Introduction to Java 8

Introduction to Java 8Introduction to Java 8 Manish Mishra, Software Consultant, Knoldus Software, LLP Manish Mishra, Software Consultant, Knoldus Software, LLP

Agenda ● Lambda Expressions ● Functional Interfaces ● Default Methods ● Method References ● Streams API ● Why Streams? ● Operations on Streams

What is Lambda Lambda is an anonymous function which can be passed around in a concise way.

Lambda Expressions: An example How about sorting a list of Apples?

Lambda Expressions: An example How about sorting a list of Apples? To sort any object in Java, we need to understand the Comparator Interface.

Lambda Expressions: An example How about sorting a list of Apples? To sort any Object in Java, we need to implement the Comparator Interface, which defines logic for: How to order two objects.

Lambda Expressions: An example How about sorting a list of Apples? Comparator<Apple> byWeight = new Comparator<Apple>() { @Override public int compare(Apple o1, Apple o2) { return o1.getWeight().compareTo(o2.getWeight()); } };

Lambda Expressions: An example How about sorting a list of Apples? Comparator<Apple> byWeight1 = (Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight());

Constructing Lambda A lambda expression is composed of three parts: A list of parameters: (Apple a1, Apple a2)

Constructing Lambda A lambda expression is composed of three parts: A list of parameters, an arrow: (Apple a1, Apple a2) ->

Constructing Lambda A lambda expression is composed of three parts: A list of parameters, an arrow and a body: (Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight());

Valid Lambda Expressions Here are some examples of valid lambda expressions: ● (String anyString) -> anyString.length()

Valid Lambda Expressions Here are some examples of valid lambda expressions: ● (String anyString) -> anyString.length() ● (Apple a) -> a.getWeight() > 150

Valid Lambda Expressions Here are some examples of valid lambda expressions: ● (String anyString) -> anyString.length() ● (Apple a) -> a.getWeight() > 150 ● () -> 47

Valid Lambda Expressions Here are some examples of valid lambda expressions: ● (String anyString) -> anyString.length() ● (Apple a) -> a.getWeight() > 150 ● () -> 47 ● (int a, int b) -> { System.out.println("Sqaure:"); System.out.println(a*b); }

Functional Interfaces A functional interface contains only a one abstract method. In Java 8 annotations, It is annotated as @FunctionalInterface

Functional Interfaces: Examples ● Runnable is a functional interface with an abstract method run(). ● Comparator<T> is a functional interface with an abstract method compareTo()

Why functional Interface A functional interface let you describe a lambda function. In other words, Lambda expressions are used to instantiate a functional interface. @FunctionalInterface interface Calculation { int calculate(int a, int b); }

Why functional Interface A functional interface let you describe a lambda function. In other words, Lambda expressions are used to instantiate a functional interface. @FunctionalInterface interface Calculation { int calculate(int a, int b); } Calculation addition = (Int a, Int b) → (a+b)

Function Descriptor The signature of an abstract method of a functional interface is called function descriptor. @FunctionalInterface interface Calculation { int calculate(int a, int b); }

Function Descriptor: Example The signature of an abstract method of a functional interface is called function descriptor. @FunctionalInterface interface Calculation { int calculate(int a, int b); } Calculation addition = (Int a, Int b) → (a+b)

Function Descriptor: Example The signature of an abstract method of a functional interface is called function descriptor. @FunctionalInterface interface Calculation { int calculate(int a, int b); } Calculation subtraction = (Int a, Int b) → (a-b)

Interface's default methods An interface can declare a method and can provide a default implementation for it.

Interface's default methods An interface can declare a method and can provide a default implementation for it. private interface InterfaceOrTrait{ default String alreadyDone() { return "Default implementation of method String"; } }

Why Interface's default methods ● Adding enhancement to language without hurting the backward compatibility.

Why Interface's default methods ● Adding enhancement to language without hurting the backward compatibility. ● Method for a default situation where any implementing class fails to implement certain methods in the Interface e.g. interface Iterator<T> { boolean hasNext(); T next(); default void remove() { throw new UnsupportedOperationException(); } }

Method References ● Provides syntax to refer directly to existing methods ● Can pass existing methods just like lambdas to functions ● It is better to re use the existing methods instead of creating lambda expressions for them.

Method References: Example Lambda Method reference equivalent (Apple a) -> a.getWeight() Apple::getWeight () -> Thread.currentThread().dumpStack() Thread.currentThread()::dumpStack (str, i) -> str.substring(i) String::substring (String s) -> System.out.println(s) System.out::println

Stream API Streams are monads which help writing programs in a declarative way, chaining multiple operations which are safe and bug free.

Why Streams ● Advantages over Collections ● No Storage ● Laziness-seeking ● Functional in nature ● No Bounds (infinite streams) ● Parallelizable

Streams API: An Example Comparator<Apple> byWeight = new Comparator<Apple>() { @Override public int compare(Apple o1, Apple o2) { return o1.getWeight().compareTo(o2.getWeight()); } }

Streams API: An Example listOfApples.add(new Apple("green",12.0)); listOfApples.add(new Apple("red",12.0)); listOfApples.add(new Apple("green",17.0)); listOfApples.add(new Apple("yellow",12.0)); listOfApples.stream() .filter( apple -> apple.getColor().equals("green")) .sorted(byWeight) .forEach(System.out::println);

Creating Streams ● From a Collections List<Apple> listOfApples = new ArrayList(); listOfApples.add(new Apple("yellow",12.0)); Stream<Apple> apples = listOfApples.stream()

Creating Streams ● From a Collections List<Apple> listOfApples = new ArrayList(); listOfApples.add(new Apple("yellow",12.0)); Stream<Apple> apples = listOfApples.stream() ● From Primitive values Stream<String> wordStream = Stream.of("Value1 ", "Value2", "Value3", "Value4");

Creating Streams ● From a Collections List<Apple> listOfApples = new ArrayList(); listOfApples.add(new Apple("yellow",12.0)); Stream<Apple> apples = listOfApples.stream() ● From Primitive values Stream<String> wordStream = Stream.of("Value1 ", "Value2", "Value3", "Value4"); ● From An Array int [] primitives = {2,4,6,8,10}; Stream<String> stream = Arrays.stream(primitives);

Operations on Streams ● Reduction Double totalWeight = listOfApples.stream().mapToDouble(Apple::getWeight).sum(); Double totalReducedWeight = listOfApples.stream().mapToDouble(Apple::getWeight).reduce(0,(a,b) -> a+b);

Operations on Streams ● Reduction Double totalWeight = listOfApples.stream().mapToDouble(Apple::getWeight).sum(); Double totalReducedWeight = listOfApples.stream().mapToDouble(Apple::getWeight).reduce(0,(a,b) -> a+b); The reduce operation in this example takes two arguments: ● identity: The identity element is both the initial value of the reduction ● accumulator: The accumulator function takes two parameters: a partial result of the reduction and the next element of the stream

Operations on Streams ● Grouping Map<String, List<Apple>> applesByColor = listOfApples.stream().collect(Collectors.groupingBy(Apple::getColor)); The above method uses groupingBy method to join the apples by color and returns a Map of type <String, List<Apple>

Operations on Streams ● Stats DoubleSummaryStatistics weightStats = listOfApples.stream().mapToDouble(Apple::getWeight).summaryStatistics(); System.out.println("The max of Double" + weightStats.getAverage()); System.out.println("The min of Double" + weightStats.getMin()); System.out.println("The max of Double" + weightStats.getMax()); The DoubleSummaryStatistics class contain the stats like count, max, min etc.

References ● Java 8 In Action: Book by: Mario-Fusco,Alan Mycroft, Raoul- Gabriel Urma

Introduction to Java 8

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Introduction to Java 8