— Scott McNealy, Sun Microsystems Chairman, President and ceo

public class Duck {
int size;
public Duck(int duckSize) {
System.out.println(“Quack”);
size = duckSize;
System.out.println(“size is “ + size);
}
}
public class UseADuck {
public static void main (String[] args) {
Duck d = new Duck(42);
}
}
File Edit Window Help Honk
% java UseADuck
Quack
size is 42
Add an int parameter to the 
Duck constructor.
Use the argument value to set 
the size instance variable.
Pass a value to the 
constructor.
This time there’s only 
one statement. We make 
the new Duck and set 
its size 
in one statement.
Let the user make a new Duck 
and set the Duck’s size all in 
one call. The call to new.
The call to the Duck 
constructor.
*Not to imply that not all Duck state is not unimportant.
initializing object 
state

---

constructors and 
gc
you are here
4
245
Make it easy to make a Duck
Be sure you have a no-arg constructor
What happens if the Duck constructor takes an argument? 
Think about it. On the previous page, there’s only one Duck 
constructor—and it takes an int argument for the size of the 
Duck. That might not be a big problem, but it does make it 
harder for a programmer to create a new Duck object, especially 
if the programmer doesn’t know what the size of a Duck should 
be. Wouldn’t it be helpful to have a default size for a Duck, so 
that if the user doesn’t know an appropriate size, he can still 
make a Duck that works?
Imagine that you want Duck users to have TWO options