• everything is an instance of something (java.lang.Object)
• class as an abstraction mechanism
• objects are defined to model the data and packaged with the methods used to act upon them (constructor, accessor, mutator, and helper)

java.lang.Object
  |
  +--java.awt.Component
        |
        +--java.awt.Container
  

• Field
  • access modifers

    public

    accessed from anywhere the class that contains it

    protected

    accessed only from within its class, classes in the same package as its class, and subclasses of its class

    private

    accessed only from within its class

    default (no access specifier)

    accessed only from within its class and classes in the same package as its class

  • existence modifiers

    static

    one exists for the entire class, rather than one for each object of its class

  • usage modifiers

    final

    cannot be changed (i.e., constant or read-only)

• Method
  • access modifers

    public

    accessed from anywhere the class that contains it

    protected

    accessed only from within its class, classes in the same package as its class, and subclasses of its class

    private

    accessed only from within its class

    default (no access specifier)

    accessed only from within its class and classes in the same package as its class

  • existence modifiers

    static

    exists to serve its class, not to serve individual objects of its class; as a consequence, static methods do not have access to instance variables, as instance variables belong to the individual objects; as a further consequence, static methods do not have access to this and super.

  • usage modifiers

    final

    cannot be overriden

    abstract

    must be overriden to be useful

• Class
  • access modifers

    public

    accessed from within any class

    default (no access specifier)

    accessed from within any class in the same package

  • usage modifiers

    final

    cannot be extended (inherited)

    abstract

    must be extended (inherited) to be useful