Why Can\'t We Use \'implements\' for Type Parameter Bounds in Java Generics?

Supertype Bound Differences in Java Generics

In Java, when defining bounds for type parameters, you may have noticed the presence of "extends" but not "implements." This disparity has sparked curiosity in understanding why this is the case.

Why No "implements T"?

Unlike "extends," which indicates a supertype relationship where the subclass shares the same interface, "implements" denotes an implementation relationship where the class implements the methods specified by the interface. This distinction becomes apparent during compilation.

Consider the example below:

<code class="java">interface C {}
class A<B extends C> {} // Allowed
class A<B implements C> {} // Not allowed</code>

In the first example, the type parameter B is bounded by the class C using "extends." This allows B to be either equal to or a subclass of C. In the second example, "implements" is used, which only permits B to be an interface implementing C. However, type parameters cannot specify implementing interfaces.

Reason for "extends T"

Java's generic constraint language focuses on supertype relationships. "extends" allows you to restrict a type parameter to a specific superclass or interface, ensuring that the type will always be assignable to that supertype. This maintains type safety and inheritance principles in Java.

On the other hand, "implements" does not fit this paradigm as it defines an implementation relationship rather than a supertype relationship. Therefore, "extends" is the preferred and sole option for defining bounds on type parameters.

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