Question

What is the concept of erasure in generics in Java?

Answer 1

It’s basically the way that generics are implemented in Java via compiler trickery. The compiled generic code actually just uses java.lang.Object wherever you talk about T (or some other type parameter) – and there’s some metadata to tell the compiler that it really is a generic type.

When you compile some code against a generic type or method, the compiler works out what you really mean (i.e. what the type argument for T is) and verifies at compile time that you’re doing the right thing, but the emitted code again just talks in terms of java.lang.Object – the compiler generates extra casts where necessary. At execution time, a List<String> and a List<Date> are exactly the same; the extra type information has been erased by the compiler.

Compare this with, say, C#, where the information is retained at execution time, allowing code to contain expressions such as typeof(T) which is the equivalent to T.class – except that the latter is invalid. (There are further differences between .NET generics and Java generics, mind you.) Type erasure is the source of many of the ‘odd’ warning/error messages when dealing with Java generics.

Other resources:

• Oracle documentation
• Wikipedia
• Gilad Bracha’s Java generics guide (PDF – highly recommended; link may need to change periodically)
• Angelika Langer’s Java Generics FAQ