Java – 配列をストリームに変換する方法

getdocs

7年前

Java 8では、
Arrays.stream`または
Stream.of`を使って配列をストリームに変換することができます。

1.オブジェクト配列

オブジェクト配列の場合、
Arrays.stream`と
Stream.of`は同じ出力を返します。

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));
    }

}

出力

a
b
c
d
e
a
b
c
d
e

JDKのソースコードを確認してください。

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.プリミティブ配列

プリミティブ配列の場合、
Arrays.stream`と
Stream.of`は異なる出力を返します。

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

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

    }

}

出力

JDKのソースコードを確認してください。

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);
    }