Funktionsprogrammierung für Java-Entwickler, Teil 2

Willkommen zurück zu diesem zweiteiligen Tutorial, in dem die funktionale Programmierung in einem Java-Kontext vorgestellt wird. In Funktionale Programmierung für Java-Entwickler, Teil 1, habe ich JavaScript-Beispiele verwendet, um Ihnen den Einstieg in fünf funktionale Programmiertechniken zu erleichtern: reine Funktionen, Funktionen höherer Ordnung, verzögerte Auswertung, Abschlüsse und Currying. Durch die Darstellung dieser Beispiele in JavaScript konnten wir uns in einer einfacheren Syntax auf die Techniken konzentrieren, ohne auf die komplexeren funktionalen Programmierfunktionen von Java eingehen zu müssen.

In Teil 2 werden wir diese Techniken mit Java-Code vor Java 8 erneut betrachten. Wie Sie sehen werden, ist dieser Code funktionsfähig, aber nicht einfach zu schreiben oder zu lesen. Sie werden auch in die neuen funktionalen Programmierfunktionen eingeführt, die vollständig in die Java-Sprache in Java 8 integriert wurden. nämlich Lambdas, Methodenreferenzen, funktionale Schnittstellen und die Streams-API.

In diesem Tutorial werden wir Beispiele aus Teil 1 erneut betrachten, um zu sehen, wie die JavaScript- und Java-Beispiele verglichen werden. Sie werden auch sehen, was passiert, wenn ich einige der Beispiele vor Java 8 mit funktionalen Sprachfunktionen wie Lambdas und Methodenreferenzen aktualisiere. Schließlich enthält dieses Tutorial eine praktische Übung, die Ihnen beim Üben des funktionalen Denkens helfen soll. Dazu wandeln Sie einen objektorientierten Java-Code in sein funktionales Äquivalent um.

download Code abrufen Laden Sie den Quellcode herunter, zum Beispiel Anwendungen in diesem Tutorial. Erstellt von Jeff Friesen für JavaWorld.

Funktionale Programmierung mit Java

Viele Entwickler wissen es nicht, aber es war möglich, funktionale Programme in Java vor Java 8 zu schreiben. Um einen umfassenden Überblick über die funktionale Programmierung in Java zu erhalten, lassen Sie uns kurz die funktionalen Programmierfunktionen überprüfen, die vor Java 8 erstellt wurden Wenn Sie diese Probleme gelöst haben, werden Sie wahrscheinlich mehr Verständnis dafür haben, wie neue Funktionen, die in Java 8 eingeführt wurden (wie Lambdas und funktionale Schnittstellen), Javas Ansatz für die funktionale Programmierung vereinfacht haben.

Grenzen der Unterstützung von Java für die funktionale Programmierung

Auch mit Verbesserungen der funktionalen Programmierung in Java 8 bleibt Java eine zwingende, objektorientierte Programmiersprache. Es fehlen Bereichstypen und andere Funktionen, die es funktionaler machen würden. Java wird auch durch nominative Eingabe behindert, was die Bedingung ist, dass jeder Typ einen Namen haben muss. Trotz dieser Einschränkungen profitieren Entwickler, die die Funktionsfunktionen von Java nutzen, weiterhin davon, präziseren, wiederverwendbaren und lesbaren Code schreiben zu können.

Funktionsprogrammierung vor Java 8

Anonyme innere Klassen sowie Schnittstellen und Schließungen sind drei ältere Funktionen, die die funktionale Programmierung in älteren Java-Versionen unterstützen:

  • Mit anonymen inneren Klassen können Sie Funktionen (beschrieben durch Schnittstellen) an Methoden übergeben.
  • Funktionsschnittstellen sind Schnittstellen, die eine Funktion beschreiben.
  • Mit Closures können Sie auf Variablen in ihren äußeren Bereichen zugreifen.

In den folgenden Abschnitten werden wir die fünf in Teil 1 vorgestellten Techniken erneut betrachten, jedoch unter Verwendung der Java-Syntax. Sie werden sehen, wie jede dieser Funktionstechniken vor Java 8 möglich war.

Schreiben von reinen Funktionen in Java

Listing 1 zeigt den Quellcode einer Beispielanwendung, DaysInMonthdie unter Verwendung einer anonymen inneren Klasse und einer funktionalen Schnittstelle geschrieben wurde. Diese Anwendung zeigt, wie man eine reine Funktion schreibt, die in Java lange vor Java 8 erreichbar war.

Listing 1. Eine reine Funktion in Java (DaysInMonth.java)

interface Function { R apply(T t); } public class DaysInMonth { public static void main(String[] args) { Function dim = new Function() { @Override public Integer apply(Integer month) { return new Integer[] { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }[month]; } }; System.out.printf("April: %d%n", dim.apply(3)); System.out.printf("August: %d%n", dim.apply(7)); } }

Die generische FunctionSchnittstelle in Listing 1 beschreibt eine Funktion mit einem einzelnen Parameter vom Typ Tund einem Rückgabetyp vom Typ R. Die FunctionSchnittstelle deklariert eine R apply(T t)Methode, die diese Funktion auf das angegebene Argument anwendet.

Die main()Methode instanziiert eine anonyme innere Klasse, die die FunctionSchnittstelle implementiert . Die apply()Methode entpackt die Box monthund indiziert damit ein Array von Ganzzahlen für Tage im Monat. Die Ganzzahl an diesem Index wird zurückgegeben. (Der Einfachheit halber ignoriere ich Schaltjahre.)

main()next führt diese Funktion zweimal aus, indem aufgerufen wird apply(), um die Tageszählungen für die Monate April und August zurückzugeben. Diese Zählungen werden anschließend gedruckt.

Wir haben es geschafft, eine Funktion zu erstellen, und zwar eine reine Funktion! Denken Sie daran, dass eine reine Funktion nur von ihren Argumenten und keinem externen Zustand abhängt. Es gibt keine Nebenwirkungen.

Kompilieren Sie Listing 1 wie folgt:

javac DaysInMonth.java

Führen Sie die resultierende Anwendung wie folgt aus:

java DaysInMonth

Sie sollten die folgende Ausgabe beachten:

April: 30 August: 31

Schreiben von Funktionen höherer Ordnung in Java

Als nächstes betrachten wir Funktionen höherer Ordnung, die auch als erstklassige Funktionen bezeichnet werden. Denken Sie daran, dass eine Funktion höherer Ordnung Funktionsargumente empfängt und / oder ein Funktionsergebnis zurückgibt. Java ordnet eine Funktion einer Methode zu, die in einer anonymen inneren Klasse definiert ist. Eine Instanz dieser Klasse wird an eine andere Java-Methode übergeben oder von dieser zurückgegeben, die als Funktion höherer Ordnung dient. Das folgende dateiorientierte Codefragment zeigt, wie eine Funktion an eine Funktion höherer Ordnung übergeben wird:

File[] txtFiles = new File(".").listFiles(new FileFilter() { @Override public boolean accept(File pathname) { return pathname.getAbsolutePath().endsWith("txt"); } });

Dieses Codefragment übergibt eine auf der Funktionsschnittstelle basierende java.io.FileFilterFunktion an die Methode der java.io.FileKlasse und File[] listFiles(FileFilter filter)weist sie an, nur die Dateien mit txtErweiterungen zurückzugeben.

Listing 2 zeigt eine andere Möglichkeit, mit Funktionen höherer Ordnung in Java zu arbeiten. In diesem Fall übergibt der Code eine Komparatorfunktion an eine Funktion sort()höherer Ordnung für eine Sortierung aufsteigender Ordnung und eine zweite Komparatorfunktion an sort()eine Sortierung absteigender Ordnung.

Listing 2. Eine Funktion höherer Ordnung in Java (Sort.java)

import java.util.Comparator; public class Sort { public static void main(String[] args) { String[] innerplanets = { "Mercury", "Venus", "Earth", "Mars" }; dump(innerplanets); sort(innerplanets, new Comparator() { @Override public int compare(String e1, String e2) { return e1.compareTo(e2); } }); dump(innerplanets); sort(innerplanets, new Comparator() { @Override public int compare(String e1, String e2) { return e2.compareTo(e1); } }); dump(innerplanets); } static  void dump(T[] array) { for (T element: array) System.out.println(element); System.out.println(); } static  void sort(T[] array, Comparator cmp) { for (int pass = 0; pass 
    
      pass; i--) if (cmp.compare(array[i], array[pass]) < 0) swap(array, i, pass); } static void swap(T[] array, int i, int j) { T temp = array[i]; array[i] = array[j]; array[j] = temp; } }
    

Listing 2 importiert die java.util.ComparatorFunktionsschnittstelle, die eine Funktion beschreibt, die einen Vergleich für zwei Objekte beliebigen, aber identischen Typs durchführen kann.

Two significant parts of this code are the sort() method (which implements the Bubble Sort algorithm) and the sort() invocations in the main() method. Although sort() is far from being functional, it demonstrates a higher-order function that receives a function--the comparator--as an argument. It executes this function by invoking its compare() method. Two instances of this function are passed in two sort() calls in main().

Compile Listing 2 as follows:

javac Sort.java

Run the resulting application as follows:

java Sort

You should observe the following output:

Mercury Venus Earth Mars Earth Mars Mercury Venus Venus Mercury Mars Earth

Lazy evaluation in Java

Lazy evaluation is another functional programming technique that is not new to Java 8. This technique delays the evaluation of an expression until its value is needed. In most cases, Java eagerly evaluates an expression that is bound to a variable. Java supports lazy evaluation for the following specific syntax:

  • The Boolean && and || operators, which will not evaluate their right operand when the left operand is false (&&) or true (||).
  • The ?: operator, which evaluates a Boolean expression and subsequently evaluates only one of two alternative expressions (of compatible type) based on the Boolean expression's true/false value.

Functional programming encourages expression-oriented programming, so you'll want to avoid using statements as much as possible. For example, suppose you want to replace Java's if-else statement with an ifThenElse() method. Listing 3 shows a first attempt.

Listing 3. An example of eager evaluation in Java (EagerEval.java)

public class EagerEval { public static void main(String[] args) { System.out.printf("%d%n", ifThenElse(true, square(4), cube(4))); System.out.printf("%d%n", ifThenElse(false, square(4), cube(4))); } static int cube(int x) { System.out.println("in cube"); return x * x * x; } static int ifThenElse(boolean predicate, int onTrue, int onFalse) { return (predicate) ? onTrue : onFalse; } static int square(int x) { System.out.println("in square"); return x * x; } }

Listing 3 defines an ifThenElse() method that takes a Boolean predicate and a pair of integers, returning the onTrue integer when the predicate is true and the onFalse integer otherwise.

Listing 3 also defines cube() and square() methods. Respectively, these methods cube and square an integer and return the result.

The main() method invokes ifThenElse(true, square(4), cube(4)), which should invoke only square(4), followed by ifThenElse(false, square(4), cube(4)), which should invoke only cube(4).

Compile Listing 3 as follows:

javac EagerEval.java

Run the resulting application as follows:

java EagerEval

You should observe the following output:

in square in cube 16 in square in cube 64

The output shows that each ifThenElse() call results in both methods executing, irrespective of the Boolean expression. We cannot leverage the ?: operator's laziness because Java eagerly evaluates the method's arguments.

Although there's no way to avoid eager evaluation of method arguments, we can still take advantage of ?:'s lazy evaluation to ensure that only square() or cube() is called. Listing 4 shows how.

Listing 4. An example of lazy evaluation in Java (LazyEval.java)

interface Function { R apply(T t); } public class LazyEval { public static void main(String[] args) { Function square = new Function() { { System.out.println("SQUARE"); } @Override public Integer apply(Integer t) { System.out.println("in square"); return t * t; } }; Function cube = new Function() { { System.out.println("CUBE"); } @Override public Integer apply(Integer t) { System.out.println("in cube"); return t * t * t; } }; System.out.printf("%d%n", ifThenElse(true, square, cube, 4)); System.out.printf("%d%n", ifThenElse(false, square, cube, 4)); } static  R ifThenElse(boolean predicate, Function onTrue, Function onFalse, T t) { return (predicate ? onTrue.apply(t) : onFalse.apply(t)); } }

Listing 4 turns ifThenElse() into a higher-order function by declaring this method to receive a pair of Function arguments. Although these arguments are eagerly evaluated when passed to ifThenElse(), the ?: operator causes only one of these functions to execute (via apply()). You can see both eager and lazy evaluation at work when you compile and run the application.

Compile Listing 4 as follows:

javac LazyEval.java

Run the resulting application as follows:

java LazyEval

You should observe the following output:

SQUARE CUBE in square 16 in cube 64

A lazy iterator and more

Neal Ford's "Laziness, Part 1: Exploring lazy evaluation in Java" provides more insight into lazy evaluation. The author presents a Java-based lazy iterator along with a couple of lazy-oriented Java frameworks.

Closures in Java

An anonymous inner class instance is associated with a closure. Outer scope variables must be declared final or (starting in Java 8) effectively final (meaning unmodified after initialization) in order to be accessible. Consider Listing 5.

Listing 5. An example of closures in Java (PartialAdd.java)

interface Function { R apply(T t); } public class PartialAdd { Function add(final int x) { Function partialAdd = new Function() { @Override public Integer apply(Integer y) { return y + x; } }; return partialAdd; } public static void main(String[] args) { PartialAdd pa = new PartialAdd(); Function add10 = pa.add(10); Function add20 = pa.add(20); System.out.println(add10.apply(5)); System.out.println(add20.apply(5)); } }

Listing 5 is the Java equivalent of the closure I previously presented in JavaScript (see Part 1, Listing 8). This code declares an add() higher-order function that returns a function for performing partial application of the add() function. The apply() method accesses variable x in the outer scope of add(), which must be declared final prior to Java 8. The code behaves pretty much the same as the JavaScript equivalent.

Compile Listing 5 as follows:

javac PartialAdd.java

Run the resulting application as follows:

java PartialAdd

You should observe the following output:

15 25

Currying in Java

You might have noticed that the PartialAdd in Listing 5 demonstrates more than just closures. It also demonstrates currying, which is a way to translate a multi-argument function's evaluation into the evaluation of an equivalent sequence of single-argument functions. Both pa.add(10) and pa.add(20) in Listing 5 return a closure that records an operand (10 or 20, respectively) and a function that performs the addition--the second operand (5) is passed via add10.apply(5) or add20.apply(5).

Currying lets us evaluate function arguments one at a time, producing a new function with one less argument on each step. For example, in the PartialAdd application, we are currying the following function:

f(x, y) = x + y

We could apply both arguments at the same time, yielding the following:

f(10, 5) = 10 + 5

However, with currying, we apply only the first argument, yielding this:

f(10, y) = g(y) = 10 + y

We now have a single function, g, that takes only a single argument. This is the function that will be evaluated when we call the apply() method.

Partial application, not partial addition

The name PartialAdd stands for partial application of the add() function. It doesn't stand for partial addition. Currying is about performing partial application of a function. It's not about performing partial calculations.

You might be confused by my use of the phrase "partial application," especially because I stated in Part 1 that currying isn't the same as partial application, which is the process of fixing a number of arguments to a function, producing another function of smaller arity. With partial application, you can produce functions with more than one argument, but with currying, each function must have exactly one argument.

Listing 5 presents a small example of Java-based currying prior to Java 8. Now consider the CurriedCalc application in Listing 6.

Listing 6. Currying in Java code (CurriedCalc.java)

interface Function { R apply(T t); } public class CurriedCalc { public static void main(String[] args) { System.out.println(calc(1).apply(2).apply(3).apply(4)); } static Function
    
     > calc(final Integer a) { return new Function
     
      >() { @Override public Function
      
        apply(final Integer b) { return new Function
       
        () { @Override public Function apply(final Integer c) { return new Function() { @Override public Integer apply(Integer d) { return (a + b) * (c + d); } }; } }; } }; } }
       
      
     
    

Listing 6 uses currying to evaluate the function f(a, b, c, d) = (a + b) * (c + d). Given expression calc(1).apply(2).apply(3).apply(4), this function is curried as follows:

  1. f(1, b, c, d) = g(b, c, d) = (1 + b) * (c + d)
  2. g(2, c, d) = h(c, d) = (1 + 2) * (c + d)
  3. h(3, d) = i(d) = (1 + 2) * (3 + d)
  4. i(4) = (1 + 2) * (3 + 4)

Compile Listing 6:

javac CurriedCalc.java

Run the resulting application:

java CurriedCalc

You should observe the following output:

21

Because currying is about performing partial application of a function, it doesn't matter in what order the arguments are applied. For example, instead of passing a to calc() and d to the most-nested apply() method (which performs the calculation), we could reverse these parameter names. This would result in d c b a instead of a b c d, but it would still achieve the same result of 21. (The source code for this tutorial includes the alternative version of CurriedCalc.)

Functional programming in Java 8

Functional programming before Java 8 isn't pretty. Too much code is required to create, pass a function to, and/or return a function from a first-class function. Prior versions of Java also lack predefined functional interfaces and first-class functions such as filter and map.

Java 8 reduces verbosity largely by introducing lambdas and method references to the Java language. It also offers predefined functional interfaces, and it makes filter, map, reduce, and other reusable first-class functions available via the Streams API.

We'll look at these improvements together in the next sections.

Writing lambdas in Java code

A lambda is an expression that describes a function by denoting an implementation of a functional interface. Here's an example:

() -> System.out.println("my first lambda")

From left to right, () identifies the lambda's formal parameter list (there are no parameters), -> signifies a lambda expression, and System.out.println("my first lambda") is the lambda's body (the code to be executed).

A lambda has a type, which is any functional interface for which the lambda is an implementation. One such type is java.lang.Runnable, because Runnable's void run() method also has an empty formal parameter list:

Runnable r = () -> System.out.println("my first lambda");

You can pass the lambda anywhere that a Runnable argument is required; for example, the Thread(Runnable r) constructor. Assuming that the previous assignment has occurred, you could pass r to this constructor, as follows:

new Thread(r);

Alternatively, you could pass the lambda directly to the constructor:

new Thread(() -> System.out.println("my first lambda"));

This is definitely more compact than the pre-Java 8 version:

new Thread(new Runnable() { @Override public void run() { System.out.println("my first lambda"); } });

A lambda-based file filter

My previous demonstration of higher-order functions presented a file filter based on an anonymous inner class. Here's the lambda-based equivalent:

File[] txtFiles = new File(".").listFiles(p -> p.getAbsolutePath().endsWith("txt"));

Return statements in lambda expressions

In Part 1, I mentioned that functional programming languages work with expressions as opposed to statements. Prior to Java 8, you could largely eliminate statements in functional programming, but you couldn't eliminate the return statement.

The above code fragment shows that a lambda doesn't require a return statement to return a value (a Boolean true/false value, in this case): you just specify the expression without return [and add] a semicolon. However, for multi-statement lambdas, you'll still need the return statement. In these cases you must place the lambda's body between braces as follows (don't forget the semicolon to terminate the statement):

File[] txtFiles = new File(".").listFiles(p -> { return p.getAbsolutePath().endsWith("txt"); });

Lambdas with functional interfaces

I have two more examples to illustrate the conciseness of lambdas. First, let's revisit the main() method from the Sort application shown in Listing 2:

public static void main(String[] args) { String[] innerplanets = { "Mercury", "Venus", "Earth", "Mars" }; dump(innerplanets); sort(innerplanets, (e1, e2) -> e1.compareTo(e2)); dump(innerplanets); sort(innerplanets, (e1, e2) -> e2.compareTo(e1)); dump(innerplanets); }

We can also update the calc() method from the CurriedCalc application shown in Listing 6:

static Function
    
     > calc(Integer a) { return b -> c -> d -> (a + b) * (c + d); }
    

Runnable, FileFilter, and Comparator are examples of functional interfaces, which describe functions. Java 8 formalized this concept by requiring a functional interface to be annotated with the java.lang.FunctionalInterface annotation type, as in @FunctionalInterface. An interface that is annotated with this type must declare exactly one abstract method.

You can use Java's pre-defined functional interfaces (discussed later), or you can easily specify your own, as follows:

@FunctionalInterface interface Function { R apply(T t); }

You might then use this functional interface as shown here:

public static void main(String[] args) { System.out.println(getValue(t -> (int) (Math.random() * t), 10)); System.out.println(getValue(x -> x * x, 20)); } static Integer getValue(Function f, int x) { return f.apply(x); }

New to lambdas?

If you're new to lambdas, you might need more background in order to understand these examples. In that case, check out my further introduction to lambdas and functional interfaces in "Get started with lambda expressions in Java." You'll also find numerous helpful blog posts on this topic. One example is "Functional programming with Java 8 functions," in which author Edwin Dalorzo shows how to use lambda expressions and anonymous functions in Java 8.

Architecture of a lambda

Every lambda is ultimately an instance of some class that's generated behind the scenes. Explore the following resources to learn more about lambda architecture:

  • "How lambdas and anonymous inner classes work" (Martin Farrell, DZone)
  • "Lambdas in Java: A peek under the hood" (Brian Goetz, GOTO)
  • "Why are Java 8 lambdas invoked using invokedynamic?" (Stack Overflow)

I think you'll find Java Language Architect Brian Goetz's video presentation of what's going on under the hood with lambdas especially fascinating.

Method references in Java

Some lambdas only invoke an existing method. For example, the following lambda invokes System.out's void println(s) method on the lambda's single argument:

(String s) -> System.out.println(s)

The lambda presents (String s) as its formal parameter list and a code body whose System.out.println(s) expression prints s's value to the standard output stream.

To save keystrokes, you could replace the lambda with a method reference, which is a compact reference to an existing method. For example, you could replace the previous code fragment with the following:

System.out::println

Here, :: signifies that System.out's void println(String s) method is being referenced. The method reference results in much shorter code than we achieved with the previous lambda.

A method reference for Sort

I previously showed a lambda version of the Sort application from Listing 2. Here is that same code written with a method reference instead:

public static void main(String[] args) { String[] innerplanets = { "Mercury", "Venus", "Earth", "Mars" }; dump(innerplanets); sort(innerplanets, String::compareTo); dump(innerplanets); sort(innerplanets, Comparator.comparing(String::toString).reversed()); dump(innerplanets); }

The String::compareTo method reference version is shorter than the lambda version of (e1, e2) -> e1.compareTo(e2). Note, however, that a longer expression is required to create an equivalent reverse-order sort, which also includes a method reference: String::toString. Instead of specifying String::toString, I could have specified the equivalent s -> s.toString() lambda.

More about method references

There's much more to method references than I could cover in a limited space. To learn more, check out my introduction to writing method references for static methods, non-static methods, and constructors in "Get started with method references in Java."

Predefined functional interfaces

Java 8 introduced predefined functional interfaces (java.util.function) so that developers don't have create our own functional interfaces for common tasks. Here are a few examples:

  • The Consumer functional interface represents an operation that accepts a single input argument and returns no result. Its void accept(T t) method performs this operation on argument t.
  • The Function functional interface represents a function that accepts one argument and returns a result. Its R apply(T t) method applies this function to argument t and returns the result.
  • The Predicate functional interface represents a predicate (Boolean-valued function) of one argument. Its boolean test(T t) method evaluates this predicate on argument t and returns true or false.
  • The Supplier functional interface represents a supplier of results. Its T get() method receives no argument(s) but returns a result.

The DaysInMonth application in Listing 1 revealed a complete Function interface. Starting with Java 8, you can remove this interface and import the identical predefined Function interface.

More about predefined functional interfaces

"Get started with lambda expressions in Java" provides examples of the Consumer and Predicate functional interfaces. Check out the blog post "Java 8 -- Lazy argument evaluation" to discover an interesting use for Supplier.

Additionally, while the predefined functional interfaces are useful, they also present some issues. Blogger Pierre-Yves Saumont explains why.

Functional APIs: Streams

Java 8 introduced the Streams API to facilitate sequential and parallel processing of data items. This API is based on streams, where a stream is a sequence of elements originating from a source and supporting sequential and parallel aggregate operations. A source stores elements (such as a collection) or generates elements (such as a random number generator). An aggregate is a result calculated from multiple input values.

A stream supports intermediate and terminal operations. An intermediate operation returns a new stream, whereas a terminal operation consumes the stream. Operations are connected into a pipeline (via method chaining). The pipeline starts with a source, which is followed by zero or more intermediate operations, and ends with a terminal operation.

Streams is an example of a functional API. It offers filter, map, reduce, and other reusable first-class functions. I briefly demonstrated this API in the Employees application shown in Part 1, Listing 1. Listing 7 offers another example.

Listing 7. Functional programming with Streams (StreamFP.java)

import java.util.Random; import java.util.stream.IntStream; public class StreamFP { public static void main(String[] args) { new Random().ints(0, 11).limit(10).filter(x -> x % 2 == 0) .forEach(System.out::println); System.out.println(); String[] cities = { "New York", "London", "Paris", "Berlin", "BrasÌlia", "Tokyo", "Beijing", "Jerusalem", "Cairo", "Riyadh", "Moscow" }; IntStream.range(0, 11).mapToObj(i -> cities[i]) .forEach(System.out::println); System.out.println(); System.out.println(IntStream.range(0, 10).reduce(0, (x, y) -> x + y)); System.out.println(IntStream.range(0, 10).reduce(0, Integer::sum)); } }

The main() method first creates a stream of pseudorandom integers starting at 0 and ending at 10. The stream is limited to exactly 10 integers. The filter() first-class function receives a lambda as its predicate argument. The predicate removes odd integers from the stream. Finally, the forEach() first-class function prints each even integer to the standard output via the System.out::println method reference.

The main() method next creates an integer stream that produces a sequential range of integers starting at 0 and ending at 10. The mapToObj() first-class function receives a lambda that maps an integer to the equivalent string at the integer index in the cities array. The city name is then sent to the standard output via the forEach() first-class function and its System.out::println method reference.

Lastly, main() demonstrates the reduce() first-class function. An integer stream that produces the same range of integers as in the previous example is reduced to a sum of their values, which is subsequently output.

Identifying the intermediate and terminal operations

Each of limit(), filter(), range(), and mapToObj() are intermediate operations, whereas forEach() and reduce() are terminal operations.

Compile Listing 7 as follows:

javac StreamFP.java

Run the resulting application as follows:

java StreamFP

I observed the following output from one run:

0 2 10 6 0 8 10 New York London Paris Berlin BrasÌlia Tokyo Beijing Jerusalem Cairo Riyadh Moscow 45 45

You might have expected 10 instead of 7 pseudorandom even integers (ranging from 0 through 10, thanks to range(0, 11)) to appear at the beginning of the output. After all, limit(10) seems to indicate that 10 integers will be output. However, this isn't the case. Although the limit(10) call results in a stream of exactly 10 integers, the filter(x -> x % 2 == 0) call results in odd integers being removed from the stream.

More about Streams

If you're unfamiliar with Streams, check out my tutorial introducing Java SE 8's new Streams API for more about this functional API.

In conclusion

Many Java developers won't pursue pure functional programming in a language like Haskell because it differs so greatly from the familiar imperative, object-oriented paradigm. Java 8's functional programming capabilities are designed to bridge that gap, enabling Java developers to write code that's easier to understand, maintain, and test. Functional code is also more reusable and more suitable for parallel processing in Java. With all of these incentives, there's really no reason not to incorporate Java's functional programming options into your Java code.

Write a functional Bubble Sort application

Funktionales Denken ist ein Begriff, der von Neal Ford geprägt wurde und sich auf den kognitiven Wechsel vom objektorientierten Paradigma zum funktionalen Programmierparadigma bezieht. Wie Sie in diesem Tutorial gesehen haben, können Sie viel über funktionale Programmierung lernen, indem Sie objektorientierten Code mithilfe funktionaler Techniken neu schreiben.

Schließen Sie das bisher Gelernte ab, indem Sie die Sortieranwendung aus Listing 2 erneut aufrufen. In diesem kurzen Tipp zeige ich Ihnen, wie Sie eine rein funktionale Blasensortierung schreiben , indem Sie zuerst Techniken vor Java 8 und dann Java 8 verwenden Funktionsmerkmale.

Diese Geschichte "Funktionale Programmierung für Java-Entwickler, Teil 2" wurde ursprünglich von JavaWorld veröffentlicht.