There has been some code that hasn't resided in our source files. Remember all those import statements at the top of every file? What they do is pull in prewritten and precompiled code from various locations so we can use it in our programs. You can do the same thing with classes you write. However to do this you do need to be aware of several conventions and restrictions.

1. No file should contain more than one public class. This means that our Hello World, Goodbye World example is no longer valid because each of the classes was public.
2. All files should have the same name as their single public class followed by the extension .java.
3. Source code files should be stored in the same directory as their compiled .class file. This is so the Java compiler can find the appropriate definitions and interfaces for a class when the class is referred to in a different file.
4. Source code and .class files should be in a directory that's part of the $CLASSPATH environment variable.

Next save the examples from the previous exercises in a separate file called ComplexExamples.java in the same directory as Complex.java. Now compile both files and run Complex Examples.java.