I know its not possible to define a constructor in a interface. But im wondering why, because i think i could be very useful.

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

For example consider the following message class:

public class MyMessage {

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

   private String receiver;

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

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. So how can i assure that every implementation of this class really has an receiver setted? If i use a method like setReceiver(String receiver) i can't be sure that this method is really called. In the constructor i could assure it.