Java Theory and Practice: Generics Gotchas

By Brian Goetz
2005-03-23


Identify and avoid some of the pitfalls in learning to use generics
Generic types, added in JDK 5.0, are a significant enhancement to type safety in the Java language. However, some aspects of generics may seem confusing, or even downright bizarre, to first-time users. In this month's Java theory and practice, Brian Goetz examines the common pitfalls that ensnare first-time users of generics.

Generic types (or generics) bear a superficial resemblance to templates in C++, both in their syntax and in their expected use cases (such as container classes). But the similarity is only skin-deep -- generics in the Java language are implemented almost entirely in the compiler, which performs type checking and type inference, and then generates ordinary, non-generic bytecodes. This implementation technique, called erasure (where the compiler uses the generic type information to ensure type safety, but then erases it before generating the bytecode), has some surprising, and sometimes confusing, consequences. While generics are a big step forward for type safety in Java classes, learning to use generics will almost certainly provide some opportunity for head-scratching (and sometimes cursing) along the way.

Note: This article assumes familiarity with the basics of generics in JDK 5.0.

Tutorial Pages:
» Identify and avoid some of the pitfalls in learning to use generics
» Generics are not covariant
» Construction delays
» Generifying existing classes
» Implications of erasure
» Summary
» Resources


First published by IBM developerWorks

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