In Java, why is there no generic type info at run time?

Question

Consider this example taken from a book, with a super class Gen and a subclass Gen2...

class Gen { }

class Gen2 extends Gen { }
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Answer 1

Generics were introduced to the Java language to provide tighter type checks at compile time and to support generic programming. [...] Type erasure ensures that no new classes are created for parameterized types; consequently, generics incur no runtime overhead.

I don't think this is related to backwards compatibility at all. If you compile your code targetting 1.5 or more, the code will be not compatible with 1.4, regardless the use of generics.