Java 8 数组转Stream

在Java8中，你可以使用Arrays.stream或Stream.of来把数组转为Stream。

1. 对象数组

对于对象数组，Arrays.stream和Stream.of返回一样的结果。

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));
    }

}

输出

a
b
c
d
e
a
b
c
d
e

查看JDK的源码后，你会发现Stream.of的内部实际上也是调用Arrays.stream方法。

Arrays.java

   /**
     * Returns a sequential {@link Stream} with the specified array as its
     * source.
     *
     * @param <T> 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 <T> 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);
    }

2. 原始类型数组

对应原始类型数组，Arrays.stream和Stream.of将会返回不同的结果。

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

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

        // 使用flatMapToInt展开Stream<int[]>
        IntStream intStream2 = temp.flatMapToInt(x -> Arrays.stream(x));
        intStream2.forEach(x -> System.out.println(x));

    }

}

输出

1
2
3
4
5
1
2
3
4
5

通过查看JDK源码可以发现，此时Arrays.stream调用的是重载函数Arrays.stream(int[] array)。

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 <T> 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);
    }