Stream Processing in Java 8

For each of the following exercises provide two solutions. The first solution must use the standard iteration constructs of Java; the second solution must use methods of the Stream interface. Futhermore, use Java generics whenever possible.

Write a static method sumOdd that given a List of integers computes the sum of the values that are odd.

Goal: Warming up!

Expected output: A properly commented Java source file containing the two requested methods.

Implement a generic class ImmutablePair<T1,T2>. Besides a suitable constructor your class should provide also methods to get each element individually. Then, write a static method someCalculation (not in ImmutablePair<T1,T2>) that given a List<Double> lst returns an object of ImmutablePair<Integer,Double> class. The first element of the pair is the number of elements of lst in the range [0.2, Math.PI], and the second element is the average of the values of lst in the range [10, 100].

Goal: Experimenting with the basics of Java's Stream API.

Expected output: Properly commented Java files for the classes and for the requested methods.

Write a static method repl that given an array xs of Object and a integer n returns an array containing the elements of xs replicated n times in any order.

Hint: For the stream-based version, consider the flatMap method of Stream.

Question: What happens if the parameter xs has type List<T>? How do you need to change repl to return an array of T in this case?

Goal: Exploring the Stream API further and its relation with types in Java.

Expected output: A properly commented Java source file implementing the requested method.

Write a static method titlecase that given a string s converts it to titlecase by upper-casing the first letter of every word.

Goal: Re-implementing an old Haskell exercise in Java, comparing the solutions.

Expected output: A properly commented Java source file implementing the requested method.

Write a static method zipWithIndex that given an array of type T returns a Stream<ImmutablePair<T, Integer>> containing the elements of the array with its indexes. Use the method just defined to implement another static method filterOdd that given an array xs returns a new array obtained from xs by removing the elements at odd positions.

Hint: Here "odd positions" means the first, third, fifth, etc position.

Goal: Re-implementing an old Haskell exercise in Java, comparing the solutions (pt. II).

Expected output: A properly commented Java source file implementing the requested method.

Write a static method replaceWord that given the name of a text file (String fileName), a word (String word) and a replacement (String repl), prints the content of fileName where each occurrence of word is replaced with repl. Note that the structure, i.e. lines, of fileName must be preserved. Test your method with the file people.csv, replacing for example true with false.

Hint: Consider the method Files.lines of the Java API.

Goal: Working with more advanced aspects of Java's Stream API.

Expected output: A properly commented Java source file implementing the requested method.

Implement a static method serialEvenSum that given a long threshold as parameter computes the sum of even numbers up to the threshold using the Stream API. Futhermore, write a static method parallelEvenSum that does the same but using a parallel stream. Finally, write another static method testSum that takes a long threshold as argument, runs the two methods just defined, measures and compares their running time. Invoke testSum for increasing values of the threshold and determine for which values of threshold using serial streams is better than using the parallel ones and vice versa.

Hint: Consider the method System.nanoTime to measure time.

Goal: Working with more advanced aspects (parallelism) of Java's Stream API.

Expected output: A properly commented Java source file implementing the requested method.

Write a method getElement implementing a partial function that given an array arr and an int index returns arr[index] if defined. Use Optional as the result type. Next write methods implementing the following partial functions:

• sqrt, that applied to an integer returns its square root as a double, if the argument is not negative;
• half, that applied to an integer returns its half as an integer, if the argument is even.

Finally write a method that composing getElement, sqrt and half, when applied to an array of integers arr and to an int index, returns the square root of the half of arr[index], if defined.

Goal: Working with partial functions in Java, exploiting the Optional class.

Expected output: A properly commented Java source file implementing the requested methods.

Write a static method listDir that given the path of a directory as argument returns a List<String> containing the names of all the subdirectories recursively contained in it.

Hint: Consider to use the methods of the Files class, e.g. list or walk.

Goal: Accessing system's informations though Java.

Expected output: A properly commented Java source file implementing the requested method.

Consider the attached csv file people.csv. This file stores information about people subscribed to a simple service. Each line of the file represents a record. The fields of each record are separated by a comma "," and have the following meaning

id,firstname,surname,title,address,town,country,postcode,subscription paid,gender,date of birth

In a record the fields id, firstname, surname, date of birth, subscription paid are mandatory, while the others are optional (denoted by "-" in the file). Implement a class Subscriber representing a subscriber, providing a property for each field. Use the Optional class to denote optional fields.

Write a static method loadDatabase that returns a List<Subscriber> containing the records of the file subscriber.csv. Furthermore, implement another static method PaymentFromGB that given a List<Subscriber> prints the subscribers from GB that have paid the annual fee.

Hint: Use the methods of Optional class to deal with optional values.

Goal: Working with more advanced aspects of Java's Stream API.

Expected output: A properly commented Java source file implementing the requested method.