Why Aren\'t Java Generics Covariant?

Java Generics Covariance

In Java, generics are not covariant, as explained in the provided article. This means that if a class extends a generic class, the child class's generic type cannot be a subtype of the parent class's generic type.

Understanding the "Alias" Concept

The author mentions that "if ln were aliased with li," it would break the type-safety promise. An alias refers to an alternative name or reference to an object. In this scenario, li is an alias of ln.

An Illustrative Example

Consider the following code:

List<Integer> li = new ArrayList<Integer>();
List<Number> ln = li; // illegal
ln.add(new Float(3.1415));

Although Integer inherits from Number, adding a Float object to ln is illegal. This is because, by aliasing li to ln, we are essentially saying that li can hold Numbers. However, li was originally declared to hold only Integers. Therefore, adding a Float to ln violates the type-safety guarantee of li.

Conclusion

This example demonstrates that generics are not covariant in Java. Attempting to assign a subclass's generic type to a superclass's generic type will result in a compile-time error.

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