Building Java Programs

Lab: Functional Programming

Lab goals

Goals for this problem set:

learn the basics of functional programming
practice working with Java streams
perform map, filter, and reduce operations

Functional Programming

functional programming: Emphasizes the use of functions (methods) to decompose a complex task into subtasks.

Key components:

Effect-free programming
Processing structured data via functions
First-class functions
Function closures
Higher-order operations on collections

side effect: A change to the state of a program or object caused by a call on a function.

Exercise : sideEffectDoubleAll

What side effect does the following function have? How could it be rewritten to avoid side effects?

// Doubles the values of all elements in an array.
public static void doubleAll(int[] a) {
    for (int i = 0; i < a.length; i++) {
        a[i] = 2 * a[i];
    }
}

It multiplies the values by 2 when it should multiply them by themselves (a[i] * a[i]).
It operates only on arrays of int when it should accept a type parameter T.
It should println the new values rather than storing them into the array.
It modifies the array that was passed in. It should return the new array state.

Lambdas

lambda: An expression that describes a function by specifying its parameters and the value that it returns. Syntax:

(parameters) -> expression

Example: a lambda that accepts an integer and returns that integer plus 1:

(x) -> x + 1

Exercise : `lambdaSquare`

Write a lambda expression that converts an integer into the square of that integer. For example, 4 would become 16.

Exercise : `lambdaLastFirst`

Write a lambda expression that accepts two strings representing a first and last name and concatenates them together into a string in "Last, First" format. For example, if passed "Cynthia" and "Lee", it would return "Lee, Cynthia".

Streams

stream: A sequence of elements from a data source that supports aggregate operations.

Common stream operations:

map: Apply a given operation to each element of the stream.
filter: Retain or remove each element from the stream based on a boolean test.
reduce: Combine the elements in some way into a single result.

// Example: sum of squares of odd integers in a stream
int sum = IntStream.of(3, 1, 4, 1, 5, 9, 2, 6, 5, 3)
    .filter(n -> n % 2 != 0)
    .map(n -> n * n)
    .sum();

Exercise : `largestEven`

Write a method largestEven that uses stream operations to find and return the largest even number from an array of integers. For example, if the array is {5, -1, 12, 10, 29, 2, 8}, your method should return 12. You may assume that the array contains at least one even integer.