Variance of Types

This refers to how the subtype relationship between complex types relates to the subtype relationship between its components.

A complex type is a nested data structure with primitive data types or other complex types. With regards to complex type, if:

• , it is covariant
• , it is contravariant
• otherwise, it is invariant

Arrays in Java are covariant.

Given an array S[] sArray = ..., and S <: T, we can do T[] tArray = sArray.

However, this causes issues, as if we do this, we can then put items of T that may not be subtypes of S into the array which is still of run-time type S[]. Thus, type safety can be compromised.