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Review Questions

Here are some exercises to get ready for the final exam. They are rejected ideas for final exam questions from last semester and this semester. :)

Question 1: Infinite lists

Implement a method called interleave that takes in two InfiniteList<T> objects, and produce another InfiniteList<T> with elements in the two lists interleave.

For instance,

list1 = InfiniteList.generate(() -> 1);
list2 = InfiniteList.generate(() -> 2);
interleave(list1, list2).limit(6).toArray(); // returns [1, 2, 1, 2, 1, 2]

The method interleave must be lazily evaluated. You can assume that the constructor

InfiniteList<T>(Supplier<T> headSupplier, Supplier<InfiniteList<T>> tailSupplier)

is available.

Question 2: Completable future

a(), b(), and c() are three methods that takes in no arguments and returns nothing (void). We want to run them asynchronously, such that a() and b() run first, in any order, concurrently. But c() can only run after either one of a() or b() completes.

Using the class CompletableFuture, write snippets of code to show how this can be done. The APIs for CompletableFuture is provided

Question 3: Lambdas

Java implements lambda expressions as anonymous classes. Suppose we have the following lambda expression Function<String,Integer>:

str -> str.indexOf(' ')

Write the equivalent anonymous class for the expression above.

Question 4: Stream of Functions

In mathematics, an iterated function is a function that is composed with itself some number of times. We denote $f^n$ = $f \cdot f \cdot ... f$ as function $f$ composed with itself $n$ times. For instance, if $f$ is $\frac{1}{1 + x}$, then $f^3$ is $\frac{1}{1 + \frac{1}{1 + \frac{1}{1 + x}}}$.

(a) Write a method that, given a Function<T,T> f, generate a stream of iterated functions of $f$. The i-th element in the stream is $f^i$.

(b) Write a method that, given a stream of iterated functions $f$, $f^2$, ..., and a value t of type T, return a stream of values where each element is the result of applying the corresponding iterated function on t.

Question 5: OO

(a) The following code is not written using inheritance nor polymorphism. Rewrite it so that it properly uses inheritance / polymorphism, eliminating the need for StoneType and the field type.

enum StoneType {
  TIME, SPACE, POWER, MIND
};

class InfinityStone {
  StoneType type;
  Color c;

  InifinityStone(StoneType type) {
    this.type = type;
    if (type == StoneType.TIME) {
      c = Color.GREEN;
    } else if (type == StoneType.SPACE) {
      c = Color.BLUE;
    } else if (type == StoneType.POWER) {
      c = Color.PURPLE;
    } else if (type == StoneType.MIND) {
      c = Color.YELLOW;
    }
  }

  void activate() {
    if (type == StoneType.TIME) {
      warpTime();
    } else if (type == StoneType.SPACE) {
      controlSpace();
    } else if (type == StoneType.POWER) {
      manipulateEnergy();
    } else if (type == StoneType.MIND) {
      controlMind();
    }
  }
}

// example of how the class will be used:
InfinityStone tesseract = new InifinityStone(StoneType.SPACE);
tesseract.activate();

(b) Suppose we want to add two new stone types REALITY and SOUL. Explain how you would do it with the original version and the OO version -- contrast how much existing code you need to modify vs. how much code you would need to add.

p/s: Adding new code is preferable over modifying existing code, since the latter is more bug prone.

Question 6: Bad Practices

Each of the following code illusrates a very bad programming practice. For each, comments on why it is bad.

(a) "Pokemon Catch"

    try {
      doSomething();
    } catch (Exception e) {

    }

(b) Switching between strings

   switch(customer.getType()) {
     case "Normal": 
       joinQueueNormal();
     case "Greedy": 
       joinQueueGreedy();
     default:
       joinQueueRandom();
   } 

(c )

void getCustomerType() {
  if (customer.isNormal()) {
    throw new NormalCustomerException();
  } else if (customer.isGreedy()) {
    throw new GreedyCustomerException();
  }
}

  :
  :

try {
  getCustomerType();
  joinQueueRandom();
} catch (NormalCustomerException e) {
  joinQueueNormal();
} catch (GreedyCustomerException e) {
  joinQueueGreedy();
}

(d)

// customers, servers, queues are arrays of Customer, 
// Server, and Queue respectively.
Customer[] customers;
Server[] servers;
Queue[] queues;
  :
  :

void handleCustomer(int q, int s, int c) {
  // if servers[s] is busy, add customers[c] into queues[q]
  // otherwise servers[s] serves customers[c]
    :
}

Question 7: Ask, Don't Tell

Suggest how we can improve the design of the classes below. Only relevant part of the code are shown for brevity.

class LabSubmission {
  private Student s;
  private int marks;

  public Student getStudent() {
    return s;
  }

  public int getMarks() {
    return marks;
  }

    :
    :
}

class Gradebook {
  List<LabSubmission> submissions;

  void print() {
    for (LabSubmission s : submissions) {
      if (s.getMarks() > 3) {
        System.out.printf(s.getStudent() + " A");
      } else if (s.getMarks() > 2) {
        System.out.printf(s.getStudent() + " B");
      } else if (s.getMarks() > 1) {
        System.out.printf(s.getStudent() + " C");
      } else {
        System.out.printf(s.getStudent() + " D");
      }
    }
  }
}