On Extending Java

closely related to closures since they too can be nested in methods and classes. Their translation scheme follows the scheme described for closures. They are crucial for implementing iterators efficiently. Iterators are quite valuable as they permit encapsu-lation of iteration strategies within containers. In other words, the iteration algorithm, complete with its initialization code and termination condition, is hidden within the container data structure. This makes client code simpler and more robust. A translator implementing our extensions is available atTranslation Iterators are implemented using the exception mechanism of Java. Two new exceptions are defined, Break and Continue /*1*/. Inside an iterator method, a break at method level or labelled with the method name is translated to a throw of the exception Break /*2*/. Similar a continue is translated to a throw of the exception Continue. Each use of an iterator method is protected by a try which catches Break and Continue exceptions /*3*/. Only one try block for each loop nesting containing iterators is generated. The exception handler contains the break or continue statement optional extended by a label /*4*/. The loop keyword is just transformed into a while (true) /*5*/. Discussion The iterator concept presented here is more expressive than Sather's iters and allows an elegant encapsulation of iteration protocols. The translation scheme relies on closures and anonymous objects presented earlier. Exceptions are an efficient implementation technique. In Java they are very frequent and are often implemented as functions with two return values: the exception and the function return value. Our iterators are just that: methods with two return values. The exception setup and the iterator call overhead can be eliminated if the iterator is inlined. 7 Conclusions This paper proposes four non-intrusive language extensions, tuples, closures, anonymous objects and iterators, designed to improve the expressive power of Java without modifying the semantics of existing programs or complicating the language unduly. Tuples are useful on their own as typed heterogeneous containers of fixed size. Their real value is for efficiently implementing functions with multiple return values. In this respect they fit well with closures. Closures are first class functions. Their presence in the language makes it easy to express functions that manipulate other functions as is commonly done in functional programming. Anonymous objects are literal objects final class Break extends Exception {} /*1*/ final class Continue extends Exception {} /*1*/