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Type variance refers to the techniques by which we can allow, or not allow, subtyping in our parameterized types. If we consider an Apple class, which is a subtype of Fruit, then is Crate<Apple> a subtype of Crate<Fruit>? The first instinct is to think of course, since Apple can be used where Fruit is required, but, generally speaking, the answer is no.
In fact, Crate<Apple> can be a subtype of Crate<Fruit>, a supertype of it, or neither, depending on which type of variance is used.
Firstly, let's discuss why Crate<Apple> might not be a subtype of Crate<Fruit> by default. Let's start by creating some classes:
class Fruit
class Apple : Fruit(...
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