JAVA EE: How to prove Java 8 Stream is lazy | Java 8 streams tutorial | Java 8 streams

Wednesday 23 August 2017

How to prove Java 8 Stream is lazy | Java 8 streams tutorial | Java 8 streams | Streams in Java 8

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How to prove Java 8 Stream is lazy | Java 8 streams tutorial | Java 8 streams | Streams in Java 8

StreamDemo1.java

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class StreamDemo1
{
    public static void main(String[] args)
    {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8,9);

        // get List from Stream Operation
        List<String> result = numbers.stream()
                .filter(i -> (i % 2) == 0).map(i -> "[" + i + "]")
                .collect(Collectors.toList());

        System.out.println(result);

    }
}

Output

[[2], [4], [6], [8]]

StreamDemo2.java

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamDemo2
{
    public static void main(String[] args)
    {
        List<Integer> newNumbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);

        Stream<Integer> lazyStream = newNumbers.stream().filter(i -> {
            System.out.println("filter: [" + i + "]");
            return (i % 2) == 0;
        }).map(i -> {
            System.out.println("map: [" + i + "]");
            return i;
        });

        for (int i = 1; i <= 3; i++)
        {
            try
            {
                Thread.sleep(1000);
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
            System.out.println("running... " + i + " sec");
        }

        System.out.println("Going to call collect method..");
        System.out.println("---------------------------------\n");
        /*
         * System only activates the command inside filter and map
         * method after we call collect method.
         */
        lazyStream.collect(Collectors.toList());

    }
}

Output

running... 1 sec
running... 2 sec
running... 3 sec
Going to call collect method..
---------------------------------

filter: [1]
filter: [2]
map: [2]
filter: [3]
filter: [4]
map: [4]
filter: [5]
filter: [6]
map: [6]
filter: [7]
filter: [8]
map: [8]
filter: [9]