You are called in by a crack team of budget astronomers who want to take advantage of a special offer and get some free telescopes. They need to concurrently send the names of stars as discount codes to an online shop, which is giving away free telescopes.

To keep things fair, the store only allows two purchases at a time from any customer. You've been asked to modify some code to use the following class's  buy(discountCode, item)  method to purchase a telescope item. 

public class AstronomyShopClient {
    public void buy(String discountCoude, String item) {
        System.out.println("Ordered a telescope");
    }
}

To order a telescope, someone needs to call the method with a star's name and a telescope argument. Eg.,  client.buy(star.name, "telescope") 

They tried to implement a solution that would split the list of planets in two and kick-off two asynchronous CompletableFuture, which loop through their respective lists and order telescopes. It hasn't worked. You've been told that it only seems to be looping through one list at a time.

You are given a FreeTelescopeGrabber class which looks like this:

public class FreeTelescopeGrabber {
    private static final String ITEM = "telescope";
    private AstronomyShopClient client = new AstronomyShopClient();
    private static final List<Star> STAR_LIST = FakeStarListFactory.make();
    
    public CompletableFuture orderTelescopes() {
        // Split our list in 2
        List<Star> leftList = stars.subList(
            0, stars.size()/2);

        List<Star> rightList = stars.subList(
            stars.size()/2, stars.size());

        
        // Submit orders for both sides of the list
        return CompletableFuture.allOf(
            CompletableFuture.runAsync(
                ()->orderWithDiscountCodes(leftList)
            ).thenRun(
                ()->orderWithDiscountCodes(rightList)
            )
        );
    }
            
    private void orderWithDiscountCodes(List<Star> starts) {
        // Use the client to order a free telescope
        // using each star as a discount code
        for (int index =0; i<stars.size(); i++) {
            client.get(star.name, ITEM);
        }
    }
}

Which of the following changes would fix this? 

The team of bargain-loving astronomers receives a notification from the store warning that you're making too many requests to their platform. While they welcome your business, for a short amount of time, they require you to restrict your application so that it only makes one request at a time on their service. Their longer-term plan is to gradually increase the number of concurrent orders that each customer can make.

Given this information, which of the following changes would limit your application to ONE ORDER at a time AND require changing a single argument if you need to modify this limit?

 Someone on your team creates a TelescopeOrderService to order those free telescopes. It watches out for the IOExceptions, which sometimes happen when ordering a telescope. Here's the code. 

public class TelescopeOrderService {
    
    // A client which will occaisonally fail
    public static class ShopClientSimulation {
        public static void orderTelescope(String discount)
            throws IOException {
                
            if (Math.random()>0.1) {
                throw new IOException("Simulated failure");
            }
        }
    }

    private Semaphore semaphore;
    
    public TelescopeOrderService(Semaphore semaphore) {
        this.semaphore = semaphore;    
    }
    
    public void orderATelescope(String starName) {
        try {
            semaphore.acquire();
            ShopClientSimulation.orderTelescope(starName);
            semaphore.release();
        } catch (InterruptedException | IOException e) {
            e.printStackTrace();
        }
    }
}

The code gets stuck from time to time.

You can try this out for yourself by starting JShell and pasting the code; although, you might have to call it several times. You can then create an instance and call it with different star names. Here's some example output:

jshell> TelescopeOrderService tos = new TelescopeOrderService(new Semaphore(1)); 
tos ==> TelescopeOrderService@73a8dfcc

jshell> tos.orderATelescope("Sirius")
 
jshell> tos.orderATelescope("Betelgeuse")

jshell> tos.orderATelescope("Rigel")
java.io.IOException: Simulated failure
    at REPL.$JShell$14$TelescopeOrderService$ShopClientSimulation.orderTelescope($JShell$14.java:13)
jshell> tos.orderATelescope("Vega")
---- THIS IS WHERE THE CODE GETS STUCK - HIT CTRL-C to GET OUT ----

How should you fix this?