Null Object Design Pattern

in object-oriented computer programming, a null object is an object with no referenced value or with defined neutral behavior
was first published in the Pattern Languages of Program Design book series

Code Example

public interface Animal {
	void makeSound() ;
}

public class Dog implements Animal {
	public void makeSound() {
		System.out.println("woof!");
	}
}

public class NullAnimal implements Animal {
	public void makeSound() {
                // silence...
	}
}