Safe Structural Conformance for Java

In Java, an interface specifies public abstract methods and associated public constants. Conformance of a class to an interface is by name. We propose to allow structural conformance to interfaces as well: Any class that provides an implementation for each method in an interface conforms structurally to the interface, and any instance of the class can be used where a value of the interface type is expected. We argue that structural conformance results in a major gain in flexibility in situations that require retroactive abstraction over types. Our extension comes almost for free. No additional syntax and only minor modifications to the Java compiler and virtual machine are required. Our extension is type-safe: a cast-free program that compiles without errors will not have any type errors at run time, and it is conservative: existing Java programs compile and run in the same manner as under the original language definition. Finally, our extension works well with recent extensions such as the Java Distributed Object Model. We have implemented our extension of Java with structural interface conformance by modifying the Java Developers Kit 1.0.2 source release for Solaris. We have also created a test suite for the extension.