java -version

javac -version

javap -version

Friendly advice: These should be the same version, or you might have a bad time :)

• Open a terminal. Navigate to any folder whatsoever. Make a new Empty.java file. Try to compile it with javac. See what happens.
• Make a class named Program. Try to compile it with javac.
• Rename the class to Empty. Compile it.
• Make a main method. Make it print "Hello world!". Compile it and run it.
• Make the main method print its arguments (String[] args). Make the for loop as you would make one in C or C++, and use the System.out.println method for printing to the console.

• Open a terminal, navigate to some folder. Make a new A.java file with a legitimate class A declared in it.

• Do the same for a file/class named B.

• Declare a public method in B.

• In class A, make a main method

• Instantiate B in it and call B's public method.

• Try, compile and run A. Does it run? Why? How does it know about B? What about import statements?

• Now move B up one folder to '..'

• Try compiling and running it.

• Add -cp ".." to your javac command and try again.

###Bundle them both###

• jar -c A.class
• Does this command look familiar to you? : )

• Disassemble Empty.class
• Does this look familiar to you? This is what Eclipse is doing when you open a look up a class file with no source attached.