2.3. Default Package¶

Prerequisite Content

This section assumes that the reader is familiar with the content of the “Executing Programs” video (Video #1 under “1301 Videos” on eLC). If you have not watched that video, please take a few minutes to do so now.

In this section, we will see how to compile a Java file using the default package as a warm-up (i.e., we will compile a class in a package-less .java file). This Java file will not be contained within a named package. In the next section, we will see how to add the file to a named package.

Remember, the src directory you created in the previous section is called the default package for source code. This directory serves as the base directory for named packages that you will create. You can think of it as the top-level directory for your package directories (more soon). The default package can also contain classes (and other types) that are not in a named package. When developing small or temporary applications, it is a convenient place for package-less .java files [2].

Note

You won’t need to use the cs1302 or hello directories (created in the previous section) in this step. You will use them in the next section.

Let’s dive in! Follow along on Odin and complete the steps below to create, compile, and execute a class in the default package. Once you are comfortable with these steps, move on to the next section to learn about named packages.

From within the cs1302-packages directory you created in the previous section, create a file in the src directory called HelloWorld.java using the following command:
```
emacs src/HelloWorld.java
```
Note the relative path used in the above command. Because we used src/HelloWorld.java, the HelloWorld.java file will be placed within the src directory (the default package for source code).
You should be presented with a blank file in the Emacs editor. Now, create a basic “Hello, World!” program in that file. The class should be called HelloWorld to match the file name. For example, the contents of the HelloWorld.java file might be:
```
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    } // main
} // HelloWorld
```
Now, we want to compile our program. We know that the source code is located in the default package for source code (src), but we want the compiled .class file to go into the default package for compiled code (bin). There are a few parts to this command but they are fairly descriptive once we break it down.

Go ahead and exit Emacs and compile the HelloWorld.java file using the command below to run the Java compiler (javac):
If you are not in the cs1302-packages directory, this command will not work¶
javac -d bin src/HelloWorld.java
Understanding the Command

In English, the command above tells the Java compiler to: “compile the HelloWorld.java file located in the src folder and place the resulting .class file into the bin folder. Note the use of relative paths in this command (bin and src/HelloWorld.java are relative to our current location.
Run the tree command from within cs1302-packages. You should see that the HelloWorld.java file is in the src folder and that the HelloWorld.class file is in the bin folder. Now, that’s organized!
Now that the class is compiled, we should note a few important things before running the program:
1. The java command requires the FQN of the class (not the name of the .class file). Since our HelloWorld class is not in a named package (we compiled to the default package), its FQN is the same as its simple class name, HelloWorld.
2. When executing the java command, Java assumes that the current working directory is the location of the default package for compiled code (where the .class files are found). Since we are in the cs1302-packages directory and not in the default package, we need to specify the default package for compiled code (bin) using a command line option called -cp for class path. As its name suggests, the class path is the path to the default package for classes.
Here is the command to run the program:
This command will not work if you aren’t in the cs1302-packages directory.¶
java -cp bin HelloWorld
Understanding the Command

In English, the command above tells Java to: “run the HelloWorld class located in the default package and, if that class is not in the present working directory, check the bin folder as well.

You can execute a Java program anywhere on the system as long as you know the fully qualified name of the class containing the main method and the location of that compiled class’s associated default package, assuming proper file permissions.

Test Yourself

Change directories into your home directory and see if you can run HelloWorld.class from that location. Note: The relative path you provide as part of the classpath -cp will have to change. What about the FQN? Write the command you used in your notes.
Test Yourself Solution (Open after attempting the question above)
Once you are in your home directory, the command to run HelloWorld will have to change. Specifically, you need to put the location of the default package for compiled code on the classpath (using a relative or absolute path). The FQN of the class does not change.
This command assumes that the cs1302-packages directory is in your home folder¶
java -cp cs1302-packages/bin HelloWorld
Important Note: bin is just the relative path to the directory where the compiled .class files will be placed. In our example, bin is a directory located inside of the present working directory. You can replace bin with any relative or absolute path to any directory that you have permission to write to. For example, if we had a directory called class_files in our present working directory, we could compile the code with -d class_files and our .class files would be placed in that directory.

PROTIP:

When you’re first starting out in a Unix environment, you might feel lost at times. Remember, it’s always important to know your present working directory (pwd) as that is the location where the commands you enter will be run. In the commands above, our pwd is cs1302-packages. Since the HelloWorld.java file is not in that directory, we had to specify the relative path src/HelloWorld.java to tell the compiler how to find that file relative to the pwd. We’re telling the compiler “first, go into the src directory, and then you’ll see the file HelloWorld.java”.
We will use the Unix tree command (or the similar find) often in this course to see all files in a directory and its subdirectories. Go ahead and execute tree from within the cs1302-packages directory. If you have followed the steps correctly up until now, you will see the following output:

The output of the tree command shows that we successfully separated the source code (src) from the compiled code (bin). If you see any tilde (~) files, those are just backup copies of older versions of your files. You can ignore those.
Let’s clean up! Delete the HelloWorld.class file that you created in the bin folder (use the rm command with a valid relative path to the file. Remember to use tab completion so you don’t have to type long filenames! In general, you don’t need to delete your .class files, but we will do so in this example because we will compile the code to a different directory in the next section.

Here is a video outlining the steps in this section if you get stuck:

Remember, source code should only be placed directly in the default package directory (src) for convenience when developing small or temporary applications or when just beginning development [2]. While types in the default package can access types in other packages, the reverse is not true. That is, types in named packages cannot access types in the default package.

In the next section, we will see how to compile our files to a named package.