar sign prompt) to run our sample sorting program.

$ java ISort 6 1 2 9 6 0

Right now, the .java files are in the default package. As a good Java programmer, you know that every class should really be in an appropriate package. So let's put these files in the package username.sorting.

a. Using your favorite text editor (Emacs, vi, vi-mangled, GEdit), add the line package username.sorting; to the top of each file.

b. What happens when you try to compile ISort.java?

c. As you've discovered, the Java compiler can no longer find the other .java files. What do we do? Two steps, believe it or not. First, we need to use the appropriate directory structure. In particular, if the pacakge is username.sorting, the code is supposed to be in the directory username/sorting. Move you .java files to that directory.

d. The other important thing to do is to set up your CLASSPATH variable, which governs where Java looks for code files. Type the following

$ export CLASSPATH=/path/to/parent/directory

For example, if the Java code is in /home/username/Java/username/sorting, use /home/username/Java as the directory in the command above.

e. What do you expect to have happen if you try to run ISort? Check your answer experimentally.

f. The full name of ISort is now username.sorting.ISort. See if you can run it with that filename.

As you may recall from your work in C, we often use library files to store utility code. In Java, library files are called “jar files” or “jars”. You build a “jar” with

$ jar cf File.jar classfiles

a. Create a jar file named Sorting.jar with the utility filename (Sorter.class, StandardIntegerComparator.class, and BuiltinSorter.class).

b. Remove those three .class files.

c. What do you expect to have happen when you try to run ISort with the following command?

$ java username.sorting.ISort  10 5 1 2 4 9 1

d. Check your answer experimentally.

e. As you probably expected, Java doesn't automatically recognize the files in the .jar. However, we can solve that problem: You can add the .jar to your classpath. Try the following command:

$ export CLASSPATH=Sorting.jar:$CLASSPATH

f. Now try to run ISort again.

What do we do when we want to ship a Java program? Usually, we put all of the files, including the class with the main method, in the same .jar file. We then have to specify where to find main. We do that with a “manifest file” that looks somethign like the following.

Main-Class: ISort

a. Create a file, manifest.txt with that line.

b. Create a jar file, ISort.jar, with the following command:

$ jar cvfm ISort.jar ISort.class Sorter.class BuiltinSorter.class StandardIntegerComparator.class

c. You can run the jar file with the following command. Try it with your own set of values.

$ java -jar ISort.jar 5 4 1 2 3

a. What do you expect to have happen if you compile the unit test, which you can find in BISTest.java?

b. Check your answer experimentally.

c. As you probably discovered, the Java compiler is upset that it doesn't know where the JUnit code is. But you can tell it. The JUnit code is in /usr/share/java/junit.jar. Add that to your CLASSPATH and see if you can get the test to compile.

d. Of course, you also want to run the unit tests. But where's the main method? Somewhere in JUnit. There are a variety of ways to run unit tests. Here's one that should work.

$ java org.junit.runner.JUnitCore username.sorting.BISTest

Give it a try.

We've seen how to compile Java files, run the compiled files, put them into libraries, and test them. What's left? Generating documentation!

You can create Javadoc documentation with the javadoc command. Try creating the documentation with

$ javadoc *.java

Right now, we're doing everything in the same directory. But that clogs the directory. Really, we should have separate diretories for our main source code, our tests, our compiled files, our Javadoc, and perhaps other things.

As you've discovered, Ant provides a way to organize all of those files. Set up an appropriate directory structure and write an Ant build file to put things in the appropriate place.