More on Overriding

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More on Overriding

More on Overriding

Changing Access Levels on Overridden Methods

You can change the access level of a method when you override it, but only to make it more accessible.

  • You can't restrict access any more than it was in the base class.
  • So, for example, you could take a method that was protected in the base class and make it public.
  • For example, if a method was public in the base class, the derived class may not override it with a method that has protected, private or package access.

This avoids a logical inconsistency:

  • Since a base class variable can reference a derived class object, the compiler will allow it to access something that was public in the base class.
  • If the derived class object actually referenced had changed the access level to private, then the element ought to be unavailable.
  • This logic could be applied to any restriction in access level, not just public to private.

As a more specific example of why this is the case, imagine that ExemptEmployee overrode public String getPayInfo() with private String getPayInfo().

The compiler would allow

Employee e = new ExemptEmployee();

// getPayInfo was public in Employee, so compiler should allow this
  • Because Employee, the type on the variable e, says that getPayInfo is public.
  • But, now at runtime, it shouldn't be accessible, since it is supposed to be private in ExemptEmployee.

Redefining Fields

A field in a derived class may be redefined, with a different type and/or more restrictive access - when you do this you are creating a second field that hides the first; this is called shadowing instead of overriding.

  • A new field is created that hides the existence of the original field.
  • Since it actually a new field being created, the access level and even data type may be whatever you want - they do not have to be the same as the original field.
  • I don't know of any good reason to do this deliberately, but it is possible.
    • A strange thing happens when you use a base class reference to such a class where the field was accessible (for example, public).
    • The base class reference sees the base class version of the field!
    But, if you were to extend a class from the Java API or other library, you wouldn't necessarily know what fields it had - this facility allows you to use whatever field names you want, and, as long as the base class versions were private, you would not get any adverse effects.

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