*** Make this class Singleton. *** public class ChocolateBoiler { private boolean empty; private boolean boiled; public ChocolateBoiler() { empty = true; boiled = false; } public void fill() { if (isEmpty()) { empty = false; boiled = false; } } public void drain() { if (!isEmpty() && isBoiled()) { // drain the boiled milk and chocolate empty = true; } } public void boil() { if (!isEmpty() && !isBoiled()) { // bring the contents to a boil boiled = true; } } public boolean isEmpty() { return empty; } public boolean isBoiled() { return boiled; } }