Question

I know it’s not possible to define a constructor in an interface, but I’m wondering why, because I think it could be very useful.

So you could be sure that some fields in a class are defined for every implementation of this interface.

For example, consider the following class:

public class MyMessage {

   private String receiver;

   public MyMessage(String receiver) {
       this.receiver = receiver;
   }

   public void send() {
       // some implementation for sending the message to the receiver
   }
}

If I define a Message interface for this class so that I can have more classes which implement the Message interface, I can only define the send method and not the constructor.

So how can I ensure that every implementation of this class really has its receiver initialized? If I define a method like setReceiver(String receiver), I can’t be sure that this method is really called. In the constructor however, I could ensure it.

Answer 1

Taking some of the things you have described:

“So you could be sure that some fields in a class are defined for
every implementation of this interface.”

“If a define a Interface for this class so that I can have more
classes which implement the message interface, I can only define the
send method and not the constructor”

…these requirements are exactly what abstract classes are for.