HELP! * GREY=local LOCAL HTML version of Foils prepared July 10 1996

Foil 72 Comments on Casting

From Basic Lectures on Java Language Applets Graphics Networking Trip to China and Icase Tutorial -- July 12-28 and June 10-13 96. by Geoffrey C. Fox * See also color IMAGE
Casting is supported between types and class types. Syntax:
  • (classname)reference
Two forms of casting are possible: widening and narrowing
Widening, where the subclass is used as an instance of the superclass, is performed implicitly
Narrowing, where the superclass is used as an instance of the subclass, must be performed explicitly
Given Parent: Dot -> DrawableDot (Child):
  • Widening: An instance of DrawableDot is used as an instance of Dot
  • Narrowing: An instance of Dot is used as an instance of DrawableDot
Casting between sibling classes is a compile-time error

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