Modern Programming in Java 8 Lambdas, Streams and Date/Time API
GANESH & HARI CODEOPS TECHNOLOGIES
[email protected]@codeops.tech
Adapt: Learn functional programming
Agenda• Introduction & Overview
• Lambdas
• Functional interfaces
• Streams
• Parallel Streams
• Date & Time package
• Refactoring to Java 8
Java meets functional programming (with lambdas)
Java is not your grandma’s language anymore!
Greek characters are scary!
He he, but lambdas are fun, not scary
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");Consumer<String> printString = string -> System.out.println(string); strings.forEach(printString);
Lambda functions!
But what are lambdas?
Lambdas is just a fancy name for functions
without a name!
What are lambda functions?
❖ (Java 8) One way to think about lambdas is “anonymous function” or “unnamed function” - they are functions without a name and are not associated with any class
❖ They don’t change external state
What is functional programming?
❖ Functional languages view programs as an entity—called a function—that accepts inputs and produces output
❖ Functions are connected together by their outputs to other functions’ inputs
❖ Underlying approach: “Evaluate an expression. Then use the results for something else.”
Perspective - for loops!List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");for(String string : strings) {
System.out.println(string);}
External Iteration
Perspective - for loops!
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");strings.forEach(string -> System.out.println(string));
Internal Iteration
Perspective - for loops!
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");strings.forEach(string -> System.out.println(string));
Internal Iteration
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");for(String string : strings) {
System.out.println(string);}
External Iteration
Perspective - for loops!
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");strings.forEach(string -> System.out.println(string));
Internal Iteration
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");for(String string : strings) {
System.out.println(string);}
External Iteration
Procedural thinking
Functional thinking
You can use lambdas for some amazing stuff
sediment
pre-carbon
ultra-filter
post-carbon
Filtered water
E.g., you can compose lambda functions as in pipes-and-filters
$ cat limerick.txt There was a young lady of Niger Who smiled as she rode on a tiger. They returned from the ride With the lady inside And a smile on the face of the tiger.
$ cat limerick.txt | tr -cs "[:alpha:]" "\n" | awk '{print length(), $0}' | sort | uniq
1 a 2 as 2 of 2 on 3 And 3 Who 3 she 3 the 3 was 4 They 4 With 4 face 4 from 4 lady 4 ride 4 rode 5 Niger 5 There 5 smile 5 tiger 5 young 6 inside 6 smiled 8 returned
List<String> lines = Files.readAllLines(Paths.get("./limerick.txt"), Charset.defaultCharset());
Map<Integer, List<String>> wordGroups = lines.stream() .map(line -> line.replaceAll("\\W", "\n").split("\n")) .flatMap(Arrays::stream) .sorted() .distinct() .collect(Collectors.groupingBy(String::length));
wordGroups.forEach( (count, words) -> { words.forEach(word -> System.out.printf("%d %s %n", count, word)); });
1 a 2 as 2 of 2 on 3 And 3 Who 3 she 3 the 3 was 4 They 4 With 4 face 4 from 4 lady 4 ride 4 rode 5 Niger 5 There 5 smile 5 tiger 5 young 6 inside 6 smiled 8 returned
Lambdas & streams help in productive programming!
public static void main(String []file) throws Exception { // process each file passed as argument
// try opening the file with FileReader try (FileReader inputFile = new FileReader(file[0])) { int ch = 0; while( (ch = inputFile.read()) != -1) { // ch is of type int - convert it back to char System.out.print( (char)ch ); } } // try-with-resources will automatically release FileReader object }
public static void main(String []file) throws Exception { Files.lines(Paths.get(file[0])).forEach(System.out::println); }
Existing APIs are enriched with lambdas and streams support
Java 8 is the new Groovy ;-)importjava.io.*;
classType{ publicsta7cvoidmain(String[]files){ //processeachfilepassedasargument for(Stringfile:files){ //tryopeningthefilewithFileReader try(FileReaderinputFile=newFileReader(file)){ intch=0; while((ch=inputFile.read())!=-1){ //chisoftypeint-convertitbacktochar System.out.print((char)ch); } }catch(FileNotFoundExcep7onfnfe){ System.err.prinR("Cannotopenthegivenfile%s",file); } catch(IOExcep7onioe){ System.err.prinR("Errorwhenprocessingfile%s;skippingit",file); } //try-with-resourceswillautoma7callyreleaseFileReaderobject } }}
args.each{printlnnewFile(it).getText()}
Agenda• Introduction & Overview
• Lambdas
• Functional interfaces
• Streams
• Parallel streams
• Date & Time package
• Refactoring to Java 8
Java 8 lambdas - “Hello world!”
interface LambdaFunction { void call(); }
class FirstLambda { public static void main(String []args) { LambdaFunction lambdaFunction = () -> System.out.println("Hello world"); lambdaFunction.call(); } }
Java 8 lambdas - “Hello world!”
interface LambdaFunction { void call(); }
class FirstLambda { public static void main(String []args) { LambdaFunction lambdaFunction = () -> System.out.println("Hello world"); lambdaFunction.call(); } }
Functional interface - provides signature for lambda functions
Lambda function/expression
Call to the lambda
Prints “Hello world” on the console when executed
Parts of a lambda expression
() -> System.out.println("Hello world");
No parameters, i.e., ()
Arrow operator that separates parameters and the body The lambda body
Return type “void” inferred from the body
Method references
Method references - “syntactic sugar” for lambda functions
They “route” function parameters
arg -> System.out.println(arg)
System.out::println
Method references
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo"); Consumer<String> printString = System.out::println; strings.forEach(printString);
Method reference
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo"); Consumer<String> printString = string -> System.out.println(string); strings.forEach(printString);
Method references
Cannot use method references when lambda functions do more than“routing” function parameters
strings.forEach(string -> System.out.println(string.toUpperCase()));
More processing here than just “routing” parameters
Method references List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo"); Consumer<String> printString = System.out::println; strings.forEach(printString);
public static void printUpperCaseString(String string) { System.out.println(string.toUpperCase()); }
strings.forEach(MethodReference::printUpperCaseString);
“Effectively final” variables
import java.util.Arrays; import java.util.List;
class PigLatin { public static void main(String []args) { String suffix = "ay"; List<String> strings = Arrays.asList("one", "two", "three", "four"); strings.forEach(string -> System.out.println(string + suffix)); } } Accessing “local variable” suffix
here; hence it is considered “effectively final”
“Effectively final” variablesimport java.util.Arrays; import java.util.List;
class PigLatin { public static void main(String []args) { String suffix = "ay"; List<String> strings = Arrays.asList("one", "two", "three", “four");
suffix = "e"; // assign to suffix variable strings.forEach(string -> System.out.println(string + suffix)); } }
PigLatinAssign.java:9: error: local variables referenced from a lambda expression must be final or effectively final strings.forEach(string -> System.out.println(string + suffix)); ^ 1 error
Agenda• Introduction & Overview
• Lambdas
• Functional interfaces
• Streams
• Parallel streams
• Date & Time package
• Refactoring to Java 8
Functional interfaces
@FunctionalInterface interface LambdaFunction { void call(); }
Functional interface
Abstract method providing the signature of the lambda function
Annotation to explicitly state that it is a functional interface
Java 8 lambdas - “Hello world!”
@FunctionalInterface interface LambdaFunction { void call(); }
class FirstLambda { public static void main(String []args) { LambdaFunction lambdaFunction = () -> System.out.println("Hello world"); lambdaFunction.call(); } }
Functional interface - provides signature for lambda functions
Lambda function/expression
Call to the lambda
Prints “Hello world” on the console when executed
Older Single Abstract Methods (SAMs)
// in java.lang package interface Runnable { void run(); }
// in java.util package interface Comparator<T> { boolean compare(T x, T y); }
// java.awt.event package: interface ActionListener { void actionPerformed(ActionEvent e) }
// java.io package interface FileFilter { boolean accept(File pathName); }
Functional interfaces: Single abstract methods
@FunctionalInterface interface LambdaFunction { void call();
// Single Abstract Method (SAM) }
Using built-in functional interfaces// within Iterable interface default void forEach(Consumer<? super T> action) { Objects.requireNonNull(action); for (T t : this) { action.accept(t);
} }
// in java.util.function package @FunctionalInterface public interface Consumer<T> {
void accept(T t); // the default andThen method elided
}
Using built-in functional interfaces
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo"); Consumer<String> printString = string -> System.out.println(string); strings.forEach(printString);
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo"); strings.forEach(string -> System.out.println(string));
Default methods in interfacespublic interface Iterator<E> {
boolean hasNext();
E next();
default void remove() { throw new UnsupportedOperationException("remove");
}
default void forEachRemaining(Consumer<? super E> action) { Objects.requireNonNull(action);
while (hasNext()) action.accept(next());
} }
“Diamond” inheritance problem?
“Diamond” inheritance problem?
interface Interface1 { default public void foo() { System.out.println("Interface1’s foo"); } }
interface Interface2 { default public void foo() { System.out.println("Interface2’s foo"); } }
public class Diamond implements Interface1, Interface2 { public static void main(String []args) { new Diamond().foo(); } }
Error:(9, 8) java: class Diamond inherits unrelated defaults for foo() from types Interface1 and Interface2
“Diamond” inheritance problem?
interface Interface1 { default public void foo() { System.out.println("Interface1’s foo"); } }
interface Interface2 { default public void foo() { System.out.println("Interface2’s foo"); } }
public class Diamond implements Interface1, Interface2 { public void foo() { Interface1.super.foo(); }
public static void main(String []args) { new Diamond().foo(); } }
Add this definition to resolve the
ambiguity
“Diamond” inheritance problem?
class BaseClass { public void foo() { System.out.println("BaseClass’s foo"); } }
interface BaseInterface { default public void foo() { System.out.println("BaseInterface’s foo”); } }
public class Diamond extends BaseClass implements BaseInterface { public static void main(String []args) { new Diamond().foo(); } }
Compiles cleanly; Java 8 rules help deal with the
diamond problem
Built-in functional interfaces
Built-in functional interfaces are a part of the java.util.function
package (in Java 8)
Built-in interfacesPredicate<T> Checks a condition and returns a
boolean value as resultIn filter() method in java.util.stream.Stream which is used to remove elements in the stream that don’t match the given condition (i.e., predicate) as Consumer<T> Operation that takes an argument but
returns nothingIn forEach() method in collections and in java.util.stream.Stream; this method is used for traversing all the elements in the collection or Function<T,
R>Functions that take an argument and return a result
In map() method in java.util.stream.Stream to transform or operate on the passed value and return a result.
Supplier<T> Operation that returns a value to the caller (the returned value could be same or different values)
In generate() method in java.util.stream.Stream to create a infinite stream of elements.
Predicate interface
Stream.of("hello", "world") .filter(str -> str.startsWith("h")) .forEach(System.out::println);
The filter() method takes a Predicate as an argument (predicates are
functions that check a condition and return a boolean value)
Predicate interface
Predicate interface
A Predicate<T> “affirms” something as true or false: it takes an argument of type T, and returns a
boolean value. You can call test() method on a Predicate object.
@FunctionalInterface public interface Predicate<T> {
boolean test(T t); // other methods elided
}
Predicate interface: Example
import java.util.function.Predicate;
public class PredicateTest { public static void main(String []args) {
Predicate<String> nullCheck = arg -> arg != null; Predicate<String> emptyCheck = arg -> arg.length() > 0; Predicate<String> nullAndEmptyCheck = nullCheck.and(emptyCheck); String helloStr = "hello"; System.out.println(nullAndEmptyCheck.test(helloStr)); String nullStr = null; System.out.println(nullAndEmptyCheck.test(nullStr));
} }
Prints: truefalse
Predicate interface: Example
import java.util.List; import java.util.ArrayList;
public class RemoveIfMethod { public static void main(String []args) {
List<String> greeting = new ArrayList<>(); greeting.add("hello"); greeting.add("world"); greeting.removeIf(str -> !str.startsWith("h")); greeting.forEach(System.out::println);
} }
Prints: hello
Consumer interface
Stream.of("hello", "world") .forEach(System.out::println);
// void forEach(Consumer<? super T> action);
Prints: helloworld
Consumer interface
Consumer interface
A Consumer<T> “consumes” something: it takes an argument (of generic type T) and returns
nothing (void). You can call accept() method on a Consumer object.
@FunctionalInterface public interface Consumer<T> {
void accept(T t); // the default andThen method elided
}
Consumer interface: Example
Consumer<String> printUpperCase = str -> System.out.println(str.toUpperCase());
printUpperCase.accept("hello");
Prints: HELLO
Consumer interface: Example
import java.util.stream.Stream; import java.util.function.Consumer;
class ConsumerUse { public static void main(String []args) {
Stream<String> strings = Stream.of("hello", "world"); Consumer<String> printString = System.out::println; strings.forEach(printString);
} }
Prints: helloworld
Function interface
import java.util.Arrays;
public class FunctionUse { public static void main(String []args) {
Arrays.stream("4, -9, 16".split(", ")) .map(Integer::parseInt) .map(i -> (i < 0) ? -i : i) .forEach(System.out::println);
} }
Prints: 4916
Function interface
Function interface
A Function<T, R> “operates” on something and returns something: it takes one argument (of
generic type T) and returns an object (of generic type R). You can call apply() method on a Function
object.
@FunctionalInterface public interface Function<T, R> {
R apply(T t); // other methods elided
}
Function interface: Example
Function<String, Integer> strLength = str -> str.length(); System.out.println(strLength.apply("supercalifragilisticexpialidocious"));
Prints: 34
Function interface: Example
import java.util.Arrays; import java.util.function.Function;
public class CombineFunctions { public static void main(String []args) {
Function<String, Integer> parseInt = Integer:: parseInt ; Function<Integer, Integer> absInt = Math:: abs ; Function<String, Integer> parseAndAbsInt = parseInt.andThen(absInt); Arrays.stream("4, -9, 16".split(", "))
.map(parseAndAbsInt)
.forEach(System. out ::println); }
}
Prints: 4916
Supplier interface
import java.util.stream.Stream; import java.util.Random;
class GenerateBooleans { public static void main(String []args) {
Random random = new Random(); Stream.generate(random::nextBoolean)
.limit(2)
.forEach(System.out::println); }
}
Prints two boolean values “true” and “false”
in random order
Supplier interface
Supplier interface
A Supplier<T> “supplies” takes nothing but returns something: it has no arguments and
returns an object (of generic type T). You can call get() method on a Supplier object
@FunctionalInterface public interface Supplier<T> {
T get(); // no other methods in this interface
}
Supplier interface: Example
Supplier<String> currentDateTime = () -> LocalDateTime.now().toString(); System.out.println(currentDateTime.get());
Prints current time: 2015-10-16T12:40:55.164
Summary of built-in interfaces in java.util.function interface
❖ There are only four core functional interfaces in this package: Predicate, Consumer, Function, and Supplier.
❖ The rest of the interfaces are primitive versions, binary versions, and derived interfaces such as UnaryOperator interface.
❖ These interfaces differ mainly on the signature of the abstract methods they declare.
❖ You need to choose the suitable functional interface based on the context and your need.
Agenda• Introduction & Overview
• Lambdas
• Functional interfaces
• Streams
• Parallel streams
• Date & Time package
• Refactoring to Java 8
Java 8 streams (and parallel streams):Excellent example of applying functional
programming in practice
But what are streams?
Arrays.stream(Object.class.getMethods()) .map(method -> method.getName()) .distinct() .forEach(System.out::println);
wait equals toString hashCode getClass notify notifyAll
Method[] objectMethods = Object.class.getMethods(); Stream<Method> objectMethodStream = Arrays.stream(objectMethods); Stream<String> objectMethodNames
= objectMethodStream.map(method -> method.getName()); Stream<String> uniqueObjectMethodNames = objectMethodNames.distinct(); uniqueObjectMethodNames.forEach(System.out::println);
Arrays.stream(Object.class.getMethods()) .map(method -> method.getName()) .distinct() .forEach(System.out::println);
Breaking up into separate (looong)
stream pipelineStreamsource
Intermediateopera1ons
Terminalopera1on
stream
stream
Examples:IntStream.range(),Arrays.stream()
Examples:map(),filter(),dis1nct(),sorted()
Examples:sum(),collect(),forEach(),reduce()
DoubleStream.of(1.0,4.0,9.0) map(Math::sqrt) .peek(System.out::
println)
StreamSource(withelements1.0,4.0,and9.0)
IntermediateOpera=on1(mapsto
elementvalues1.0,2.0,and3.0)
IntermediateOpera=on2
(prints1.0,2.0,and3.0)
.sum();
TerminalOpera=on(returnsthesum6.0)
DoubleStream.of(1.0, 4.0, 9.0) .map(Math::sqrt) .peek(System.out::println) .sum();
IntStream.range(1, 6)
You can use range or iterate factory methods in the
IntStream interface
IntStream.iterate(1, i -> i + 1).limit(5)
1 2 3 4 5
1 4 9 16 25
map(i->i*i)
IntStream.range(1, 5).map(i -> i * i).forEach(System.out::println);
Using streams instead of imperative for i = 1 to 5, print i * i
Stream.of (1, 2, 3, 4, 5) .map(i -> i * i) .peek(i -> System.out.printf("%d ", i)) .count();
prints: 1 4 9 16 25
stream can be infinite
IntStream.iterate(0, i -> i + 2).forEach(System.out::println);
This code creates infinite stream of even numbers!
IntStream .iterate(0, i -> i + 2) .limit(5) .forEach(System.out::println);
Using the “limit” function to limit the stream to 5 integers
IntStream chars = "bookkeep".chars(); System.out.println(chars.count()); chars.distinct().sorted().forEach(ch -> System.out.printf("%c ", ch));
Cannot “reuse” a stream; this code throws IllegalStateException
Streams are lazy!
Files.lines(Paths.get("FileRead.java")).forEach(System.out::println);
This code prints the contents of the file “FileRead.java” in the
current directory
Pattern.compile(" ").splitAsStream("java 8 streams").forEach(System.out::println);
This code splits the input string “java 8 streams” based on whitespace and hence
prints the strings “java”, “8”, and “streams” on the console
new Random().ints().limit(5).forEach(System.out::println);
Generates 5 random integers and prints them on the console
"hello".chars().sorted().forEach(ch -> System.out.printf("%c ", ch));
Extracts characters in the string “hello”, sorts the chars and prints the chars
Agenda• Introduction & Overview
• Lambdas
• Functional interfaces
• Streams
• Parallel streams
• Date & Time package
• Refactoring to Java 8
Parallel Streams
race conditions
deadlocks
I really really hate concurrency problems
Parallel code
Serial code
Sometimes, dreams do come true even at 86 :-)
So, keep dreaming till you become 86!
long numOfPrimes = LongStream.rangeClosed(2, 100_000) .filter(PrimeNumbers::isPrime) .count();
System.out.println(numOfPrimes);
Prints 9592
2.510 seconds
Parallel code
Serial code
Let’s flip the switch by calling parallel() function
long numOfPrimes = LongStream.rangeClosed(2, 100_000) .parallel() .filter(PrimeNumbers::isPrime) .count();
System.out.println(numOfPrimes);
Prints 9592
1.235 seconds
Wow! That’s an awesome flip switch!
Internally, parallel streams make use of fork-join framework
import java.util.Arrays;
class StringConcatenator { public static String result = ""; public static void concatStr(String str) { result = result + " " + str; } }
class StringSplitAndConcatenate { public static void main(String []args) { String words[] = "the quick brown fox jumps over the lazy dog".split(" "); Arrays.stream(words).forEach(StringConcatenator::concatStr); System.out.println(StringConcatenator.result); } }
Gives wrong results with with parallel() call
Agenda• Introduction & Overview
• Lambdas
• Functional interfaces
• Streams
• Parallel streams
• Date & Time package
• Refactoring to Java 8
–Craig Larman
"The critical design tool for software development is a mind well educated in design principles"
Design Smells: Example
Discussion Example
// using java.util.Date Date today = new Date(); System.out.println(today);
$ java DateUse Wed Dec 02 17:17:08 IST 2015
Why should we get the time and timezone details if I only want a date? Can
I get rid of these parts? No!
So What?!Date today = new Date(); System.out.println(today); Date todayAgain = new Date(); System.out.println(todayAgain);
System.out.println(today.compareTo(todayAgain) == 0);
Thu Mar 17 13:21:55 IST 2016 Thu Mar 17 13:21:55 IST 2016 false
What is going on here?
Joda API
JSR 310: Java Date and Time API
Stephen Colebourne
Refactoring for Date
Replace inheritance with delegation
java.time package!
Date, Calendar, and TimeZone Java 8 replaces these types
Refactored SolutionLocalDate today = LocalDate.now(); System.out.println(today); LocalDate todayAgain = LocalDate.now(); System.out.println(todayAgain); System.out.println(today.compareTo(todayAgain) == 0);
2016-03-17 2016-03-17 true
Works fine now!
Refactored Example … You can use only date, time, or even timezone, and combine them as
needed!
LocalDate today = LocalDate.now(); System.out.println(today); LocalTime now = LocalTime.now(); System.out.println(now);
ZoneId id = ZoneId.of("Asia/Tokyo"); System.out.println(id);
LocalDateTime todayAndNow = LocalDateTime.now(); System.out.println(todayAndNow);
ZonedDateTime todayAndNowInTokyo = ZonedDateTime.now(ZoneId.of("Asia/Tokyo")); System.out.println(todayAndNowInTokyo);
2016-03-17 13:28:06.927 Asia/Tokyo 2016-03-17T13:28:06.928 2016-03-17T16:58:06.929+09:00[Asia/Tokyo]
“Fluent interfaces”
❖ Code is more readable and easier to use:
❖ Classes in this package have numerous static methods (many of them factory methods)
❖ Methods in the classes follow a common naming convention (for example, they use the prefixes plus and minus to add or subtract date or time values)
java.time Sub-packages
❖ java.time.temporal —Accesses date/time fields and units
❖ java.time.format —Formats the input and output of date/time objects
❖ java.time.zone —Handles time zones
❖ java.time.chrono —Supports calendar systems such as Japanese and Thai calendars
ISO-8601 Calendar System Format
❖ The Java 8 date and time API uses ISO 8601 as the default calendar format.
❖ In this internationally accepted format, the date and time values are sorted from the largest to the smallest unit of time: year, month/week, day, hour, minute, second, and millisecond/nanosecond.
❖ Example: LocalDate is represented in the in a year-month-day format (YYYY-MM-DD), as in, 2015-10-26.
java.time.LocalDate
LocalDate newYear2016 = LocalDate.of(2016, 1, 1); System.out.println("New year 2016: " + newYear2016);
New year 2016: 2016-01-01
java.time.LocalDate
LocalDate valentinesDay = LocalDate.of(2016, 14, 2); System.out.println("Valentine's day is on: " + valentinesDay);
Exception in thread "main" java.time.DateTimeException: Invalid value
for MonthOfYear(valid values 1 - 12): 14
java.time.LocalDate
long visaValidityDays = 180L; LocalDate currDate = LocalDate.now(); System.out.println("My Visa expires on: " + currDate.plusDays(visaValidityDays));
My Visa expires on: 2016-04-23
Important Methods in LocalDate
java.time.LocalTime
LocalTime currTime = LocalTime.now(); System.out.println("Current time is: " + currTime);
Current time is: 12:23:05.072
java.time.LocalTime
System.out.println(LocalTime.of(18,30));
prints: 18:30
java.time.LocalTime
long hours = 6; long minutes = 30; LocalTime currTime = LocalTime.now(); System.out.println("Current time is: " + currTime); System.out.println("My meeting is at: " + currTime.plusHours(hours).plusMinutes(minutes));
Current time is: 12:29:13.624My meeting is at: 18:59:13.624
Important Methods in LocalTime
java.time.LocalDateTime
LocalDateTime currDateTime = LocalDateTime.now(); System.out.println("Today's date and current time is: " + currDateTime);
Today's date and current time is: 2015-10-29T21:04:36.376
java.time.LocalDateTime
LocalDateTime christmas = LocalDateTime.of(2015, 12, 25, 0, 0); LocalDateTime newYear = LocalDateTime.of(2016, 1, 1, 0, 0); System.out.println("New Year 2016 comes after Christmas 2015”
+ newYear.isAfter(christmas));
New Year 2016 comes after Christmas 2015? true
java.time.LocalDateTime
LocalDateTime dateTime = LocalDateTime.now(); System.out.println("Today's date and current time: " + dateTime); System.out.println("The date component is: " + dateTime.toLocalDate()); System.out.println("The time component is: " + dateTime.toLocalTime());
Today's date and current time: 2015-11-04T13:19:10.497
The date component is: 2015-11-04The time component is: 13:19:10.497
java.time.Instant
import java.time.Instant;
public class UsingInstant { public static void main(String args[]){
// prints the current timestamp with UTC as time zone Instant currTimeStamp = Instant.now(); System.out.println("Instant timestamp is: "+ currTimeStamp); // prints the number of seconds as Unix timestamp from epoch time System.out.println("Number of seconds elapsed: " + currTimeStamp.getEpochSecond()); // prints the Unix timestamp in milliseconds System.out.println("Number of milliseconds elapsed: " + currTimeStamp.toEpochMilli());
} }
Instant timestamp is: 2015-11-02T03:16:04.502ZNumber of seconds elapsed: 1446434164
Number of milliseconds elapsed: 1446434164502
java.time.Period
LocalDate manufacturingDate = LocalDate.of(2016, Month.JANUARY, 1); LocalDate expiryDate = LocalDate.of(2018, Month.JULY, 18); Period expiry = Period.between(manufacturingDate, expiryDate); System.out.printf("Medicine will expire in: %d years, %d months, and %d days (%s)\n", expiry.getYears(), expiry.getMonths(), expiry.getDays(), expiry);
Medicine will expire in: 2 years, 6 months, and 17 days (P2Y6M17D)
Important Methods in Period
The Java 8 date and time API differentiates how humans and computers use date- and time-related information.
For example, the Instant class represents a Unix timestamp and internally uses long and int variables.
Instant values are not very readable or usable by humans because the class does not support methods
related to day, month, hours, and so on (in contrast, the Period class supports such methods).
java.time.Duration
LocalDateTime comingMidnight = LocalDateTime.of(LocalDate.now().plusDays(1), LocalTime.MIDNIGHT);
LocalDateTime now = LocalDateTime.now(); Duration between = Duration.between(now, comingMidnight); System.out.println(between);
PT7H13M42.003S
Important Methods in Duration
Summary of Instant, Period and Duration
TemporalUnit
import java.time.temporal.ChronoUnit;
public class ChronoUnitValues { public static void main(String []args) {
System.out.println("ChronoUnit DateBased TimeBased Duration"); System.out.println("---------------------------------------"); for(ChronoUnit unit : ChronoUnit.values()) {
System.out.printf("%10s \t %b \t\t %b \t\t %s %n”, unit, unit.isDateBased(), unit.isTimeBased(), unit.getDuration());
} }
}
ZoneId
System.out.println("My zone id is: " + ZoneId.systemDefault());
My zone id is: Asia/Kolkata
ZoneId AsiaKolkataZoneId = ZoneId.of("Asia/Kolkata");
ZonedDateTime
LocalDate currentDate = LocalDate.now(); LocalTime currentTime = LocalTime.now(); ZoneId myZone = ZoneId.systemDefault(); ZonedDateTime zonedDateTime = ZonedDateTime.of(currentDate, currentTime, myZone); System.out.println(zonedDateTime);
2015-11-05T11:38:40.647+05:30[Asia/Kolkata]
ZonedDateTime
ZoneId myZone = ZoneId.of("Asia/Kolkata"); LocalDateTime dateTime = LocalDateTime.now(); ZonedDateTime zonedDateTime = dateTime.atZone(myZone); ZoneOffset zoneOffset = zonedDateTime.getOffset(); System.out.println(zoneOffset);
+05:30
ZonedDateTimeZoneId singaporeZone = ZoneId.of(“Asia/Singapore"); ZonedDateTime dateTimeInSingapore =
ZonedDateTime.of(LocalDateTime.of(2016, Month.JANUARY, 1, 6, 0), singaporeZone);
ZoneId aucklandZone = ZoneId.of("Pacific/Auckland"); ZonedDateTime sameDateTimeInAuckland =
dateTimeInSingapore.withZoneSameInstant(aucklandZone);
Duration timeDifference = Duration.between( dateTimeInSingapore.toLocalTime(),
sameDateTimeInAuckland.toLocalTime());
System.out.printf("Time difference between %s and %s zones is %d hours”, singaporeZone, aucklandZone, timeDifference.toHours());
Time difference between Asia/Singapore and Pacific/Auckland zones is 5 hours
Daylight Savings
ZoneId kolkataZone = ZoneId.of("Asia/Kolkata"); Duration kolkataDST = kolkataZone.getRules().getDaylightSavings(Instant.now()); System.out.printf("Kolkata zone DST is: %d hours %n", kolkataDST.toHours());
ZoneId aucklandZone = ZoneId.of("Pacific/Auckland"); Duration aucklandDST = aucklandZone.getRules().getDaylightSavings(Instant.now()); System.out.printf("Auckland zone DST is: %d hours", aucklandDST.toHours());
Kolkata zone DST is: 0 hoursAuckland zone DST is: 1 hours
DateTimeFormatter
Predefined formatters: • ISO_DATE (2015-11-05) • ISO_TIME (11:25:47.624) • RFC_1123_DATE_TIME (Thu, 5 Nov 2015 11:27:22 +0530) • ISO_ZONED_DATE_TIME (2015-11-05T11:30:33.49+05:30[Asia/Kolkata])
DateTimeFormatter
Wake up time: 06:00:00
LocalTime wakeupTime = LocalTime.of(6, 0, 0); System.out.println("Wake up time: " + DateTimeFormatter.ISO_TIME.format(wakeupTime));
01 Jan 2016
DateTimeFormatter customFormat = DateTimeFormatter.ofPattern("dd MMM yyyy"); System.out.println(customFormat.format(LocalDate.of(2016, Month.JANUARY, 01)));
Uppercase and lowercase letters can have similar or different meanings when used in format strings fordates and times. Read the Javadoc for these patterns
carefully before trying to use these letters. For example,in dd-MM-yy, MM refers to month; however, in dd-mm-
yy, mm refers to minutes !
Formatting Dates
• G (era: BC, AD) • y (year of era: 2015, 15) • Y (week-based year: 2015, 15) • M (month: 11, Nov, November) • w (week in year: 13) • W (week in month: 2) • E (day name in week: Sun, Sunday) • D (day of year: 256) • d (day of month: 13)
Custom Date Patternspublic class CustomDatePatterns {
public static void main(String []args) { // patterns from simple to complex ones String [] dateTimeFormats = {
"dd-MM-yyyy", /* d is day (in month), M is month, y is year */ "d '('E')' MMM, YYYY", /*E is name of the day (in week), Y is year*/ "w'th week of' YYYY", /* w is the week of the year */ "EEEE, dd'th' MMMM, YYYY" /*E is day name in the week */
};
LocalDateTime now = LocalDateTime.now(); for(String dateTimeFormat : dateTimeFormats) {
System.out.printf("Pattern \"%s\" is %s %n", dateTimeFormat, DateTimeFormatter.ofPattern(dateTimeFormat).format(now));
} }
} Pattern "dd-MM-yyyy" is 05-11-2015Pattern "d '('E')' MMM, YYYY" is 5 (Thu) Nov, 2015Pattern "w'th week of' YYYY" is 45th week of 2015Pattern "EEEE, dd'th' MMMM, YYYY" is Thursday, 05th November, 2015
Formatting Times
• a (marker for the text a.m./p.m. marker) • H (hour: value range 0–23) • k (hour: value range 1–24) • K (hour in a.m./p.m.: value range 0–11) • h (hour in a.m./p.m.: value range 1–12) • m (minute) • s (second) • S (fraction of a second) • z (time zone: general time-zone format)
Custom Time Patternsclass CustomTimePatterns {
public static void main(String []args) { // patterns from simple to complex ones String [] timeFormats = {
"h:mm", /* h is hour in am/pm (1-12), m is minute */ "hh 'o''clock'", /* '' is the escape sequence to print a single quote */ "H:mm a", /* H is hour in day (0-23), a is am/pm*/ "hh:mm:ss:SS", /* s is seconds, S is milliseconds */ "K:mm:ss a" /* K is hour in am/pm(0-11) */
};
LocalTime now = LocalTime.now(); for(String timeFormat : timeFormats) {
System.out.printf("Time in pattern \"%s\" is %s %n", timeFormat, DateTimeFormatter.ofPattern(timeFormat).format(now));
} }
} Time in pattern "h:mm" is 12:27Time in pattern "hh 'o''clock'" is 12 o'clockTime in pattern "H:mm a" is 12:27 PMTime in pattern "hh:mm:ss:SS" is 12:27:10:41Time in pattern "K:mm:ss a" is 0:27:10 PM
Flight Travel - Time Calculation - Example
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("dd MMM yyyy hh.mm a");
// Leaving on 1st Jan 2016, 6:00am from "Singapore" ZonedDateTime departure = ZonedDateTime.of(
LocalDateTime.of(2016, Month.JANUARY, 1, 6, 0), ZoneId.of("Asia/Singapore"));
System.out.println("Departure: " + dateTimeFormatter.format(departure));
// Arrival on the same day in 10 hours in "Auckland" ZonedDateTime arrival =
departure.withZoneSameInstant(ZoneId.of("Pacific/Auckland")).plusHours(10); System.out.println("Arrival: " + dateTimeFormatter.format(arrival));
Departure: 01 Jan 2016 06.00 AMArrival: 01 Jan 2016 09.00 PM
Agenda• Introduction & Overview
• Lambdas
• Functional interfaces
• Streams
• Parallel streams
• Date & Time package
• Refactoring to Java 8
Examples of refactorings (to Java 8)
❖ Convert anonymous inner classes to lambda expressions (when they are functional interfaces)
❖ Convert for/foreach loops to streams (i.e., external iteration to internal iteration)
Refactoring loops to streams
❖ Replace if conditions with ‘filter’ and calls to methods that return a boolean value (Predicate)
❖ Replace accumulation operations with reduce (or its special forms like sum, count, etc).
Source: LAMBDAFICATOR: From Imperative to Functional Programming through Automated Refactoring. Lyle Franklin; Alex Gyori; Jan Lahoda; Danny Dig. 35th International Conference on Software Engineering (ICSE), 2013.
Tool support for refactoring
❖ Most Java IDEs provide suggestions to automatically refactor to lambdas and streams
❖ IDEs that support Java 8 refactoring include: Eclipse, IntelliJ Idea and NetBeans
Refactoring suggestions in NetbeansImage source: http://refactoring.info/tools/LambdaFicator/
Java 8 Migration Aids in IntelliJ IDEAImage source: https://www.jetbrains.com/help/img/idea/ij_java_8_inspection_results_migration_runnable.png
Java 8 Refactorings in IntelliJ IDEAImage source: https://www.jetbrains.com/help/img/idea/ij_java_8_replace_method_reference_with_lambda.png
https://www.jetbrains.com/help/img/idea/ij_java_8_can_be_replaced_with_method_ref.png
Refactoring suggestions in NetbeansImage source: http://refactoring.info/tools/LambdaFicator/
Refactoring suggestions in NetbeansImage source: http://refactoring.info/tools/LambdaFicator/
Suggested Reading
❖ Refactoring with Loops and Collection Pipelines (Martin Fowler, 2015)
❖ Pragmatic Functional Refactoring with Java 8 (Raoul-Gabriel Urma & Richard Warburton, 2015)
❖ Migrating to Java 8 (IntelliJ IDEA, 2016)
Meetups
hYp://www.meetup.com/JavaScript-Meetup-Bangalore/hYp://www.meetup.com/Container-Developers-Meetup-Bangalore/hYp://www.meetup.com/So^ware-Cra^smanship-Bangalore-Meetup/hYp://www.meetup.com/Core-Java-Meetup-Bangalore/hYp://www.meetup.com/Technical-Writers-Meetup-Bangalore/hYp://www.meetup.com/CloudOps-Meetup-Bangalore/hYp://www.meetup.com/Bangalore-SDN-IoT-NetworkVirtualiza7on-Enthusiasts/hYp://www.meetup.com/So^wareArchitectsBangalore/
Image credits❖ http://mayhemandmuse.com/wp-content/uploads/2013/04/This-optical-illusion-drawing-by-WE-
Hill-shows-both-his-wife-and-his-mother-in-law.jpg
❖ http://www.webtrafficroi.com/wp-content/uploads/2012/10/mahatma-gandhi-apple-think-different.jpg
❖ http://rexx-language-association-forum.44760.x6.nabble.com/file/n2236/Ruby-lambda-function.jpg
❖ http://www.ibm.com/developerworks/library/j-jn16/figure1.png
❖ http://www.ibm.com/developerworks/library/j-jn16/figure2.png
❖ http://img.viralpatel.net/2014/01/java-lambda-expression.png
❖ http://www.codercaste.com/wp-content/uploads/2011/01/animals.png
❖ http://blog.takipi.com/wp-content/uploads/2014/03/blog_lambada_2.png
❖ http://quotespictures.com/wp-content/uploads/2014/01/it-is-not-the-strongest-of-the-species-that-survive-nor-the-most-intelligent-but-the-one-most-responsive-to-change-charles-darwin.jpg
❖ http://7-themes.com/data_images/out/27/6859733-surfing-wallpaper.jpg
[email protected] @GSamarthyam
www.codeops.tech slideshare.net/sgganesh
+91 98801 64463 bit.ly/sgganesh