StackDemo2.java

// stack demo with type derivation based on the article from http://www.25hoursaday.com/CsharpVsJava.html#same
import java.util.Stack;

interface Mammal {
 // default speak() implementation
 default void speak() {
 System.out.println("*Generic mammal sound*");
 }
}

class Dog implements Mammal {
 // Dog provides its own speak() implementation
 public void speak() {
 System.out.println("Woof! Woof!");
 }
}

class Cat implements Mammal {
 // Cat provides its own speak() implementation
 public void speak() {
 System.out.println("Meow! Meow!");
 }
}

class MuteKoala implements Mammal {
 // mute koala is mute is no speak() implementation :(
}

class StackDemo2 {
 // wildcard with upper bound to capture all Mammal-derived stacks
 // (keyword 'extends' used for classes and interfaces!)
 public static void annoyNeighbors(Stack<? extends Mammal> pets) {
 while (!pets.empty()) {
 Mammal animal = pets.pop();
 animal.speak();
 }
 }

 public static void main(String[] args) {
 // stack of different mammals
 Stack<Mammal> pets = new Stack<Mammal>();
 // add some animals to the stack (both push() and add() allowed)
 pets.add(new Dog());
 pets.add(new Cat());
 pets.add(new MuteKoala());
 // make some animal noise!
 System.out.println("Your pets are annoying the neighbors:");
 annoyNeighbors(pets);
 System.out.println("Your neighbors are furious...");
 }
}