As most Java programmers know, Java generics are erased at compile-time. This has trade-offs, but the two main reasons for this are:
- Compatibility - Java 1.4 and earlier dealt exclusively in raw types at the VM level. Generics were able to compile without introducing significant new typing to the bytecode and classfile format spec, and having to deal with older classes generated without that typing.
- Simplicity - By erasing to raw types, the JVM doesn’t have to understand specialization; something that has its own complexities and downsides. For example, specialized types are much more challenging to optimize with a just-in-time compiler.
However, knowing the type parameters used at runtime can have real value, and it’s something Java simply doesn’t offer. Kotlin would like to help in this area, but there are many challenges in doing so.