Java 8 Intro - Core Features

Java 8 Intro - Core Features

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  Java 8 intro: Core features Spring 2016 Arkadii Tetelman
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  2 Java 8 NewFeatures (Core and Collections) • Interface’s Default (default methods) • Functional Interfaces • Lambdas • Method References • Streams (Stream API) • Optional
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  3 Interface’s Default (defaultmethods) Java 8 extends interface declarations with two new items: • default (default method implementation) • static methods • functional interface
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  4 Default Method Default methodsadds default method implementation that would be used in the case when the implementer class won’t implement this method . default is a new keyword in Java8 syntax When you extend an interface that contains a default method, you can do the following: • Not mention the default method at all, which lets your extended interface inherit the default method. • Re-declare the default method, which makes it abstract. • Redefine the default method, which overrides it.
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  5 Default Method codeexample public interface DefaultExample { default String defaultMethod() { return "Default implementation"; } } private class DefaultableImpl implements DefaultExample { } protected abstract class DefaultableAbstractImpl implements DefaultExample { public abstract String defaultMethod(); } class OverrideImpl implements DefaultExample { @Override public String defaultMethod() { return "Overridden implementation"; } }
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  6 Static Methods ininterfaces with example Another greate feature delivered by Java 8 is that interfaces can declare (and provide implementation) of static methods. Here is an example: public interface TimeClient { static public ZoneId getZoneId (String zoneString) { try { return ZoneId.of(zoneString); } catch (DateTimeException e) { return ZoneId.systemDefault(); } } default public ZonedDateTime getZonedDateTime(String zoneString) { return ZonedDateTime.of(getLocalDateTime(), getZoneId(zoneString)); } LocalDateTime getLocalDateTime(); // Should be implemented!!!!!!! }
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  7 Functional Interfaces In Java8 a functional interface is defined as an interface with exactly one abstract method. New annotation @FunctionalInterface //Invalid @FunctionalInterface public interface SomeInterfaceWithZeroAbstractMethods {} //Invalid @FunctionalInterface public interface SomeInterfaceWithTWOAbstractMethods { void firstAbstractMethodMethod(); int second(int k); } @FunctionalInterface public interface SomeInterfaceWithONEAbstractMethods { void f(); // abstract static int g(int k) {return 0;} // static default void h() {} // default }
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  8 Built-in functional interfaces@ Java 8 Function Method Example Supplier<T> T get() Supplier<String> stringSupplier = String::new; String newInteger = stringSupplier.get(); // creates new String Consumer<T> void accept(T t) List<String> one = Arrays.asList("A","AB","ABA","ABBA"); Consumer<String> style = (String s) -> System.out.println("Item:"+s); one.forEach(style); Predicate<T> boolean test(T t); Predicate<Double> isNegative = x -> x < 0; System.out.println(isNegative.test(new Double(1))); Function<T,R> R apply(T t); Function<String, Integer> toInteger = Integer::valueOf; Integer test = toInteger.apply("2016");
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  9 Lamdas (Lambda expressions) Alambda expression (lambda) is a short-form replacement for an anonymous class. Lambdas simplify the use of interfaces that declare single abstract methods, that known as functional interfaces. Core points: • A lambda expression is a block of code with optional parameters • Lambda expression is the best choice whenever you want a block of code executed at a later point in time • Lambda expression reduces amount of code • Lambda expression used everywhere in Java8 (StreamAPI, Optionals, etc)
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  10 Lambda expressions syntax LambdaDescription () -> 4; takes no value and returns 4; x -> 3 * x; takes a number and returns the result of tripling it (x, y) -> x – y; takes two numbers and returns their difference (int x, int y) -> x + y; takes two integers and returns their sum (String s) -> { System.out.print("YES"+s); System.err.print("YES"+s); } takes a string and prints it to console and err output with “YES” suffix New keyword -> (minus with bigger, AKA structure dereference in C++) (parameters) ->expression or (parameters) ->{ statements;} Lambda Expression and Statements examples:
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  11 Method References List<String> one= Arrays.asList("A","AB","ABA","ABBA"); //Reference to a static method Collections.sort(one, Comparator.comparing((Function<String, Integer>) (s) -> s.length())); Collections.sort(one, Comparator.comparing(String::length)); List<String> two = Arrays.asList("1","2","3","4"); //Reference to an instance method of a particular object two.forEach(System.out::println); two.forEach((x) -> System.out.println(x)); //Reference to an instance method of an arbitrary object of a particular type two.forEach(String::toString); two.forEach((s) -> s.toString()); //Reference to a constructor List<Integer> integers = two.stream().map(Integer::new).collect(Collectors.toList()); integers = two.stream().map((s) -> new Integer(s)).collect(Collectors.toList());
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  12 Streams (Stream API) Anewly added Stream API (java.util.stream) introduces real-world functional-style programming into the Java. A stream represents a sequence of elements and supports different kind of operations to perform computations upon those elements. Stream operations are • intermediate • terminal Intermediate operations return a stream so we can chain multiple intermediate operations without using semicolons. Terminal operations are either void or return a non-stream result. In the above example filter, map and sorted are intermediate operations whereas forEach is a terminal operation. For a full list of all available stream operations see the Stream Javadoc.
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  13 • Streams inCollection with some examples Streams can be created from various data sources ,especially collections Kind of Streams: • stream() – process elements one by one in single thread • parallelStream() – process elements in several threads Operation Code Example Filter (Intermediate) stringCollection.stream().filter((s) -> s.startsWith(“A")) Sorted (Intermediate) stringCollection.parallelStream().sorted((a, b) -> b.compareTo(a)); Map (Intermediate) stringCollection.stream().map(String::toUpperCase); Match (Terminal) stringCollection.stream().anyMatch((s) -> s.startsWith("a")) stringCollection.stream().noneMatch((s) -> s.startsWith("z")) Count (Terminal) stringCollection.stream().filter((s) -> s.startsWith(“B")).count();
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  14 • Streams inCollection with some examples #2 Operation Code Example Collect (Terminal) stringCollection.stream().map(String::toUpperCase). .collect(Collectors.toList()); Reduce (Terminal) List<Integer> list= Arrays.asList(1,2,3,4); System.out.println(list.stream().reduce(0,Integer::sum)); Distinct (Intermediate) List<Integer> list= Arrays.asList(1,2,3,4,5,4,5); System.out.println(list.stream().distinct().reduce(0,Integer::sum)); Peek (Intermediate) List<String> list= Arrays.asList(“1”,”2”,”3”,”4”,”5”); list.stream().map(String::toUpperCase).peek((e)->System.out.print(e)). collect(Collectors.toList()) Skip (Intermediate) List<String> list= Arrays.asList(“1”,”2”,”3”,”4”,”5”); list.stream().map(String::toUpperCase).skip(2). collect(Collectors.toList())
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  15 Optional vs NullPointerException The… NullPointerException is by far the most popular cause of Java application failures. Long time ago the great Google Guava project introduced the Optional as a solution to NullPointerException, discouraging codebase pollution with null checks and encouraging developers to write cleaner code. Inspired by Google Guava, the Optional is now a part of Java 8 library Here is basic example Optional<String> optional = Optional.of("Some value"); optional.isPresent(); // true optional.get(); // "Some value" optional.orElse("fallback"); // "Some value" optional.ifPresent((s) -> System.out.println(s.charAt(2))); // “m"
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  16 Questions? Email: arkadii_tetelman@globallogic.com