How Can I Combine Class and Interface Bounds in Java Generics?

Java Generics: Combining Class and Interface Bounds

In Java, generics offer the capability to define types that can operate on different types of objects. A common challenge arises when you want to create a class object that is restricted to extend a specific class while implementing a specific interface.

Let's consider a use case where we want to create a Class object that represents a class extending ClassA and implementing the InterfaceB interface.

Can't Do Both?

Initially, you might attempt to use the following wildcards:

Class<? extends ClassA>

or

Class<? extends InterfaceB>

However, using either of these will not allow you to enforce both requirements simultaneously.

A Solution

The key to achieving this is to use multiple bounds in your wildcard. The syntax for such a wildcard is:

<T extends Class & Interface>

Applying this to our scenario, we get:

Class<? extends ClassA & InterfaceB>

Bounded Type Parameters

As explained in the Java Generics Tutorial, you can use multiple bounds to specify that a type parameter must extend a given class and implement one or more interfaces. The use of the & operator separates the bounds.

Complex Examples

While this solution allows you to achieve your goal, it can become complex. For instance, consider the Collections#max method in Java, which has the following declaration:

public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll)

This sophisticated syntax ensures binary compatibility while enforcing specific type constraints.

Using a Generic Class

In your use case, you can create a generic class with the desired bounds:

public class MyClass<T extends ClassA & InterfaceB> {
    Class<T> variable;
}

This approach allows you to create a variable that has the desired restrictions, such as:

MyClass<ClassB>.variable

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