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Subtyping in Java Generics

Consider the following block of Java code, which we all know as valid -

We can do this because Long is a subtype of Number. However, the following will fail to compile -

Allowing such assignments would have easily let programmers break the type safety guarantee provided by the Generics. One would then be able to do -

From the above example, it is clear that Subtyping in Java Generics works differently than the usual class based Subtyping. A list of numbers cannot point directly to a list of longs even though Long is a subtype of Number. In order to get around this restriction, we will have to use an upper bounded wildcard -

which will also allow us to refer to a list of floats as well.

A List<? extends Number>  is then treated as something like a super type of both List<Long> and List<Number>. In fact, as long as a type X is a subtype of Number, List<? extends Number> will be able to refer to List<X> without any compilation errors.

Using an upper bounde…