Generic extends and implements

Question

I don't understand why Company compiled. I thought it checked for extends but not for implements?

public interface Employee

public class HourlyEmployee implements Employee

public class Company<T extends Employee>

Company<HourlyEmployee> company = new Company<>();

Answer 1

The extends keyword in Generics has a slightly different semantics than the general extends keyword.

When using extends in the context of Generics, for example T extends Something, this means that T should be a type that either implements the interface Something (in cases when Something is interface), or is a subclass of Something (in case Something is a class).

Probably the reason for this is that if the implements keyword was supported in Generics, this would have made type-parameter declaration too verbose.

For example, you'd have:

<T extends SomeClass implements Serializable & Observable>

Instead, the valid syntax for this would be:

<T extends SomeClass & Serializable & Observable>

And you don't need to have the implements keyword, actually. When defining the bounds of a type T, you just need to point out which types does your type T derive from, without caring if those are interfaces or classes.

The type definition is not a class definition. You can consider type definition as joining few data sets, where the resulting set is your type T.

Answer 2

The notation T extends Employee in the declaration of a type parameter refers to either extending a class or implementing an interface.

public class Company<T implements Employee> is not a valid syntax.

Therefore public class Company<T extends Employee> means that the generic type parameter T of your Company class must implement the Employee interface.