Check if a generic T implements an interface

Question

so I have this class in Java:

public class Foo<T>{ } 

and inside this class I want to know if T implements certain interface.

The following code DOES NOT work but it's the idea of what I want to accomplish:

if(T.class implements SomeInterface){     // do stuff } 

so I want to check if the class T that was passed to Foo have implements SomeInterface on its signature.

Is it possible? How?

Answer 1

Generics, oddly enough, use extends for interfaces as well.¹ You'll want to use:

public class Foo<T extends SomeInterface>{     //use T as you wish } 

This is actually a requirement for the implementation, not a true/false check.

For a true/false check, use unbounded generics(class Foo<T>{) and make sure you obtain a Class<T> so you have a refiable type:

if(SomeInterface.class.isAssignableFrom(tClazz)); 

where tClazz is a parameter of type java.lang.Class<T>.

If you get a parameter of refiable type, then it's nothing more than:

if(tParam instanceof SomeInterface){ 

but this won't work with just the generic declaration.

¹If you want to require extending a class and multiple interfaces, you can do as follows: <T extends FooClass & BarInterface & Baz> The class(only one, as there is no multiple inheritance in Java) must go first, and any interfaces after that in any order.

Answer 2

you can check it using isAssignableFrom

if (YourInterface.class.isAssignableFrom(clazz)) {     ... } 

or to get the array of interface as

Class[] intfs = clazz.getInterfaces();