Why is \"extends T\" allowed for type parameter bounds in Java, but not \"implements T\"?

Extends vs. Implements in Generic Type Parameter Bounds

Question:

In Java, why is "extends T" permitted for defining type parameter bounds, but "implements T" is not allowed?

For instance, the following code is prohibited:

<code class="java">public interface C {}
public class A<B implements C> {}</code>

While this code is valid:

<code class="java">public class A<B extends C> {}</code>

Answer:

Semantically, there is no distinction between "extends" and "implements" within the generic constraint language. The constraint possibilities are limited to "extends" and "super," reflecting the direction of inheritance or assignment compatibility.

• extends T: The class associated with the type parameter can be assigned to or extended from the type T.
• super T: The class associated with the type parameter can be assigned from the type T.

In the case of the invalid code example, it attempts to use "implements" to define a constraint on the type parameter B. However, "implements" is not a valid constraint type and is therefore not permitted.

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