Why do some claim that Java's implementation of generics is bad?

Bad:

• Type information is lost at compile time, so at execution time you can't tell what type it's "meant" to be
• Can't be used for value types (this is a biggie - in .NET a List<byte> really is backed by a byte[] for example, and no boxing is required)
• Syntax for calling generic methods sucks (IMO)
• Syntax for constraints can get confusing
• Wildcarding is generally confusing
• Various restrictions due to the above - casting etc

Good:

• Wildcarding allows covariance/contravariance to be specified at calling side, which is very neat in many situations
• It's better than nothing!

The biggest problem is that Java generics are a compile-time only thing, and you can subvert it at run-time. C# is praised because it does more run-time checking. There is some really good discussion in this post, and it links to other discussions.

The main problem is that Java doesn't actually have generics at runtime. It's a compile time feature.

When you create a generic class in Java they use a method called "Type Erasure" to actually remove all of the generic types from the class and essentially replace them with Object. The mile high version of generics is that the compiler simply inserts casts to the specified generic type whenever it appears in the method body.

This has a lot of downsides. One of the biggest, IMHO, is that you can't use reflection to inspect a generic type. Types are not actually generic in the byte code and hence can't be inspected as generics.

Great overview of the differences here: http://www.jprl.com/Blog/archive/development/2007/Aug-31.html