Generics super vs. extends

Question

Just when I thought I finally understood generics, I came across the following example:

public class Organic {
          void react(E e) { }
            


        

Answer 1

Your first declaration

Organic compound

means that compound could be an Organic (since Aliphatic extends Organic, Hexane extends Aliphatic and SomeSubtypeOfHexane extends Hexane).

In that case, compound.react(new Organic()), compound.react(new Aliphatic()) and compound.react(new Hexane()) would lead to a type error, since E in compound must be a SomeSubtypeOfHexane (or subtype thereof).

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Your second declaration

Organic compound

means that compount could be an Organic.

In that case compound.react(new Organic()) would lead to a type error, since E must be an Aliphatic (or subtype thereof).

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Remember that declaring a variable using A or A

• extends the amount of objects that can be assigned to it, and, in consequence,
• restricts what can be done with the variable.

Since the exact type of the class is unknown (only a constraint is known), the compiler has to err on the side of safety and disallow certain operations that are either not co- or contravariant. (If you are not already familiar with it, Co- and contravariance is the scientific background of these types of generics.)