Question

If you have one interface called A that has one method signature called print; Now if you have 3 classes implementing A and you call A.print how do you know which class method gets invoked. THERE IS NO NEWING OF AN OBJECT

public interface A()
{
  public void print(){}
}
@Component
public class B implements A
{
 public void print()
 {
   system.out.print("B");
 }
}
@Component
public class c implements A
{
 public void print()
 {
   system.out.print("C");
 }
}
@Component
public class d implements A
{
 public void print()
 {
   system.out.print("d");
 }
}


public class runner()
{
    @Autowired
    private A aThing_;

    aThing_.print();
}

Answer 1

An interface defines an interaction contract or, in other words, defines a set of methods that every class implementing that interface should provide.

Oracle’s answer to the question, What’s an Interface? is:

As you’ve already learned, objects define their interaction with the
outside world through the methods that they expose. Methods form the
object’s interface with the outside world; the buttons on the front of
your television set, for example, are the interface between you and
the electrical wiring on the other side of its plastic casing. You
press the “power” button to turn the television on and off.

The invocation depends on the type of the object implementing the interface.

A interface = new B();

You’ll be invoking B‘s print method’s implementation for the print method defined in the A interface.

EDIT: The point of an interface is defining the interaction with an object regardless of its actual type. That code seems to be the autoscanning of a group of components behind the same interface to show that you can define a set of different components to handle the same situation in a different way, given the context.

AFAIK the autowire defaults to the field’s name. You can define which interface implementation you want to inject with the @Qualifier("CLASS_NAME_HERE") annotation alongside with @Autowire.

You might want to check this.