What is the difference between ‘E’, ‘T’, and ‘?’ for Java generics?

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Well there’s no difference between the first two – they’re just using different names for the type parameter (E or T).

The third isn’t a valid declaration – ? is used as a wildcard which is used when providing a type argument, e.g. List<?> foo = ... means that foo refers to a list of some type, but we don’t know what.

All of this is generics, which is a pretty huge topic. You may wish to learn about it through the following resources, although there are more available of course:

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