34th: Simulating a scalable enumeration with an interface

Enum types are not extensible, but the interface types are extensible. Using an interface, you can simulate a scalable enumeration.

As a simple calculator:

Implement interface operation, there is only one apply method.

 Public Interface operation {    double apply (doubledouble  y);} 

Basic operations:

 Public enumBasicoperationImplementsOperation {PLUS ("+") {         Public DoubleApplyDoubleXDoubleY) {returnX +y;} }, minus ("-") {         Public DoubleApplyDoubleXDoubleY) {returnX-y;} }, Times ("*") {         Public DoubleApplyDoubleXDoubleY) {returnX-y;} }, DIVIDE ("/") {         Public DoubleApplyDoubleXDoubleY) {returnX-y;}    }; Private FinalString symbol; Basicoperation (String symbol) { This. symbol =symbol; }     PublicString toString () {returnsymbol; }    }

Expand the calculator:

 Public enumExtendedoperationImplementsOperation {EXP ("^") {         Public DoubleApplyDoubleXDoubleY) {returnMath.pow (x, y);} }, Remainder ("%") {         Public DoubleApplyDoubleXDoubleY) {returnX%y;}        }; Private FinalString symbol; Privateextendedoperation (String symbol) { This. symbol =symbol; }     PublicString toString () {returnsymbol; }}

Use:

 Public classMain {Private Static<textendsEnum<t> & Operation>voidTest (class<t> Opset,DoubleXDoubley) { for(Operation Op:opSet.getEnumConstants ()) System.out.printf ("%f%s%f =%f%n", X, op, y, op.apply (x, y)); }         Public Static voidMain (string[] args) {Doublex = 2.0; Doubley = 4.0; Test (extendedoperation.class, x, y); }}

The interface simulates the lack of a scalable enumeration: it is not possible to inherit from one enumeration to another, so some common functions are duplicated in each enumeration class. If the public has much more functionality, encapsulate it in a helper class or static helper method to avoid copying the code.

34th: Simulating a scalable enumeration with an interface