Arrays.asList vs. Arrays.stream to use forEach()

Question

If you have an Array and you want to use the Java8 forEach() method, which approach is better or more efficient:

Arrays.asList(new String[]{"hallo","hi"}).forEach(System.out::println);

or

Arrays.stream(new String[]{"hallo","hi"}).forEach(System.out::println);

Is the difference significant or are there any better solutions to solve this?

Answer 1

Neither. If you already had an array,

String[] array;

I would use:

Arrays.stream(array).forEach(System.out::println);

because you leave the conversion of the array to a stream to the JDK - let it be responsible for efficiency etc.

But, since you don't have an array, I would use Stream.of()'s varargs to create a stream of the values:

Stream.of("hallo","hi").forEach(System.out::println);

Which again lets the JDK take the responsibility of efficiently streaming the values as it sees fit.

Answer 2

It seems to make almost absolutely no difference. I created a test class for this. Over the course of five runs, my output was this:

Run 1:
Arrays.asList() method................: 3231 ms
Arrays.stream() method................: 3111 ms
Stream.of() method....................: 3031 ms
Arrays.asList() (premade array) method: 3086 ms
Arrays.stream() (premade array) method: 3231 ms
Stream.of() (premade array) method....: 3191 ms

Run 2:
Arrays.asList() method................: 3270 ms
Arrays.stream() method................: 3072 ms
Stream.of() method....................: 3086 ms
Arrays.asList() (premade array) method: 3002 ms
Arrays.stream() (premade array) method: 3251 ms
Stream.of() (premade array) method....: 3271 ms

Run 3:
Arrays.asList() method................: 3307 ms
Arrays.stream() method................: 3092 ms
Stream.of() method....................: 2911 ms
Arrays.asList() (premade array) method: 3035 ms
Arrays.stream() (premade array) method: 3241 ms
Stream.of() (premade array) method....: 3241 ms

Run 4:
Arrays.asList() method................: 3630 ms
Arrays.stream() method................: 2981 ms
Stream.of() method....................: 2821 ms
Arrays.asList() (premade array) method: 3058 ms
Arrays.stream() (premade array) method: 3221 ms
Stream.of() (premade array) method....: 3214 ms

Run 5:
Arrays.asList() method................: 3338 ms
Arrays.stream() method................: 3174 ms
Stream.of() method....................: 3262 ms
Arrays.asList() (premade array) method: 3064 ms
Arrays.stream() (premade array) method: 3269 ms
Stream.of() (premade array) method....: 3275 ms

From the output, It looks like the Stream.of() method is very marginally (but consistently) the most efficent, and

Stream.of("hallo","hi").forEach(System.out::println);

is very readable code. Stream.of has the advantage in that it doesn't have to convert the array into a list, or create an array and then create a stream, but can create a stream directly from the elements. What was mildly surprising to me, was that because of the way I did my tests, it was faster to instantiate a new array stream each time with Stream.of() than it was to pass in a pre-made array, probably because "capturing" lambdas - those that reference an external variable - are a little less efficient.

Here's the code for my test class:

import java.util.Arrays;
import java.util.function.Consumer;
import java.util.stream.Stream;


public class StreamArrayTest {

    public static void main(String[] args){
        System.out.println("Arrays.asList() method................: " + arraysAsListMethod() + " ms");
        System.out.println("Arrays.stream() method................: " + arraysStreamMethod() + " ms");
        System.out.println("Stream.of() method....................: " + streamOfMethod() + " ms");
        System.out.println("Arrays.asList() (premade array) method: " + presetArraysAsListMethod() + " ms");
        System.out.println("Arrays.stream() (premade array) method: " + presetArraysStreamMethod() + " ms");
        System.out.println("Stream.of() (premade array) method....: " + presetStreamsOfMethod() + " ms");
    }

    private static Long timeOneMillion(Runnable runner){
        MilliTimer mt = MilliTimer.start();
        for (int i = 0; i < 1000000; i++){
            runner.run();
        }
        return mt.end();
    }

    private static Long timeOneMillion(String[] strings, Consumer<String[]> consumer){
        MilliTimer mt = MilliTimer.start();
        for (int i = 0; i < 1000000; i++){
            consumer.accept(strings);
        }
        return mt.end();        
    }

    public static Long arraysAsListMethod(){
        return timeOneMillion(()->Arrays.asList(new String[]{"hallo","hi","test","test2","test3","test4","test5","test6","test7","test8"}).forEach(StreamArrayTest::doSomething));
    }

    public static Long arraysStreamMethod(){
        return timeOneMillion(()->Arrays.stream(new String[]{"hallo","hi","test","test2","test3","test4","test5","test6","test7","test8"}).forEach(StreamArrayTest::doSomething));
    }

    public static Long streamOfMethod(){
        return timeOneMillion(()->Stream.of("hallo","hi","test","test2","test3","test4","test5","test6","test7","test8").forEach(StreamArrayTest::doSomething));        
    }   

    public static Long presetArraysAsListMethod(){
        String[] strings = new String[]{"hallo","hi","test","test2","test3","test4","test5","test6","test7","test8"};
        return timeOneMillion(strings, (s)->Arrays.asList(s).forEach(StreamArrayTest::doSomething));    
    }

    public static Long presetArraysStreamMethod(){
        String[] strings = new String[]{"hallo","hi","test","test2","test3","test4","test5","test6","test7","test8"};
        return timeOneMillion(strings, (s)->Arrays.stream(s).forEach(StreamArrayTest::doSomething));    
    }

    public static Long presetStreamsOfMethod(){
        String[] strings = new String[]{"hallo","hi","test","test2","test3","test4","test5","test6","test7","test8"};
        return timeOneMillion(strings, (s)->Stream.of(s).forEach(StreamArrayTest::doSomething));    
    }

    public static void doSomething(String s){
        String result = s;
        for (int i = 0; i < 10; i++){
            result = result.concat(s);
        }
    }
}

And the MilliTimer class I used:

public class MilliTimer {
    private long startTime = 0L;

    private MilliTimer(long startTime){
        this.startTime = startTime;
    }

    public static MilliTimer start(){
        return new MilliTimer(System.currentTimeMillis());
    }

    public long end() throws IllegalArgumentException {
        return System.currentTimeMillis() - startTime;
    }
}

Answer 3

Arrays.asList() method................: 22 ms
Arrays.stream() method................: 26 ms
Stream.of() method....................: 26 ms
Arrays.asList() (premade array) method: 8 ms
Arrays.stream() (premade array) method: 30 ms
Stream.of() (premade array) method....: 17 ms

When you change doSomething to actually do nothing as follows:

public static void doSomething(String s){
}

Then you're measuring the actual speed of these operations instead of the operation String = String + String; This is what doSomething was doing, and of course it is about the same speed consistently. However, the actual speed isn't the same and asList with a premade array is much faster.

The real result here has already been noted by others that you should beware of the stream as it is typically 4 times slower than the plain old java (non-lambda) approach.