Extending Generic Classes

Let’s look at this definition:

public class Extend1<T, E> extends MyGeneric<T, E> {}

Here T and E are each present twice and in two different roles

• in Extend1<T,E> you define type arguments. This means that the type Extend1 has two (unbounded) type arguments T and E. This tells the Java compiler that those who use Extend1 need to specify the types.
• in extends MyGeneric<T,E> you use the previously defined type arguments. If T and E were not known to be type arguments here, then T and E would be simple type references, i.e. the compiler would look for classes (or interfaces, …) named T and E (and most likely not find them).

Yes, type arguments follow the same syntactic rules as any other identifier in Java, so you can use multiple letters ABC or even names that can be confusing (using a type argument called String is legal, but highly confusing).

Single-letter type argument names are simply a very common naming strategy.