Java™ Application Development on Linux - Dator

90Chapter 3An Experienced Programmer’s Introduction to JavaTIPA common technique used with public static final constants is to putthem in an interface definition. (This is the exception to the rule that interfacesdefine method signatures but contain no data.) When a class wants to use oneor more of those constants, it is declared to implement that interface:public MyClassextends BigClassimplements Comparable, LotsaConstants{...}In defining MyClass we have declared that it implements LotsaConstants(not a name that we recommend you using). That makes all the constants thatwe have defined inside the LotsaConstants interface available to theMyClass class. Since classes can implement many different interfaces, thisdoesn’t interfere with the use of other “real” interfaces, such as Comparable.WARNINGThe keyword enum is new to Java 5.0, so older programs that may have usedenum as a variable name and will now cause an error when recompiled forJava 5.0.The enum will look very familiar to C/C++ programmers, but there aresome important differences. In C/C++ the values of the enum elements are, inreality, integer values. Not so in Java. Here they are their own type, but can beconverted to a String via the toString() method, with a value that matchesthe name, for easy reading and debugging.Enumerated values can be used in == comparisons since they will bedefined only once (like other static final constants) and it would only bereferences that are passed around. They would be referenced by the name ofthe enumeration followed by dot followed by the particular value (e.g.,WallMods.WINDOW) and used as an object. (We have used uppercase for thenames not out of any syntax requirement, but only to follow the typical namingconvention for constants.)