I have an ArrayList of byte[] and I'm wondering if it's possible to convert this to a byte[] using stream from Java 8. All the arrays inside the ArrayList have the same size.
ArrayList<byte[]> buffer = new ArrayList();
byte[] output = buffer.stream(...)
Try this.
List<byte[]> list = Arrays.asList("abc".getBytes(), "def".getBytes());
byte[] result = list.stream()
.collect(
() -> new ByteArrayOutputStream(),
(b, e) -> b.write(e, 0, e.length),
(a, b) -> {}).toByteArray();
System.out.println(new String(result));
// -> abcdef
b.writeTo(a); suffices.
b.write(e, 0, e.length), you don’t need to deal with IOException, so the entire lambda expression simplifies to (b, e) -> b.write(e, 0, e.length). Further, I can only second @user167019, don’t write broken combiner functions. Especially when there is a simple straight-forward implementation like b.writeTo(a).This is a possible solution with the Guava library:
List<byte[]> list = Arrays.asList("abc".getBytes(), "def".getBytes());
byte[] res = Bytes.toArray(list.stream()
.map(byteArray -> Bytes.asList(byteArray))
.flatMap(listArray -> listArray.stream())
.collect(Collectors.toList()));
flatMap should be what you are looking for, ideally it should look like this:
byte[] output = buffer.stream().flatMap(x -> Arrays.stream(x)).toArray(n -> new byte[n])
But it does not compile.
With some helper methods:
private Byte[] box(final byte[] arr) {
final Byte[] res = new Byte[arr.length];
for (int i = 0; i < arr.length; i++) {
res[i] = arr[i];
}
return res;
}
private byte[] unBox(final Byte[] arr) {
final byte[] res = new byte[arr.length];
for (int i = 0; i < arr.length; i++) {
res[i] = arr[i];
}
return res;
}
The following should work (but not very nice or efficient):
byte[] output = unBox(buffer.stream().flatMap(x -> Arrays.stream(box(x))).toArray(n -> new Byte[n]));
toArray can't be used to output a primitive array (generics don't allow it). This still won't compile.
You can use Guava library, it has Bytes which supports converting byte[] to List<Byte> and back via:
public static List<Byte> asList(byte... backingArray)
and
public static byte[] toArray(Collection<? extends Number> collection)
Another option is to simply iterate and copy the arrays, one by one, to one big byte[], it seems to me simpler and more straightforward that the code in the accepted answer...
public static void main(String[] args) {
List<byte[]> list = Arrays.asList("abc".getBytes(), "def".getBytes());
byte[] flattened= flatByteList(list);
System.out.println(new String(flattened)); // abcdef
}
private static byte[] flatByteList(List<byte[]> list) {
int byteArrlength = list.get(0).length;
byte[] result = new byte[list.size() * byteArrlength]; // since all the arrays have the same size
for (int i = 0; i < list.size(); i++) {
byte[] arr = list.get(i);
for (int j = 0; j < byteArrlength; j++) {
result[i * byteArrlength + j] = arr[j];
}
}
return result;
}
