When you do if __name__ == "__main__": you are checking to see if you are in main already.

It's different because the entire python script is considered to be "main" in the way that public static void main is considered to be "main" in java.

When you run a python file, it starts from the very top and works its way down looking for executable statements (it bypasses declarations such as function definitions and classes).

When you run a java class it looks for the main method and starts from there.

The reason for having this if __name__ == "__main__": is so that it only executes when you run the python file directly.

As you will learn soon, python files can also be considered as "modules" to be included from other python scripts. In such cases you would not want this 'main' logic to be implicitly executed.

Much like you almost never invoked a classes main when importing it as part of a larger app in java.

You could also use Py4J. There is an example on the frontpage and lots of documentation, but essentially, you just call Java methods from your python code as if they were python methods:

from py4j.java_gateway import JavaGateway
gateway = JavaGateway()                        # connect to the JVM
java_object = gateway.jvm.mypackage.MyClass()  # invoke constructor
other_object = java_object.doThat()
other_object.doThis(1,'abc')
gateway.jvm.java.lang.System.out.println('Hello World!') # call a static method

As opposed to Jython, one part of Py4J runs in the Python VM so it is always "up to date" with the latest version of Python and you can use libraries that do not run well on Jython (e.g., lxml). The other part runs in the Java VM you want to call.

The communication is done through sockets instead of JNI and Py4J has its own protocol (to optimize certain cases, to manage memory, etc.)

Disclaimer: I am the author of Py4J