Usage patterns for private, static, final, public, abstract keywords in java

Question

I know what all of these do except for abstract. I'm currently in the process of teaching myself java with what I consider a middle-school-level education (my highschool was in a bad neighborhood so I got shafted)...

But what exactly are the usage patterns for these keywords? When do I use what? When do I omit them? Putting 'public' in front of my classes makes every class that uses it require a new file, can I just omit that if I want to create a monolithic source file?

Every bit of information I look up, explains exactly WHAT these do, just doesn't give a clear view of when/why/where I should use them.

Thanks in advance, Anthony

Answer 1

Sources tell what do these keywords mean because when/why/where they are used follows from that. My explanations have the "when" word, for example, but they follow directly from the semantics of the keywords.

• private should be used when something is not used outside of a given class
  • for methods and fields - when they are used only within the same class
  • for classes - only on nested classes, when used in the same class
• protected should be used when
  • for methods and field - when you need to make them accessible to subclasses only
  • for classes - again only nested classes, accessible by subcalsses
• public is used when something is accessible by every other class

The above three are "visibility modifiers". They are used when you want to limit the usage of some methods/fields/classes to a group of objects, and hide them from other objects. There is another visibility modifier - the default one (when no other is present). It is used when you want your class/method/field to be accessible only to classes from the same package.

• static is used when you don't need an instance of a class (i.e. object) to use it:
  • for fields - when you want to have a global field
  • for methods - when you need utility functions that do not depend on object state
  • for nested classes - when you want to access them without an instance of the enclosing class.
• abstract when you don't want to provide implementations in the current class:
  • on methods - when subclasses have to provide the actual implementation, but you want to invoke these methods (no matter how they are implemented) in this class.
  • on classes - to denote that the class may have abstract methods.
• final - when you don't want something to change.
  • on fields, when you want to assign the value only once. It is useful when you want to pass a local variable to an inner class - you have to declare it final.
  • on classes and methods - when you don't want subclasses to be able to extend / override them.