Java – 怎样将 Array 转为 Stream

   在java 8中，您可以使用arrays.stream或Stream.of将 Arrays Array into a Stream.

1. Object Arrays

For object arrays, both Arrays.stream and Stream.of returns the same output.

TestJava8.java

package com.mkyong.java8;

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

public class TestJava8 {

    public static void main(String[] args) {

        String[] array = {"a", "b", "c", "d", "e"};

        //Arrays.stream
        Stream<String> stream1 = Arrays.stream(array);
        stream1.forEach(x -> System.out.println(x));

        //Stream.of
        Stream<String> stream2 = Stream.of(array);
        stream2.forEach(x -> System.out.println(x));
    }

}

Output

a
b
c
d
e
a
b
c
d
e

检查JDK源代码.

Arrays.java

/**
  * Returns a sequential {@link Stream} with the specified array as its
  * source.
  *
  * @param  The type of the array elements
  * @param array The array, assumed to be unmodified during use
  * @return a {@code Stream} for the array
  * @since 1.8
  */
 public static <T> Stream<T> stream(T[] array) {
     return stream(array, 0, array.length);
 }

Stream.java

/**
  * Returns a sequential ordered stream whose elements are the specified values.
  *
  * @param  the type of stream elements
  * @param values the elements of the new stream
  * @return the new stream
  */
 @SafeVarargs
 @SuppressWarnings("varargs") // Creating a stream from an array is safe
 public static<T> Stream<T> of(T... values) {
     return Arrays.stream(values);
 }

注
对象数组的stream.of方法调用arrays.stream内部。

2. 原始数组

对原始数组, 用Arrays.stream 和Stream.of 将返回不同的输出结果.

TestJava8.java

package com.mkyong.java8;

import java.util.Arrays;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class TestJava8 {

    public static void main(String[] args) {

        int[] intArray = {1, 2, 3, 4, 5};

        // 1. Arrays.stream -> IntStream
        IntStream intStream1 = Arrays.stream(intArray);
        intStream1.forEach(x -> System.out.println(x));

        // 2. Stream.of -> Stream
        Stream<int[]> temp = Stream.of(intArray);

        // Cant print Stream directly, convert / flat it to IntStream
        IntStream intStream2 = temp.flatMapToInt(x -> Arrays.stream(x));
        intStream2.forEach(x -> System.out.println(x));

    }

}

Output

1
2
3
4
5
1
2
3
4
5

Review the JDK source code.

Arrays.java

/**
  * Returns a sequential {@link IntStream} with the specified array as its
  * source.
  *
  * @param array the array, assumed to be unmodified during use
  * @return an {@code IntStream} for the array
  * @since 1.8
  */
 public static IntStream stream(int[] array) {
     return stream(array, 0, array.length);
 }

Stream.java

/**
  * Returns a sequential {@code Stream} containing a single element.
  *
  * @param t the single element
  * @param  the type of stream elements
  * @return a singleton sequential stream
  */
 public static<T> Stream<T> of(T t) {
     return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
 }

选择哪一个？
对象的数组，都是调用同一个arrays.stream（参见例1，JDK源代码）。原始数组，我更喜欢arrays.stream为好，因为它返回固定大小的intstream，容易操作。

P.S Tested with Oracle JDK 1.8.0_77

References

1. Arrays JavaDoc
2. Stream JavaDoc
3. How to print an array, java and java 8 examples