Sorry for the lengthiness of this post, but in order to illustrate my
question, I needed to include all the relevant classes.

I am working through a book on design patterns (Head First Design
Patterns)  and the first set of code  sets up a duck system.  The
original MallardDuck class:

public class MallardDuck extends Duck {

    /** Creates a new instance of MallardDuck */
    public MallardDuck() {

        quackBehavior = new Quack();
        flyBehavior = new FlyWithWings();
    }
    public void display()
    {
        System.out.println("I'm a real mallard duck");
    }

}

I was just curious so I added a second flyBehavior:

public class MallardDuck extends Duck {

    /** Creates a new instance of MallardDuck */
    public MallardDuck() {

        quackBehavior = new Quack();
        flyBehavior = new FlyWithWings();
        flyBehavior = new FlyNoWay();
    }
    public void display()
    {
        System.out.println("I'm a real mallard duck");
    }

}

wanting to know what would happen.

Here are the remaining pieces
---------------------------
public abstract class Duck {

    FlyBehavior flyBehavior;
    QuackBehavior quackBehavior;
    public Duck() {

    }

    public abstract void display();

    public void performFly() {
        flyBehavior.fly();
    }
    public void performQuack() {
        quackBehavior.quack();
    }
    public void swim() {
        System.out.println("All ducks float, even decoys");
    }
}

----------------------
public class FlyWithWings implements FlyBehavior {
    public void fly() {
        System.out.println("I'm flying");
    }

}
----------------------
public class FlyNoWay implements FlyBehavior{

    public void fly() {
        System.out.println("I can't fly");
    }
}

---------------------------Here is the main class -------------
public class MiniDuckSimulator {

    /** Creates a new instance of MiniDuckSimulator */
    public static void main(String [] args) {
        Duck mallard = new MallardDuck();
        mallard.performQuack();
        mallard.performFly();
    }

}
-----------------------------------------
I wasn't quite sure what would happen with the second reference variable
 flyBehavior.  When I ran the program,  I get this

init:
deps-jar:
compile-single:
run-single:
Quack
I'm flying
BUILD SUCCESSFUL (total time: 1 second)

Why would it not show the results from both flyBehavior references like such

init:
deps-jar:
compile-single:
run-single:
Quack
I'm flying
I can't fly
BUILD SUCCESSFUL (total time: 1 second)