java - Constructor in an Interface?

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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
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 an 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 an receiver set? 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 ensure it.

Answer

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 href="http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html"
rel="noreferrer">abstract classes are for.