Running Java Applications

Running Java App

In order to run your Java Applications you must first compile all of the source code into it's .class byte code file respectively. Then in order to run your code you have to specify the fully qualified name for the class to the java command.

When you are running the code, the current working directory doesn't really matter as you can change the search path using the -cp or --classpath option

Fully qualified name

The fully qualified name is the class under the corresponding package. A package is just a list of related files, think of it as a folder. When you write a Java class say under the src/com/yy directory call it Circle.java, then Circle.class's fully qualified name is com.yy.Circle after you have compiled it into byte code.

src is not part of the package name, src itself is not a package, but inside the src directory all of it is considered to be java packages, and they have their respectively qualified name starting from inside src.

How does java run the program?

java will take a fully qualified name, not a directory to the .class file! Keep in mind!

When you use java along with a fully qualified name it is going to do the following:

1. Search for the compiled version of the fully qualified named class
2. Load the class
3. Check that the class has a main method
4. Call that method and passing it the command line arguments if any

Why Java cannot find the class

Remember that fully qualified name is not a file path! It is the name to refer to that class underneath src.

If your .class that you want to run is under the fully qualified name com.yy.Circle then you must specify that as the fully qualified name, not Circle or circle.

Wrong classpath

Now, when you execute java and provide a fully qualified name, it is going to search for that .class in couple of places. If you set the CLASSPATH environment variable then it is going to search through that list of directory to find .class. If not then it is going to check if you have provided any -cp/-classpath argument to the java command and check through those. If you didn't specify it then the current directory is going to be used as the classpath.

• To make it clear say you are executing com.acme.example.Foon, the Foon.class class
  
• The full file path to the Foon.class is /usr/local/acme/classes/com/acme/example/Foon.class
• Your current working directory is /usr/local/acme/classes/com/acme/example/

Then in order to run Foon.class you have to run

java -cp /usr/local/acme/classes com.acme.example.Foon

Now Java is able to find the correct Foon.class file by following the classpath you have specified, and looking it under /usr/local/acme/classes/com/acme/example directory to find that Foon.class file correctly.

Basically you have to provide the source directory path to the .class that you have compiled. That's all. It will follow the fully qualified name underneath classes directory to find com folder, then follow acme folder then example folder and finally find the correctly file.

It is as if it cd to the classpath you have provided, then try to locate that file by following your qualified name like a folder traversal.

How do I import classes I wrote in same package?

You do not need to import it, you can just use it directly because they exist in the same package.

Wait I wrote a class directly under src, how do I import it?

You cannot directly import classes you have written directly under src folder because it technically does not have a package name. However, it is able to be exposed to outside.