One motto underpinning the design of generics for Java is evolution, not revolution. It must be possible to migrate a large, existing body of code to use generics gradually (evolution) without requiring a radical, all-at-once change (revolution). The generics design ensures that old code compiles against the new Java libraries, avoiding the unfortunate situation in which half of your code needs old libraries and half of your code needs new libraries.
The requirements for evolution are much stronger than the usual backward compatibility. With simple backward compatibility, one would supply both legacy and generic versions for each application; this is exactly what happens in C#, for example. If you are building on top of code supplied by multiple suppliers, some of whom use legacy collections and some of whom use generic collections, this might rapidly lead to a versioning nightmare.
What we require is that the same client code works with both the legacy and generic versions of a library. This means that the supplier and clients of a library can make completely independent choices about when to move from legacy to generic code. This is a much stronger requirement than backward compatibility; it is called migration compatibility or platform compatibility.
Java implements generics via erasure, which ensures that legacy and generic versions usually generate identical class files, save for some auxiliary information about types. It is possible to replace ...