Java 8 Streams: Declaratieve benadering voor dataverwerking

Streams genereren

Met Java 8 heeft de Collection-interface twee methodes om een Stream te genereren:

• stream() – Retourneert een sequentiële stream met de collection als bron.

• parallelStream() – Retourneert een parallelle stream met de collection als bron.

List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());

forEach

Stream biedt een nieuwe methode forEach om elk element van de stream te itereren. Het volgende code-segment print 10 willekeurige getallen met forEach.

Random random = new Random();
random.ints().limit(10).forEach(System.out::println);

map

De map-methode wordt gebruikt om elk element af te beelden op het bijbehorende resultaat. Het volgende segment print unieke kwadraten van getallen met map.

List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
List<Integer> squaresList = numbers.stream().map(i -> i*i).distinct().collect(Collectors.toList());

filter

De filter-methode elimineert elementen op basis van een criterium. Het volgende segment telt lege strings met filter.

List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
int count = strings.stream().filter(string -> string.isEmpty()).count();

limit

De limit-methode vermindert de grootte van de stream. Voorbeeld: 10 willekeurige getallen printen.

Random random = new Random();
random.ints().limit(10).forEach(System.out::println);

sorted

De sorted-methode sorteert de stream. Voorbeeld: 10 willekeurige getallen in gesorteerde volgorde.

Random random = new Random();
random.ints().limit(10).sorted().forEach(System.out::println);

Parallelle verwerking

parallelStream is de alternatief voor stream voor parallelle verwerking. Voorbeeld: aantal lege strings tellen met parallelStream.

List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
long count = strings.parallelStream().filter(string -> string.isEmpty()).count();

Het is heel eenvoudig om te schakelen tussen sequentiële en parallelle streams.

Collectors

Collectors combineren het resultaat van verwerking op de elementen van een stream. Collectors kunnen een lijst of een string retourneren.

List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());
System.out.println("Filtered List: " + filtered);
String mergedString = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.joining(", "));
System.out.println("Merged String: " + mergedString);

Statistieken

Met Java 8 zijn statistics collectors geïntroduceerd om alle statistieken te berekenen tijdens stream-verwerking.

List numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
IntSummaryStatistics stats = numbers.stream().mapToInt((x) -> x).summaryStatistics();
System.out.println("Highest number in List : " + stats.getMax());
System.out.println("Lowest number in List : " + stats.getMin());
System.out.println("Sum of all numbers : " + stats.getSum());
System.out.println("Average of all numbers : " + stats.getAverage());

Stream Voorbeeld

Maak het volgende programma in C:\JAVA:

Java8Tester.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Random;
import java.util.stream.Collectors;
import java.util.Map;

public class Java8Tester {
    public static void main(String args[]) {
        System.out.println("Using Java 7: ");
        List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
        System.out.println("List: " + strings);
        long count = getCountEmptyStringUsingJava7(strings);
        System.out.println("Empty Strings: " + count);
        count = getCountLength3UsingJava7(strings);
        System.out.println("Strings of length 3: " + count);
        List<String> filtered = deleteEmptyStringsUsingJava7(strings);
        System.out.println("Filtered List: " + filtered);
        String mergedString = getMergedStringUsingJava7(strings,", ");
        System.out.println("Merged String: " + mergedString);
        List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
        List<Integer> squaresList = getSquares(numbers);
        System.out.println("Squares List: " + squaresList);
        List<Integer> integers = Arrays.asList(1,2,13,4,15,6,17,8,19);
        System.out.println("List: " + integers);
        System.out.println("Highest number in List : " + getMax(integers));
        System.out.println("Lowest number in List : " + getMin(integers));
        System.out.println("Sum of all numbers : " + getSum(integers));
        System.out.println("Average of all numbers : " + getAverage(integers));
        System.out.println("Random Numbers: ");
        Random random = new Random();
        for(int i = 0; i < 10; i++) {
            System.out.println(random.nextInt());
        }
        System.out.println("Using Java 8: ");
        System.out.println("List: " + strings);
        count = strings.stream().filter(string -> string.isEmpty()).count();
        System.out.println("Empty Strings: " + count);
        count = strings.stream().filter(string -> string.length() == 3).count();
        System.out.println("Strings of length 3: " + count);
        filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());
        System.out.println("Filtered List: " + filtered);
        mergedString = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.joining(", "));
        System.out.println("Merged String: " + mergedString);
        squaresList = numbers.stream().map(i -> i*i).distinct().collect(Collectors.toList());
        System.out.println("Squares List: " + squaresList);
        System.out.println("List: " + integers);
        IntSummaryStatistics stats = integers.stream().mapToInt((x) -> x).summaryStatistics();
        System.out.println("Highest number in List : " + stats.getMax());
        System.out.println("Lowest number in List : " + stats.getMin());
        System.out.println("Sum of all numbers : " + stats.getSum());
        System.out.println("Average of all numbers : " + stats.getAverage());
        System.out.println("Random Numbers: ");
        random.ints().limit(10).sorted().forEach(System.out::println);
        count = strings.parallelStream().filter(string -> string.isEmpty()).count();
        System.out.println("Empty Strings: " + count);
    }
    // Java 7 helper methodes (wegge laten voor beknoptheid)
    private static int getCountEmptyStringUsingJava7(List<String> strings) { ... }
    private static int getCountLength3UsingJava7(List<String> strings) { ... }
    private static List<String> deleteEmptyStringsUsingJava7(List<String> strings) { ... }
    private static String getMergedStringUsingJava7(List<String> strings, String separator) { ... }
    private static List<Integer> getSquares(List<Integer> numbers) { ... }
    private static int getMax(List<Integer> numbers) { ... }
    private static int getMin(List<Integer> numbers) { ... }
    private static int getSum(List numbers) { ... }
    private static int getAverage(List<Integer> numbers) { ... }
}

Resultaat verifiëren

Compileer met javac:

C:\JAVA> javac Java8Tester.java

Voer uit:

C:\JAVA> java Java8Tester

Uitvoer (verkort):

Using Java 7: ...
Using Java 8: ...
Empty Strings: 2
Strings of length 3: 3
Filtered List: [abc, bc, efg, abcd, jkl]
Merged String: abc, bc, efg, abcd, jkl
Squares List: [9, 4, 49, 25]
Highest number in List : 19
Lowest number in List : 1
Sum of all numbers : 85
Average of all numbers : 9.444444444444445
Random Numbers: (gesorteerd)
Empty Strings: 2