What are some examples of variance in generics?

What are some examples of variance in generics?

Variance in generics refers to how subtyping between more complex types relates to subtyping between their component types. For example, in a programming language like Java, you can have covariant and contravariant types. A common example is using a List of a subtype, like List<Dog>, where you can assign it to a List<Animal> if it's covariant.

Can you explain covariant and contravariant with examples?

Sure! Covariance allows you to use a derived type where a base type is expected. For instance, if you have a method that accepts List<? extends Animal>, you can pass a List<Dog>. Contravariance, on the other hand, allows you to use a base type where a derived type is expected. For example, a method that accepts List<? super Dog> can take a List<Animal>.

What about invariance? How does it fit in?

Invariance means that you cannot substitute a subtype for a supertype or vice versa. For example, if you have a List<Dog>, you cannot assign it to a List<Animal> or vice versa. This is common in languages that do not support variance.

Are there any practical applications of these concepts?

Yes, understanding variance is crucial for designing APIs and libraries. It helps ensure type safety while allowing flexibility in how types are used, especially in collections.