使用forEach()的Arrays.asList与Arrays.stream

Question

如果您有一个数组，并且希望使用Java8 forEach()方法，哪种方法更好或更有效：

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

或

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

差异是否显着，或者有没有更好的解决方案？

Answer 1

都不是。如果你已经有了一个数组，

String[] array;

我将使用：

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

因为您将数组到流的转换留给了JDK -让它负责效率等。

但是，由于您没有数组，我将使用Stream.of()的varargs来创建一个值流：

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

这又一次让JDK负责在它认为合适的时候有效地流式传输这些值。

Answer 2

这似乎完全没有什么不同。我为此创建了一个测试类。在五次运行过程中，我的输出是：

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

从输出看，Stream.of()方法看起来是最有效的(但始终如此)，并且

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

是可读性很强的代码。Stream.of的优势在于，它不必将数组转换为列表，也不必创建数组然后创建流，而是可以直接从元素创建流。让我有点惊讶的是，由于我做测试的方式，每次用Stream.of()实例化一个新的数组流比传递一个预制的数组更快，可能是因为“捕获”lambdas那些引用外部变量的lambdas的效率有点低。

下面是我的测试类的代码：

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);
        }
    }
}

和我使用的MilliTimer类：

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

当您将doSomething更改为实际不执行任何操作时，如下所示：

public static void doSomething(String s){
}

然后，您将测量这些操作的实际速度，而不是操作字符串=字符串+字符串；这就是doSomething正在做的事情，当然，它的速度大致相同。然而，实际的速度并不相同，使用预制数组的asList要快得多。

这里的实际结果已经被其他人注意到了，你应该当心流，因为它通常比普通的老式java (非lambda)方法慢4倍。