Effective Java

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ITEM 38: EMULATE EXTENSIBLE ENUMS WITH INTERFACES
177
private final String symbol;
BasicOperation(String symbol) {
this.symbol = symbol;
}
@Override public String toString() {
return symbol;
}
}
While the enum type (
BasicOperation
) is not extensible, the interface type
(
Operation
) is, and it is the interface type that is used to represent operations in
APIs. You can define another enum type that implements this interface and use
instances of this new type in place of the base type. For example, suppose you
want to define an extension to the operation type shown earlier, consisting of the
exponentiation and remainder operations. All you have to do is write an enum
type that implements the 
Operation
interface:
// Emulated extension enum
public enum ExtendedOperation implements Operation {
EXP("^") {
public double apply(double x, double y) {
return Math.pow(x, y);
}
},
REMAINDER("%") {
public double apply(double x, double y) {
return x % y;
}
};
private final String symbol;
ExtendedOperation(String symbol) {
this.symbol = symbol;
}
@Override public String toString() {
return symbol;
}
}
You can now use your new operations anywhere you could use the basic oper-
ations, provided that APIs are written to take the interface type (
Operation
), not
the implementation (
BasicOperation
). Note that you don’t have to declare the

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CHAPTER 6
ENUMS AND ANNOTATIONS
178
abstract 
apply
method in the enum as you do in a nonextensible enum with
instance-specific method implementations (page 162). This is because the abstract
method (
apply
) is a member of the interface (
Operation
).
Not only is it possible to pass a single instance of an “extension enum” any-
where a “base enum” is expected, but it is possible to pass in an entire extension
enum type and use its elements in addition to or instead of those of the base type.
For example, here is a version of the test program on page 163 that exercises all of
the extended operations defined previously:
public static void main(String[] args) {
double x = Double.parseDouble(args[0]);
double y = Double.parseDouble(args[1]);
test(