Preface

Like many Java developers, the first time I heard about lambda expressions it piqued my interest. Also like many others, I was disappointed when it was set back. However, it is better late than never.

Java 8 is a giant step forward for the Java language. Writing this book has forced me to learn a lot more about it. In Project Lambda, Java gets a new closure syntax, method-references, and default methods on interfaces. It manages to add many of the features of functional languages without losing the clarity and simplicity Java developers have come to expect.

Aside from Project Lambda, Java 8 also gets a new Date and Time API (JSR 310), the Nashorn JavaScript engine, and removes the Permanent Generation from the HotSpot virtual machine, among other changes.

I would like to acknowledge the following people for providing valuable resources:

Brian Goetz – “State of the Lambda”

Aleksey Shipilev – jdk8-lambda-samples

Richard Warburton – “Java 8 Lambdas”

Julien Ponge – “Oracle Nashorn” in the Jan./Feb. 2014 issue of Java Magazine.

Venkat Subramaniam – agiledeveloper.com

All of the developers behind Java 8.

The developers of Guava, joda-time, Groovy, and Scala.

1. Overview

This book is a short introduction to Java 8. After reading it, you should have a basic understanding of the new features and be ready to start using it.

This book assumes that you have a good understanding of Java the language and the JVM. If you’re not familiar with the language, including features of Java 7, it might be hard to follow some of the examples. For a more comprehensive introduction to programming in Java you should check out Modern Programming Made Easy or Modern Java: Second Edition.

Java 8 includes the following:

Lambda expressions

Method references

Default Methods (Defender methods)

A new Stream API.

Optional

A new Date/Time API.

Nashorn, the new JavaScript engine

Removal of the Permanent Generation

and more…

The best way to read this book is with a Java 8 supporting IDE running so you can try out the new features.

Code examples can be found on github.

2. Lambda Expressions

The biggest new feature of Java 8 is language level support for lambda expressions (Project Lambda). A lambda expression is like syntactic sugar for an anonymous class1 with one method whose type is inferred. However, it will have enormous implications for simplifying development.

2.1 Syntax

The main syntax of a lambda expression is “parameters -> body”. The compiler can usually use the context of the lambda expression to determine the functional interface2 being used and the types of the parameters. There are four important rules to the syntax:

Declaring the types of the parameters is optional.

Using parentheses around the parameter is optional if you have only one parameter.

Using curly braces is optional (unless you need multiple statements).

The “return” keyword is optional if you have a single expression that returns a value.

Here are some examples of the syntax:

1 () -> System . out . println ( this ) 2 ( String str ) -> System . out . println ( str ) 3 str -> System . out . println ( str ) 4 ( String s1 , String s2 ) -> { return s2 . length () - s1 . length (); } 5 ( s1 , s2 ) -> s2 . length () - s1 . length ()

The last expression could be used to sort a list; for example:

1 Arrays . sort ( strArray , 2 ( String s1 , String s2 ) -> s2 . length () - s1 . length ());

In this case the lambda expression implements the Comparator interface to sort strings by length.

2.2 Scope

Here’s a short example of using lambdas with the Runnable interface:

1 import static java.lang.System.out ; 2 3 public class Hello { 4 Runnable r1 = () -> out . println ( this ); 5 Runnable r2 = () -> out . println ( toString ()); 6 7 public String toString () { return "Hello, world!" ; } 8 9 public static void main ( String ... args ) { 10 new Hello (). r1 . run (); //Hello, world! 11 new Hello (). r2 . run (); //Hello, world! 12 } 13 }

The important thing to note is both the r1 and r2 lambdas call the toString() method of the Hello class. This demonstrates the scope available to the lambda.

You can also refer to final variables or effectively final variables. A variable is effectively final if it is only assigned once.

For example, using Spring’s HibernateTemplate:

1 String sql = "delete * from User" ; 2 getHibernateTemplate (). execute ( session -> 3 session . createSQLQuery ( sql ). uniqueResult ());

In the above, you can refer to the variable sql because it is only assigned once. If you were to assign to it a second time, it would cause a compilation error.

2.3 Method references

Since a lambda expression is like an object-less method, wouldn’t be nice if we could refer to existing methods instead of using a lamda expression? This is exactly what we can do with method references.

For example, imagine you frequently need to filter a list of Files based on file types. Assume you have the following set of methods for determining a file’s type:

1 public class FileFilters { 2 public static boolean fileIsPdf ( File file ) { /*code*/ } 3 public static boolean fileIsTxt ( File file ) { /*code*/ } 4 public static boolean fileIsRtf ( File file ) { /*code*/ } 5 }

Whenever you want to filter a list of files, you can use a method reference as in the following example (assuming you already defined a method getFiles() that returns a Stream ):

1 Stream < File > pdfs = getFiles (). filter ( FileFilters :: fileIsPdf ); 2 Stream < File > txts = getFiles (). filter ( FileFilters :: fileIsTxt ); 3 Stream < File > rtfs = getFiles (). filter ( FileFilters :: fileIsRtf );

Method references can point to:

Static methods.

Instance methods.

Methods on particular instances.

Constructors (ie. TreeSet::new )

For example, using the new java.nio.file.Files.lines method:

1 Files . lines ( Paths . get ( "Nio.java" )) 2 . map ( String :: trim ) 3 . forEach ( System . out :: println );

The above reads the file “Nio.java”, calls trim() on every line, and then prints out the lines.

Notice that System.out::println refers to the println method on an instance of PrintStream .

2.4 Functional Interfaces

In Java 8 a functional interface is defined as an interface with exactly one abstract method. This even applies to interfaces that were created with previous versions of Java.

Java 8 comes with several new functional interfaces in the package, java.util.function .

Function <T,R> - takes an object of type T and returns R.

- takes an object of type T and returns R. Supplier <T> - just returns an object of type T.

- just returns an object of type T. Predicate <T> - returns a boolean value based on input of type T.

- returns a boolean value based on input of type T. Consumer <T> - performs an action with given object of type T.

- performs an action with given object of type T. BiFunction - like Function but with two parameters.

BiConsumer - like Consumer but with two parameters.

It also comes with several corresponding interfaces for primitive types, such as:

IntConsumer

IntFunction <R>

IntPredicate

IntSupplier

See the java.util.function Javadocs for more information.

The coolest thing about functional interfaces is that they can be assigned to anything that would fulfill their contract. Take the following code for example:

1 Function < String , String > atr = ( name ) -> { return "@" + name ;}; 2 Function < String , Integer > leng = ( name ) -> name . length (); 3 Function < String , Integer > leng2 = String :: length ;

This code is perfectly valid Java 8. The first line defines a function that prepends “@” to a String. The last two lines define functions that do the same thing: get the length of a String.

The Java compiler is smart enough to convert the method reference to String’s length() method into a Function (a functional interface) whose apply method takes a String and returns an Integer. For example:

1 for ( String s : args ) out . println ( leng2 . apply ( s ));

This would print out the lengths of the given strings.

Any interface can be functional interface, not merely those that come with Java. To declare your intention that an interface is functional, use the @FunctionalInterface annotation. Although not necessary, it will cause a compilation error if your interface does not satisfy the requirements (ie. one abstract method).

Github See jdk8-lambda-samples for more examples.

2.5 Comparisons to Java 7

To better illustrate the benefit of Lambda-expressions, here are some examples of how code from Java 7 can be shortened in Java 8.

Creating an ActionListener

1 // Java 7 2 ActionListener al = new ActionListener () { 3 @Override 4 public void actionPerformed ( ActionEvent e ) { 5 System . out . println ( e . getActionCommand ()); 6 } 7 }; 8 // Java 8 9 ActionListener al8 = e -> System . out . println ( e . getActionCommand ());

Printing out a list of Strings

1 // Java 7 2 for ( String s : list ) { 3 System . out . println ( s ); 4 } 5 //Java 8 6 list . forEach ( System . out :: println );

Sorting a list of Strings

1 // Java 7 2 Collections . sort ( list , new Comparator < String >() { 3 @Override 4 public int compare ( String s1 , String s2 ) { 5 return s1 . length () - s2 . length (); 6 } 7 }); 8 //Java 8 9 Collections . sort ( list , ( s1 , s2 ) -> s1 . length () - s2 . length ()); 10 // or 11 list . sort ( Comparator . comparingInt ( String :: length ));

Sorting

For the sorting examples, assume you have the following Person class:

1 public static class Person { 2 3 String firstName ; 4 String lastName ; 5 6 public String getFirstName () { 7 return firstName ; 8 } 9 10 public String getLastName () { 11 return lastName ; 12 } 13 }

Here’s how you might sort this list in Java 7 by last-name and then first-name:

1 Collections . sort ( list , new Comparator < Person >() { 2 @Override 3 public int compare ( Person p1 , Person p2 ) { 4 int n = p1 . getLastName (). compareTo ( p2 . getLastName ()); 5 if ( n == 0 ) { 6 return p1 . getFirstName (). compareTo ( p2 . getFirstName ()); 7 } 8 return n ; 9 } 10 });

In Java 8, this can be shortened to the following:

1 list . sort ( Comparator . comparing ( Person :: getLastName ) 2 . thenComparing ( Person :: getFirstName ));

This example uses a static method on an interface ( comparing ) and a default method ( thenComparing ) which are discussed in the next chapter.

3. Default Methods

In order to add the stream method (or any others) to the core Collections API, Java needed another new feature, Default methods (also known as Defender Methods or Virtual Extension methods). This way they could add new methods to the List interface for example without breaking all the existing implementations (backwards compatibility).

Default methods can be added to any interface. Like the name implies, any class that implements the interface but does not override the method will get the default implementation.

For example, the stream method in the Collection interface is defined something like the following:

1 default public Stream stream () { 2 return StreamSupport . stream ( spliterator ()); 3 }

See the Java docs for more on Spliterators.

You can always override a default method if you need different behavior.

3.1 Default and Functional

An interface can have one or more default methods and still be functional.

For example, take a look at the Iterable interface:

1 @FunctionalInterface 2 public interface Iterable { 3 Iterator iterator (); 4 default void forEach ( Consumer <? super T > action ) { 5 Objects . requireNonNull ( action ); 6 for ( T t : this ) { 7 action . accept ( t ); 8 } 9 } 10 }

It has both the iterator() method and the forEach method.

3.2 Multiple Defaults

In the unlikely case that your class implements two or more interfaces that define the same default method, Java will throw a compilation error. You will need to override the method and choose from one of the methods. For example:

1 interface Foo { 2 default void talk () { 3 out . println ( "Foo!" ); 4 } 5 } 6 interface Bar { 7 default void talk () { 8 out . println ( "Bar!" ); 9 } 10 } 11 class FooBar implements Foo , Bar { 12 @Override 13 void talk () { Foo . super . talk (); } 14 }

In the above code, talk is overridden and calls Foo ’s talk method. This is similar to the way you refer to a super class in pre-Java-8.

3.3 Static Methods on Interface

Although not strictly related to default methods, the ability to add static methods to interfaces is a similar change to the Java language.

For example, there are many static methods on the new Stream interface. This makes “helper” methods easier to find since they can be located directly on the interface, instead of a different class such as StreamUtil or Streams.

Here’s an example in the new Stream interface:

1 public static < T > Stream < T > of ( T ... values ) { 2 return Arrays . stream ( values ); 3 }

The above method creates a new stream based on the given values.

4. Streams

The Stream interface is such a fundamental part of Java 8 it deserves its own chapter.

4.1 What is a Stream?

The Stream interface is located in the java.util.stream package. It represents a sequence of objects somewhat like the Iterator interface. However, unlike the Iterator, it supports parallel execution.

The Stream interface supports the map/filter/reduce pattern and executes lazily, forming the basis (along with lambdas) for functional-style programming in Java 8.

There are also corresponding primitive streams (IntStream, DoubleStream, and LongStream) for performance reasons.

4.2 Generating Streams

There are many ways to create a Stream in Java 8. Many of the existing Java core library classes have Stream returning methods in Java 8.

Streaming Collections

The most obvious way to create a stream is from a Collection .

The Collection interface has two default methods on it for creating streams:

stream(): Returns a sequential Stream with the collection as its source.

parallelStream(): Returns a possibly parallel Stream with the collection as its source.

The ordering of the Stream relies on the underlying collection just like an Iterator.

Streaming Files

The BufferedReader now has the lines() method which returns a Stream; for example3:

1 try ( FileReader fr = new FileReader ( "file" ); 2 BufferedReader br = new BufferedReader ( fr )) { 3 br . lines (). forEach ( System . out :: println ); 4 }

You can also read a file as a Stream using Files.lines(Path filePath) ; for example:

1 try ( Stream st = Files . lines ( Paths . get ( "file" ))) { 2 st . forEach ( System . out :: println ); 3 }

Note this populates lazily; it does not read the entire file when you call it.

Files.lines(Path): Any IOException that is thrown while processing the file (after the file is opened) will get wrapped in an UncheckedIOException and thrown.

Streaming File Trees

There are several static methods on the Files class for navigating file trees using a Stream.

list(Path dir) – Stream of files in the given directory.

– Stream of files in the given directory. walk(Path dir) 4 – Stream that traverses the file tree depth-first starting at the given directory.

– Stream that traverses the file tree depth-first starting at the given directory. walk(Path dir, int maxDepth) – Same as walk(dir) but with a maximum depth.

Streaming Text Patterns

The Pattern class now has a method, splitAsStream(CharSequence) , which creates a Stream.

For example:

1 import java.util.regex.Pattern ; 2 // later on... 3 Pattern patt = Pattern . compile ( "," ); 4 patt . splitAsStream ( "a,b,c" ) 5 . forEach ( System . out :: println );

The above uses a very simple pattern, a comma, and splits the text into a stream and prints it out. This would produce the following output:

1 a 2 b 3 c

Infinite Streams

Using the generate or iterate static methods on Stream, you can create a Stream of values including never ending streams. For example, you could call generate in the following way to create an infinite supply of objects:

1 Stream . generate (() -> new Dragon ());

For example, you could use this technique to produce a stream of CPU load or memory usage. However, you should use this with caution. It is similar to an infinite loop.

You could also use generate to create an infinite random number supply; for example:

1 Stream . generate (() -> Math . random ());

However, the java.util.Random class does this for you with the following new methods: ints() , longs() , and doubles() . Each of those methods is overloaded with definitions similar to the following:

ints() : An infinite Stream of random integers.

: An infinite Stream of random integers. ints(int n, int m) : An infinite Stream of random integers from n (inclusive) to m (exclusive).

: An infinite Stream of random integers from n (inclusive) to m (exclusive). ints(long size) : A Stream of given size of random integers.

: A Stream of given size of random integers. ints(long size, int n, int m) : A Stream of given size of random integers with given bounds.

The iterate method is similar to generate except it takes an initial value and a Function that modifies that value. For example, you can iterate over the Integers using the following code:

1 Stream . iterate ( 1 , i -> i + 1 ) 2 . forEach ( System . out :: print );

This would print out “1234…” continuously until you stop the program.

There are ways to limit an infinite stream which we will cover later ( filter and limit ).

Ranges

There are also new methods for creating ranges of numbers as Streams.

For example, the static method, range , on the IntStream interface:

1 IntStream . range ( 1 , 11 ) 2 . forEach ( System . out :: println );

The above would print out the numbers one through ten.

Each primitive Stream (IntStream, DoubleStream, and LongStream) has a corresponding range method.

Streaming Anything

You can create a Stream from any number of elements or an array using the two following methods:

1 Stream < Integer > s = Stream . of ( 1 , 2 , 3 ); 2 Stream < Object > s2 = Arrays . stream ( array );

Stream.of can take any number of parameters of any type.

4.3 For Each

The most basic thing you can do with a Stream is loop through it using the forEach method.

For example, to print out all of the files in the current directory, you could do the following:

1 Files . list ( Paths . get ( "." )) 2 . forEach ( System . out :: println );

For the most part, this replaces the “for loop”. It is more concise, and more object-oriented since you are delegating the implementation of the actual loop.

4.4 Map/Filter/Reduce

Lambda expressions and default methods allow us to implement map/filter/reduce in Java 8. Actually it is already implemented for us in the standard library.

For example, imagine you want to get the current point scores from a list of player-names and find the player with the most points. You have a simple class, PlayerPoints , and a getPoints method defined as the following:

1 public static class PlayerPoints { 2 public final String name ; 3 public final long points ; 4 5 public PlayerPoints ( String name , long points ) { 6 this . name = name ; 7 this . points = points ; 8 } 9 10 public String toString () { 11 return name + ":" + points ; 12 } 13 } 14 15 public static long getPoints ( final String name ) { 16 // gets the Points for the Player 17 }

Finding the highest player could be done very simply in Java 8 as shown in the following code:

1 PlayerPoints highestPlayer = 2 names . stream (). map ( name -> new PlayerPoints ( name , getPoints ( name ))) 3 . reduce ( new PlayerPoints ( "" , 0.0 ), 4 ( s1 , s2 ) -> ( s1 . points > s2 . points ) ? s1 : s2 );

This could also be done in Java 7 with the dollar library (or similarly with Guava or Functional-Java), but it would be much more verbose as shown in the following:

1 PlayerPoints highestPlayer = 2 $ ( names ). map ( new Function < String , PlayerPoints >() { 3 public PlayerPoints call ( String name ) { 4 return new PlayerPoints ( name , getPoints ( name )); 5 } 6 }) 7 . reduce ( new PlayerPoints ( "" , 0.0 ), 8 new BiFunction < PlayerPoints , PlayerPoints , PlayerPoints >() { 9 public PlayerPoints call ( PlayerPoints s1 , PlayerPoints s2 ) { 10 return ( s1 . points > s2 . points ) ? s1 : s2 ; 11 } 12 });

The major benefit to coding this way (apart from the reduction in lines of code) is the ability to hide the underlying implementation of map/reduce. For example, it’s possible that map and reduce are implemented concurrently, allowing you to easily take advantage of multiple processors. We’ll describe one way to do this (ParallelArray) in the following section.

4.5 Parallel Array

The ParallelArray was part of JSR-166, but ended up being excluded from the standard Java lib. It does exist and was released to the public domain (you can download it from the JSR website).

Although it was already out there, it really wasn’t easy to use until closures were included in the Java language. In Java 7 using the ParallelArray looks like the following:

1 // with this class 2 public class Student { 3 String name ; 4 int graduationYear ; 5 double gpa ; 6 } 7 // this predicate 8 final Ops . Predicate < Student > isSenior = 9 new Ops . Predicate <>() { 10 public boolean op ( Student s ) { 11 return s . graduationYear == Student . THIS_YEAR ; 12 } 13 }; 14 // and this conversion operation 15 final Ops . ObjectToDouble < Student > selectGpa = 16 new Ops . ObjectToDouble <>() { 17 public double op ( Student student ) { 18 return student . gpa ; 19 } 20 }; 21 // create a fork-join-pool 22 ForkJoinPool fjPool = new ForkJoinPool (); 23 ParallelArray < Student > students = new ParallelArray <>( fjPool , data ); 24 // find the best GPA: 25 double bestGpa = students . withFilter ( isSenior ) 26 . withMapping ( selectGpa ) 27 . max ();

In Java 8, you can do the following:

1 // create a fork-join-pool 2 ForkJoinPool pool = new ForkJoinPool (); 3 ParallelArray < Student > students = new ParallelArray <>( pool , data ); 4 // find the best GPA: 5 double bestGpa = students 6 . withFilter (( Student s ) -> ( s . graduationYear == THIS_YEAR )) 7 . withMapping (( Student s ) -> s . gpa ) 8 . max ();

However, Java 8’s addition of stream() and parallelStream() make this even easier:

1 double bestGpa = students 2 . parallelStream () 3 . filter ( s -> ( s . graduationYear == THIS_YEAR )) 4 . mapToDouble ( s -> s . gpa ) 5 . max (). getAsDouble ();

This makes it extremely simple to switch between a sequential implementation and a concurrent one.

Groovy GPars You can do something similar to this right now if you use Groovy with the GPars library in the following way: 1 GParsPool . withPool { 2 // a map-reduce functional style (students is a Collection) 3 def bestGpa = students . parallel 4 . filter { s -> s . graduationYear == Student . THIS_YEAR } 5 . map { s -> s . gpa } 6 . max () 7 } The static method GParsPool.withPool takes in a closure and augments any Collection with several methods (using Groovy’s Category mechanism). The parallel method actually creates a ParallelArray (JSR-166) from the given Collection and uses it with a thin wrapper around it.

4.6 Peek

You can peek into a stream to do some action without interrupting the stream.

For example you could print out elements to debug code:

1 Files . list ( Paths . get ( "." )) 2 . map ( Path :: getFileName ) 3 . peek ( System . out :: println ) 4 . forEach ( p -> doSomething ( p ));

You can use any action you want, but you should not try to modify elements; you should use map instead.

4.7 Limit

The limit(int n) method can be used to limit a stream to the given number of elements. For example:

1 Random rnd = new Random (); 2 rnd . ints (). limit ( 10 ) 3 . forEach ( System . out :: println );

The above would print out ten random integers.

4.8 Sort

Stream also has the sorted() method for sorting a stream. Like all intermediate methods on Stream (such as map , filter , and peek ), the sorted() method executes lazily. Nothing happens until a terminating operation (such as reduce or forEach ) is called. However, you should call a limiting operation like limit before calling sorted() on an infinite stream.

For example, the following would throw a runtime exception (using build 1.8.0-b132):

1 rnd . ints (). sorted (). limit ( 10 ) 2 . forEach ( System . out :: println );

However, the following code works just fine:

1 rnd . ints (). limit ( 10 ). sorted () 2 . forEach ( System . out :: println );

Also, you should call sorted() after any calls to filter . For example, this code prints out the first five Java file-names in the current directory:

1 Files . list ( Paths . get ( "." )) 2 . map ( Path :: getFileName ) // still a path 3 . map ( Path :: toString ) // convert to Strings 4 . filter ( name -> name . endsWith ( ".java" )) 5 . sorted () // sort them alphabetically 6 . limit ( 5 ) // first 5 7 . forEach ( System . out :: println );

The code above does the following:

Lists the files in the current directory.

Maps those files to file names.

Finds names that end with “.java”.

Takes only the first five (sorted alphabetically).

Prints them out.

4.9 Collectors and Statistics

Since Streams are lazily evaluated and support parallel execution, you need a special way to combine results; this is called a Collector.

A Collector represents a way to combine the elements of a Stream into one result. It consists of three things:

A supplier of an initial value.

An accumulator which adds to the initial value.

A combiner which combines two results into one.

There are two ways to do this: collect(supplier,accumulator,combiner) , or collect(Collector) (types left off for brevity).

Luckily, Java 8 comes with several Collectors built in. Import them the following way:

1 import static java.util.stream.Collectors.* ;

Simple Collectors

The simplest collectors are things like toList() and toCollection():

1 // Accumulate names into a List 2 List < String > list = dragons . stream () 3 . map ( Dragon :: getName ) 4 . collect ( toList ()); 5 6 // Accumulate names into a TreeSet 7 Set < String > set = dragons . stream () 8 . map ( Dragon :: getName ) 9 . collect ( toCollection ( TreeSet :: new ));

Joining

If you’re familiar with Apache Commons’ StringUtil.join , the joining collector is similar to it. It combines the stream using a given delimiter. For example:

1 String names = dragons . stream () 2 . map ( Dragon :: getName ) 3 . collect ( joining ( "," ));

This would combine all of the names into one String separated by commas.

Statistics

More complex collectors resolve to a single value. For example, you can use an “averaging” Collector to get the average; for example:

1 System . out . println ( "

----->Average line length:" ); 2 System . out . println ( 3 Files . lines ( Paths . get ( "Nio.java" )) 4 . map ( String :: trim ) 5 . filter ( s -> ! s . isEmpty ()) 6 . collect ( averagingInt ( String :: length )) 7 );

The above code calculates the average length of non-empty lines in the file “Nio.java”.

Sometimes you want to collect multiple statistics about a collection. Because Streams are consumed when you call collect , you need to calculate all of your statistics at once. This is where SummaryStatistics comes in. First import the one you want to use:

1 import java.util.IntSummaryStatistics ;

Then use the summarizingInt collector; for example:

1 IntSummaryStatistics stats = Files . lines ( Paths . get ( "Nio.java" )) 2 . map ( String :: trim ) 3 . filter ( s -> ! s . isEmpty ()) 4 . collect ( summarizingInt ( String :: length )); 5 6 System . out . println ( stats . getAverage ()); 7 System . out . println ( "count=" + stats . getCount ()); 8 System . out . println ( "max=" + stats . getMax ()); 9 System . out . println ( "min=" + stats . getMin ());

The above code performs the same average as before, but also computes the maximum, minimum, and count of the elements.

There’s also summarizingLong and summarizingDouble .

Equivalently, you can map your stream to a primitive type and then call summaryStatistics() . For example:

1 IntSummaryStatistics stats = Files . lines ( Paths . get ( "Nio.java" )) 2 . map ( String :: trim ) 3 . filter ( s -> ! s . isEmpty ()) 4 . mapToInt ( String :: length ) 5 . summaryStatistics ();

4.10 Grouping and Partitioning

The groupingBy collector groups elements based on a function you provide. For example:

1 // Group by first letter of name 2 List < Dragon > dragons = getDragons (); 3 Map < Character , List < Dragon >> map = dragons . stream () 4 . collect ( groupingBy ( dragon -> dragon . getName (). charAt ( 0 )));

Similarly, the partitioningBy method creates a map with a boolean key. For example:

1 // Group by whether or not the dragon is green 2 Map < Boolean , List < Dragon >> map = dragons . stream () 3 . collect ( partitioningBy ( Dragon :: isGreen ));

Parallel Grouping To execute grouping in parallel (if you don’t care about ordering) you should use the groupingByConcurrent method. The underlying stream should be unordered to allow grouping to occur in parallel; for example: dragons.parallelStream().unordered().collect(groupingByConcurrent(Dragon::getColor)); .

4.11 Comparisons to Java 7

To better illustrate the benefit of Streams in Java 8, here are some examples of code from Java 7 compared to their new versions.

Finding a maximum

1 // Java 7 2 double max = 0 ; 3 4 for ( Double d : list ) { 5 if ( d > max ) { 6 max = d ; 7 } 8 } 9 //Java 8 10 max = list . stream (). reduce ( 0.0 , Math :: max ); 11 // or 12 max = list . stream (). mapToDouble ( Number :: doubleValue ). max (). getAsDouble ();

Calculating an average

1 double total = 0 ; 2 double ave = 0 ; 3 // Java 7 4 for ( Double d : list ) { 5 total += d ; 6 } 7 ave = total / (( double ) list . size ()); 8 //Java 8 9 ave = list . stream (). mapToDouble ( Number :: doubleValue ). average (). getAsDouble ();

Printing the numbers one through ten

1 // Java 7 2 for ( int i = 1 ; i < 11 ; i ++) { 3 System . out . println ( i ); 4 } 5 // Java 8 6 IntStream . range ( 1 , 11 ) 7 . forEach ( System . out :: println ); 8 //or 9 Stream . iterate ( 1 , i -> i + 1 ). limit ( 10 ) 10 . forEach ( System . out :: println );

Joining Strings

1 // Java 7 using commons-util 2 List < String > names = new LinkedList <>(); 3 for ( Dragon dragon : dragons ) 4 names . add ( dragon . getName ()); 5 String names = StringUtils . join ( names , "," ); 6 // Java 8 7 String names = dragons . stream () 8 . map ( Dragon :: getName ) 9 . collect ( Collectors . joining ( "," ));

5. Optional

Java 8 comes with the Optional class in the java.util package for avoiding null return values (and thus NullPointerException ). It is very similar to Google Guava’s Optional, which is similar to Nat Pryce’s Maybe class and Scala’s Option class.

The Billion Dollar Mistake Tony Hoare, the inventor of null, has gone on record calling it his “billion-dollar mistake”. Despite your opinion of null, many efforts have been made to make null-checks part of the compilation or automated-code-check process; for example, the @Nonnull annotation of JSR-305. Optional makes it very simple for API designers to avoid null.

You can use Optional.of(x) to wrap a non-null value, Optional.empty() to represent a missing value, or Optional.ofNullable(x) to create an Optional from a reference that may or may not be null.

After creating an instance of Optional, you then use isPresent() to determine if the there is a value and get() to get the value. Optional provides a few other helpful methods for dealing with missing values:

orElse(T) – Returns the given default value if the Optional is empty.

– Returns the given default value if the Optional is empty. orElseGet(Supplier<T>) – Calls on the given Supplier to provide a value if the Optional is empty.

– Calls on the given Supplier to provide a value if the Optional is empty. orElseThrow(Supplier<X extends Throwable>) – Calls on the given Supplier for an exception to throw if the Optional is empty.

It also includes functional style (lambda friendly) methods, like the following:

filter(Predicate<? super T> predicate) – Filters the value and returns a new Optional.

– Filters the value and returns a new Optional. flatMap(Function<? super T,Optional<U>> mapper) – Performs a mapping operation which returns an Optional.

– Performs a mapping operation which returns an Optional. ifPresent(Consumer<? super T> consumer) – Executes the given Consumer only if there is a value present (no return value).

– Executes the given Consumer only if there is a value present (no return value). map(Function<? super T,? extends U> mapper) – Uses the given mapping Function and returns a new Optional.

Stream Optional The new Stream interface has multiple methods which return Optional (in case there are no values in the Stream): reduce(BinaryOperator<T> accumulator) – Reduces the stream to a single value.

– Reduces the stream to a single value. max(Comparator<? super T> comparator) – Finds the maximum value.

– Finds the maximum value. min(Comparator<? super T> comparator) – Finds the minimum value.

6. Nashorn

Nashorn replaces Rhino as the default JavaScript engine for the Oracle JVM. Nashorn is much faster since it uses the invokedynamic feature of the JVM. It also includes a command line tool ( jjs ).

6.1 jjs

JDK 8 includes the command line tool jjs for running JavaScript.

You can run JavaScript files from the command line (assuming you have Java 8’s bin in your PATH ):

1 $ jjs script . js

This can be useful for running scripts; for example, let’s say you wanted to quickly find the sum of some numbers:

1 var data = [ 1 , 3 , 5 , 7 , 11 ] 2 var sum = data . reduce ( function ( x , y ) { return x + y }, 0 ) 3 print ( sum )

Running the above code should print out 27 .

6.2 Scripting

Running jjs with the -scripting option starts up an interactive shell where you can type and evaluate JavaScript.

You can also embed variables into strings and have them evaluate; for example:

1 jjs > var date = new Date () 2 jjs > print ( "${date}" )

This would print out the current date and time.

6.3 ScriptEngine

You can also run JavaScript dynamically from Java.

First, you need to import the ScriptEngine:

1 import javax.script.ScriptEngine ; 2 import javax.script.ScriptEngineManager ;

Second, you use the ScriptEngineManager to get the Nashorn engine:

1 ScriptEngineManager engineManager = new ScriptEngineManager (); 2 ScriptEngine engine = engineManager . getEngineByName ( "nashorn" );

Now you can evaluate javascript at any point:

1 engine . eval ( "function p(s) { print(s) }" ); 2 engine . eval ( "p('Hello Nashorn');" );

The eval method can also take a FileReader as input:

1 engine . eval ( new FileReader ( ' library . js ' ));

This way you can include and run any JavaScript. However, keep in mind that the typical variables available to you in the browser (window, document, etc.) are not available.

6.4 Importing

You can import and use Java classes and packages using the JavaImporter.

For example, import java.util , the IO, and NIO file packages:

1 var imports = new JavaImporter ( java . util , java . io , java . nio . file ); 2 with ( imports ) { 3 var paths = new LinkedList (); 4 print ( paths instanceof LinkedList ); //true 5 paths . add ( Paths . get ( "file1" )); 6 paths . add ( Paths . get ( "file2" )); 7 paths . add ( Paths . get ( "file3" )); 8 print ( paths ) // [file1, file2, file3] 9 }

The above demonstrates that paths is an instance of LinkedList and prints out the list.

Later on you could add the following code to write text into the files:

1 for ( var i = 0 ; i < paths . size (); i ++ ) 2 Files . newOutputStream ( paths . get ( i )) 3 . write ( "test

" . getBytes ());

We can use existing Java classes, but we can also create new ones.

6.5 Extending

You can extend Java classes and interfaces using the Java.type and Java.extend functions. For example, you can extend the Callable interface and implement the call method:

1 var concurrent = new JavaImporter ( java . util , java . util . concurrent ); 2 var Callable = Java . type ( "java.util.concurrent.Callable" ); 3 with ( concurrent ) { 4 var executor = Executors . newCachedThreadPool (); 5 var tasks = new LinkedHashSet (); 6 for ( var i = 0 ; i < 200 ; i ++ ) { 7 var MyTask = Java . extend ( Callable , { call : function () { print ( "task " + i )}}) 8 var task = new MyTask (); 9 tasks . add ( task ); 10 executor . submit ( task ); 11 } 12 }

6.6 Invocable

You can also invoke JavaScript functions directly from Java.

Firstly, you need to cast the engine to the Invocable interface:

1 Invocable inv = ( Invocable ) engine ;

Then, to invoke any function, simple use the invokeFunction method, for example:

1 engine . eval ( "function p(s) { print(s) }" ); 2 inv . invokeFunction ( "p" , "hello" );

Lastly, you can use the getInterface method to implement any interface in JavaScript.

For example, if you have the following JPrinter interface, you can use it like so:

1 public static interface JPrinter { 2 void p ( String s ); 3 } 4 // later on... 5 JPrinter printer = inv . getInterface ( JPrinter . class ); 6 printer . p ( "Hello again!" );

Notes