— Scott McNealy, Sun Microsystems Chairman, President and ceo

for making a Duck—one where they supply the Duck 
size (as the constructor argument) and one where they 
don’t specify a size and thus get your default Duck size.
You can’t do this cleanly with just a single constructor. 
Remember, if a method (or constructor—same rules) has 
a parameter, you must pass an appropriate argument when 
you invoke that method or constructor. You can’t just say, “If 
someone doesn’t pass anything to the constructor, then use 
the default size”, because they won’t even be able to compile 
without sending an int argument to the constructor call. You 
could do something clunkly like this:
But that means the programmer making a new Duck object has 
to know that passing a “0” is the protocol for getting the default 
Duck size. Pretty ugly. What if the other programmer doesn’t 
know that? Or what if he really does want a zero-size Duck? 
(Assuming a zero-sized Duck is allowed. If you don’t want 
zero-sized Duck objects, put validation code in the constructor 
to prevent it.) The point is, it might not always be possible 
to distinguish between a genuine “I want zero for the size” 
constructor argument and a “I’m sending zero so you’ll give 
me the default size, whatever that is” constructor argument.
public class Duck2 {
int size;
public Duck2() {
// supply default size
size = 27;
}
public Duck2(int duckSize) {
// use duckSize parameter
size = duckSize;
}
}
To make a Duck when you know the size:
Duck2 d = new Duck2(15);
To make a Duck when you do not know 
the size:
Duck2 d2 = new Duck2();
You really want TWO ways to 
make a new Duck: