Java: JUnit Testing a Method with an Interface as the parameter

as the title states, i want to test the following Method:

Class A:

private List<String> s;
    
public A() {
    s = new ArrayList<String>();
    s.add("Gregor");
    s.add("Example");
    s.add("Example2");
}

public String argr(B b) { //B-> Interface
        int t = b.t();
        String r = "Hello ";
        for(String z:s ) {
            boolean x = b.f(t, 5);
            if(x) {
                r += z;
            }
        }
        return r;
    }

With the Interface B:

public interface B {
    int t();
    boolean f(int t, int i);
}

The Testing:

class Example{
  public B b;
  private A example = new A();

  @Test
  void test() {
    //assert(example .s() == "Hello GregorExampleExample2");
        
    System.out.println(example .s(b));
        
    //bsp.A(c);
    }

}

The Issue: method: b, requires an Interface as a parametere, the interface could be implemented via this Method: Class A implements B{}

Problem: When testing it doesn't recognize the interface implemented Methods in the Class A.

Question 1: How do you jUnit test an Method with an interface as a parameter?

Question 2: Do you need to implement the interface method in the jUnit test class, for it to work?