Java – How do I define type constraints for method parameters?

I have class A and interface B, and one method:

void foo(A x) {}

What I want is to define a constraint on parameter x, which must implement interface B.

Although I know it can be done at runtime, for example:

void foo(A x) {
    if (!( x instanceof B)) {
        throw new IllegalStateException();
    }
}

Or define another class and change the method signature, for example:

class C extends A implements B {}

void foo(C x) {}

I don’t

accept this solution because in my case I have many subclasses of A in the 3rd party jar and I don’t want to modify them.

I

wonder if it can be implemented at the language level, enabling the constraint on the fly when I write the calling code.

If you must do this, you can use generics such as Bounded Type Parameters . Documentation, Multiboundary section:

public interface A {}
public interface B {}
public static class C implements A, B {}

public static <T extends A & B> void foo(T type) {

}

public static void main(String[] args) {
  foo(new A() {});  compile error
  foo(new B() {});  compile error
  foo(new C() {});
}