CS1021 - Functional Programming

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.title[
      # Functional Programming
    ]

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---
    # OO versus Functional

* Object-oriented: **group** data with operations
    --

* Functional: treat operations **as** data

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## OO

* modular and encapsulated
    * Code reuse through inheritance and composition
    * Functionality and data tightly coupled

## Functional

* Can define functionality and pass it as a parameter
    * Well suited for concurrent and event driven

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# Java 8 
    ## Added many functional features

---
    # Functional Characteristics

* Function call can be replace by the result
    --

* Functions are not allowed to have side effects
    --

* Compiler is allowed to rearrange function invocations
    --
    ... or run on different threads
    --

* Program flow driven by data dependencies, not order of instructions

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* Data must be immutable
    * If changed, new copy is created

---
    # First-Class Functions

Functions can be
    * Assigned to variables
    --

* Passed as arguments to other functions
    --

* Returned by other functions

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# Lambda Functions

```
    Button button = new Button("Run");
    button.setOnAction(`event -> run(event)`);
    ```
    --

* `run(event)` is a function that is passed to a method
    * Under the hood it is really the implementation of an interface
    --

```
    Button button = new Button("Run");
    button.setOnAction(`new EventHandler<ActionEvent>() {`
        `@Override`
        `private void handle(ActionEvent event) {`
        `    run(event);`
        `}}`);
    ```

---
    # Functional Interface

```
    @FunctionalInterface
    public interface EventHandler<T> {
        void handle(T event);
    }
    ```

* An interface with only one method
    --

* Annotated as a `@FunctionalInterface`
    --

* Can use lambda expression

---
    # `Collection` and `Stream` Interfaces

* `Collection` 
      - Efficient management of and access to their elements 
    --

* `Stream` 
      - A sequence of elements supporting sequential and parallel aggregate operations
    --

- A declarative description of their source and computational operations which will be performed in aggregate on that source

---
    # `Stream` Methods

* `count()` - returns the number of elements in the stream
    * `distinct()` - returns a stream with duplicates removed
    * `limit(long maxSize)` - returns a stream with no more than `maxSize` elements
    * `skip(long count)` - returns a stream without the first `count` elements
    * `sorted()` - returns a stream in sorted order

---
    # Example - `count()` and `distinct()`

```
    List<String> words = Arrays.asList("a", "function", "that", "returns", "a", "function");

long count = words.stream()
                      .count();
    long numberUnique = words.stream()
                             .distinct()
                             .count();

System.out.println("Count: " + count + " Unique: " + numberUnique);
    ```
    --

#### Output:
    ```diff
    Count: 6 Unique: 4
    ```

---
    # Higher-Order Functions
    * Accept other functions as argument(s)
    * Return a function as a result
    --

* E.g., `setOnAction(EventHandler<ActionEvent> e)`

---
    # Higher-Order Functions in `Stream`
    * `allMatch(Predicate<? super T> predicate)`
    * `anyMatch(Predicate<? super T> predicate)`
    * `noneMatch(Predicate<? super T> predicate)`
    * `filter(Predicate<? super T> predicate)`

---
    # `Predicate` Interface

```
    @FunctionalInterface
    public interface Predicate<T> {
        boolean test(T t);
    }
    ```
    .right[$ ^* $Not showing `default` methods]

---
    # Higher-Order Functions in `Stream` ... again
    * `allMatch(Predicate<? super T> predicate)`
    * `anyMatch(Predicate<? super T> predicate)`
    * `noneMatch(Predicate<? super T> predicate)`
    * `filter(Predicate<? super T> predicate)`

---
    # Example - `filter()`

```
    List<String> words = Arrays.asList("a", "function", "that", "returns", "a", "function");

Predicate<String> isShort = (word -> word!=null && word.length()<5);

Object[] shortWords = words.stream()
                               .filter(isShort)
                               .toArray();

for(Object word : shortWords) {
        System.out.print(word + " ");
    }
    System.out.println();
    ```
    --

#### Output:
    ```diff
    a that a
    ```

---
    # More Higher-Order Functions in `Stream`
    * `map(Function<? super T, ? extends R> mapper)`
    * `mapToDouble(Function<? super T, ? extends R> mapper)`
    * `mapToInt(Function<? super T, ? extends R> mapper)`

---
    # `Function` Interface

```
    @FunctionalInterface
    public interface Function<T, R> {
        R apply(T t);
    }
    ```
    .right[$ ^* $Not showing `default` methods]

---
    # Example - `map()`

```
    List<String> words = Arrays.asList("a", "function", "that", "returns", "a", "function");

List<String> lengths = words.stream()
                                .map(word -> word + ": " + word.length())
                                .collect(Collectors.toList());

System.out.println(lengths);
    ```
    --

#### Alternative:
    ```
    Function<String, String> toLength = (word -> word + ": " + word.length());
    words.stream().map(toLength);
    ```
    --

#### Output:
    ```diff
    [a: 1, function: 8, that: 4, returns: 7, a: 1, function: 8]
    ```

---
    # Numeric Streams

.center[`IntStream`, `DoubleStream`, and `LongStream`]

* `sum()`
    * `max()`
    * `min()`
    * `average()`
    * `summaryStatistics()`
    * `reduce()`

---
    # Example - `mapToInt()`

```
    int sum = words.stream()
                   .mapToInt(w -> w.length());
                   .sum();
    ```
    --

#### Output:
    ```diff
    Sum of lengths of all strings: 29
    ```
    ---
    # `Consumer` Interface

```
    @FunctionalInterface
    public interface Consumer<T> {
        void accept(T t);
    }
    ```
    .right[$ ^* $Not showing `default` methods]

---
    # Example - `forEach()`

```
    List<String> words = Arrays.asList("a", "function", "that", "returns", "a", "function");

words.stream()
         .forEach(w -> System.out.println(w + ": " + w.length()));
    ```
    --

#### Output:
    ```diff
    a: 1
    function: 8
    that: 4
    returns: 7
    a: 1
    function: 8
    ```

Note: we can do `forEach()` directly on a `List`:
    ```
    words.forEach(w -> System.out.println(w + ": " + w.length()));
    ```

---
    # Extract Even Numbers

```
    Stream<Integer> numbers = Stream.of(3, 8, 7, 2, 1, 9, 8);
    List<Integer> evens = numbers.filter(i -> i%2==0)
                                 .collect(Collectors.toList());

System.out.println(evens);
    ```
    --

#### Output:
    ```
    [8, 2, 8]
    ```

---
    # Extract Based on Suffix

```
    System.out.println(Stream.of("happy", "discussion" /* ... */, "locomotion")
                             .filter(word -> word.endsWith("tion"))
                             .count());
    ```
    --

#### Output:
    ```
    1
    ```

---
    # Extract Circles

```
    Stream<Shape> shapes = Stream.of(new Circle(0, 0), new Square(0, 0, 4), new Circle(0, 0), 
                                     new Triangle(0, 0, 1, 5));

List<Circle> circles = shapes.filter(shape -> shape instanceof Circle)
                                 .map(shape -> (Circle)shape)
                                 .collect(Collectors.toList());
    ```

---
    # Sum

```
    double totalArea = shapes.mapToDouble(shape -> shape.getArea())
                             .sum();

System.out.println("Total area: " + totalArea);
    ```
    --

#### Output:
    ```
    Total area: 38.239208802178716
    ```

---
    # Average

```
    OptionalDouble averageArea = shapes.mapToDouble(Shape::getArea)
                                       .average();

if(averageArea.isPresent()) {
        System.out.println("Average area: " + averageArea.getAsDouble());
    }
    ```
    --

#### Output:
    ```
    Average area: 9.559802200544679
    ```

---
    # Summary Statistics

```
    DoubleSummaryStatistics areaStats = shapes.mapToDouble(Shape::getArea)
                                              .summaryStatistics();

System.out.println("Number of shapes: " + areaStats.getCount());
    System.out.println("Total area: " + areaStats.getSum());
    System.out.println("Average area: " + areaStats.getAverage());
    System.out.println("Maximum area: " + areaStats.getMax());
    System.out.println("Minimum area: " + areaStats.getMin());
    ```
    --

#### Output:
    ```
    Number of shapes: 4
    Total area: 38.239208802178716
    Average area: 9.559802200544679
    Maximum area: 16.0
    Minimum area: 2.5
    ```

---
    # Average with `reduce()`

```
    double area = shapes.map(Shape::getArea)
                        .reduce(0.0, (a, b) -> a+b);

System.out.println("Total area: " + area);
    ```
    --

#### Output:
    ```
    Total area: 38.239208802178716
    ```