Some Rules in Java:

• A source code file can have only one public class and may have multiple non-public class
  
• A public class  file name must match the public class name.
• A file can have only one package statement and it must be the first line in the source code.
• A file can have multiple imports.
• Import statement is applicable to all classes in the source code file
• If a class cannot be accessed, its members (methods and instance variables) cannot be accessed either.
• A subclass can inherit a member of its super-class.A subclass can only extend one parent.
• A class access determines its members' access. If a  class cannot be access, its member's cannot be accessed either.
• 'this.' means currently executing object.
• Private members cannot be inherited.