Java Modifiers and Casting Reference

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Java Modifiers

• 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

four possible ways to mix and match superclass references and subclass reference with superclass objects and subclass objects:

1. referring to a superclass object with a superclass reference is o.k.
2. referring to a subclass object with a subclass reference is o.k.
3. referring to a subclassd object with a superclass reference is safe. Such code can only refer to superclass members. If this code refers to subclass-only member through the superclass reference, the compiler will report a syntax error. Instance methods from the superclass that are overridden by the subclass are not accessed when the superclass reference is used, unlike the case for hidden data and hidden static methods. Regardless of the superclass reference used, the subclass's copy is accessed.
4. referring to a superclass object with a subclass reference is a syntaz error.

the references of superclass must be cast in order to access the fields/methods of subclass objects

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