Convert ArrayList into a byte[]

Question

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How to convert an ArrayList containing Integers to primitive int array?

How to convert an ArrayList<Byte> into a byte[]?

ArrayList.toArray() gives me back a Byte[].

ArrayList is about the most inefficient way to store Bytes there is. If you don't know how many bytes to will have using ByteArrayOutputStream with write() and toByteArray(), don't use List if you can avoid it at all. — Peter Lawrey, Jul 28 '11 at 15:55

score 17 · Answer 1 · edited Aug 12 '14 at 09:27

17

byte[] result = new byte[list.size()];
for(int i = 0; i < list.size(); i++) {
    result[i] = list.get(i).byteValue();
}

Yeah, Java's collections are annoying when it comes to primitive types.

edited Aug 12 '14 at 09:27

meisterluk

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answered Jul 28 '11 at 13:55

Michael Borgwardt

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missing brackets in size () – piggyback Nov 14 '13 at 00:18

JustinKSU · Accepted Answer · 2021-08-13T14:03:44.863

9

After calling toArray() you can pass the result into the Apache Commons toPrimitive method:

https://commons.apache.org/proper/commons-lang/javadocs/api-release/org/apache/commons/lang3/ArrayUtils.html#toPrimitive-java.lang.Byte:A->

edited Aug 13 '21 at 14:03

answered Jul 28 '11 at 13:54

JustinKSU

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For this you would still have to use `toArray()` in the list beforehand. – Marcelo Jul 28 '11 at 13:57
1

This method takes an array as parameter. Not an ``ArrayList``. It would require conversion from the ``ArrayList`` to an array, and then converting the ``Byte`` array to ``byte`` array. Not sure it would be the most efficient way to do the conversion. – Vivien Barousse Jul 28 '11 at 13:57
@Vivien I agree it may not be the most efficient, but probably the simplest to write. If performance is a concern, I would use Michael's or Vivien's answer. – JustinKSU Jul 28 '11 at 17:33
2

byte[] b_arr= ArrayUtils.toPrimitive(arrayList.toArray(new Byte[0])); – jsanmarb Aug 01 '15 at 20:32
this solution doesn't work. toPrimitiveArray returns array of int – coolcool1994 Jul 25 '20 at 11:29
Read the last comment @coolcool1994 – JustinKSU Jul 26 '20 at 18:00
link is broken :( – Andrew Aug 12 '21 at 12:19
@Andrew I fixed the link for you. – JustinKSU Aug 13 '21 at 14:04
@JustinKSU, thank you so much. I think this edition will be useful for all users, not only for me :) – Andrew Aug 13 '21 at 17:10

NPE · Answer 3 · 2011-07-28T16:06:54.670

No built-in method comes to mind. However, coding one up is pretty straightforward:

public static byte[] toByteArray(List<Byte> in) {
    final int n = in.size();
    byte ret[] = new byte[n];
    for (int i = 0; i < n; i++) {
        ret[i] = in.get(i);
    }
    return ret;
}

Note that this will give you a NullPointerException if in is null or if it contains nulls. It's pretty obvious how to change this function if you need different behaviour.

score 2 · Answer 4 · answered Jul 28 '11 at 13:54

byte[] data = new byte[list.size()];
for (int i = 0; i < data.length; i++) {
    data[i] = (byte) list.get(i);
}

Please note that this can take some time due to the fact that Byte objects needs to be converted to byte values.

Also, if your list contains null values, this will throw a NullPointerExcpetion.

Convert ArrayList into a byte[]

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