Q4.java

public class Q4 {
    public final static int PIZZA_SLICE = 1; // One piece of pizza
    public final static int PIZZA_REORDER = 0; // Pizza finished. Signals to reorder
    public final static int PIZZA_FINISHED = -1; // Pizza finished

    public static void main(String[] args) {
        Plate pizzaPlate = new Plate(new Pizza()); // Create a plate to hold the pizza
        Runnable[] students = new Runnable[15];

        // Runnable implementation for a pizza order
        Runnable pizzaOrder = () -> {
            synchronized (pizzaPlate) {
                if (pizzaPlate.isEmpty()) {
                    pizzaPlate.addNewPizza(new Pizza());
                    System.out.println("Kamal brought a new pizza");
                    pizzaPlate.notifyAll();
                }
            }
        };

        for (int i = 0; i < students.length; i++) {
            students[i] = () -> {
                while (true) {
                    int gotPizza;
                    synchronized (pizzaPlate) {
                        gotPizza = pizzaPlate.takePizzaSlice(); // Try to take a slice of pizza

                        if (gotPizza == PIZZA_REORDER) {
                            System.out.println("Pizza finished. " +
                                    Thread.currentThread().getName() + " is placing a new order.");
                            new Thread(pizzaOrder).start(); // Order a new pizza

                            try {
                                pizzaPlate.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        } else if (gotPizza == PIZZA_FINISHED) {
                            try {
                                System.out.println("No pizza. " +
                                        Thread.currentThread().getName() + " is going to sleep.");
                                pizzaPlate.wait(); // If there is no pizza, go sleep until there is a new pizza
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                    }

                    // If you get a slice of pizza, go on ahead, eating & studying
                    if (gotPizza == PIZZA_SLICE) {
                        System.out.println(Thread.currentThread().getName() + " is eating pizza & studying");
                        study();
                    }
                }
            };
            new Thread(students[i], "Student #" + (i + 1)).start();
        }
    }

    public static void study() {
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

class Plate {
    private Pizza pizza;

    public Plate(Pizza pizza) {
        this.pizza = pizza;
    }

    public int takePizzaSlice() {
        pizza.slices--;

        if (pizza.slices >= 0)
            return 1;
        else if (pizza.slices == -1)
            return 0;
        else
            return -1;
    }

    // Replace the current pizza with a new one
    public void addNewPizza(Pizza pizza) {
        this.pizza = pizza;
    }

    public boolean isEmpty() {
        return pizza == null || pizza.slices <= 0;
    }
}

class Pizza {
    int slices = 8;
}