The Java 5 Generics Compromise Orthogonality to Keep Compatibility

Radenski, A., J. Furlong, V. Zanev. The Java 5 Generics Compromise Orthogonality to Keep Compatibility. Journal of Systems and Software, Elsevier Science, Vol. 81, Issue 11, 2008, 2069-2078.

In response to a long-lasting anticipation by the Java community, version 1.5 of the Java 2 platform - referred to as Java 5 - introduced generic types and methods to the Java language. The Java 5 generics are a significant enhancement to the language expressivity because they allow straightforward composition of new generic classes from existing ones while reducing the need for a plethora of type casts. While the Java 5 generics are expressive, the chosen implementation method, type erasure, has triggered undesirable orthogonality violations. This paper identifies six cases of orthogonality violations in the Java 5 generics and demonstrates how these violations are mandated by the use of type erasure. The paper also compares the Java 5 cases of orthogonality violations to compatible cases in C# 2 and NextGen 2 and analyzes the trade-offs in the three approaches. The conclusion is that Java 5 users face new challenges: a number of generic type expressions are forbidden, while others that are allowed are left unchecked.

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Last updated: September 2008