Java: what is the best way to convert ArrayList<Byte> to byte[]?

Question

ArrayList<Byte> bytes = new ArrayList<Byte>();
try {
    int data = putObjectRequest.getInputStream().read();
    bytes.add((byte) data);
    while (data != -1) {
        data = putObjectRequest.getInputStream().read();
        bytes.add((byte)data);
    }
} catch (IOException e) {
    e.printStackTrace();
}

I want to convert this to byte[]. is this this the only way?

byte[] byteArray = new byte[bytes.size()];
for (int i = 0; i < bytes.size(); i++) {
   byteArray[i] = bytes.get(i);
}

Ted Hopp · Accepted Answer · 2012-09-06 20:00:34Z

4

I'd suggest using a ByteArrayOutputStream instead of an ArrayList<Byte> to collect your input:

ByteArrayOutputStream bos = new ByteArrayOutputStream();
try {
    int data = putObjectRequest.getInputStream().read();
    while (data != -1) {
        bos.write(data);
        data = putObjectRequest.getInputStream().read();
    }
} catch (IOException e) {
    e.printStackTrace();
}
byte[] byteArray = bos.toByteArray();

This avoids the horrible overhead of boxing and unboxing every byte. (I also fixed a small bug in your original code where you would write -1 if putObjectRequest was empty.)

edited Sep 6, 2012 at 20:00

answered Sep 6, 2012 at 19:14

Ted Hopp

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main-- · Accepted Answer · 2012-09-06 18:52:50Z

2

byte[] byteArray = new byte[bytes.size()];
for (int i = 0; i < bytes.size(); i++) {
   byteArray[i] = bytes.get(i);
}

Yes, this is the only way.

byte[] byteArray = bytes.toArray(new byte[bytes.size()]);

Using toArray() as proposed in another answer does not work because the method can't automatically convert the wrapper type Byte to the primitive byte.

answered Sep 6, 2012 at 18:52

main--

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2 Comments

Sam Grondahl Over a year ago

With an ArrayList implementation this is fine but it will run in polynomial time on any linked list implementation. Writing to object array and then iterating through with static casts is safer because guarantees linear time.

Ted Hopp Over a year ago

@SamGrondahl - OP is using ArrayList, which is not a linked list implementation.

Reimeus · Accepted Answer · 2012-09-06 19:09:30Z

2

Using ArrayUtils in Apache Commons:

byte[] byteArray = ArrayUtils.toPrimitive(bytes.toArray(new Byte[bytes.size()]));

answered Sep 6, 2012 at 19:09

Reimeus

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1 Comment

duffymo Over a year ago

Which uses the code in main-- answer, I'll bet. You'll have to decide whether you prefer three lines of code or one line of code and a JAR dependency.

Sam Grondahl · Accepted Answer · 2012-09-06 19:10:12Z

1

Nope. Easier:

Byte[] byteArray = bytes.toArray(new Byte[bytes.size()]);

And if you really want primitives:

byte[] primitives = new byte[byteArray.length]
for (int i = 0; i < byteArray.length; i++) {
  primitives [i] = (byte)byteArray[i];
}

This guarantees you linear time complexity for both linked list and resizing array implementations.

It's been supported since 5.0:

http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#toArray(T[])

http://docs.oracle.com/javase/6/docs/api/java/util/ArrayList.html

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/ArrayList.html

edited Sep 6, 2012 at 19:10

answered Sep 6, 2012 at 18:48

Sam Grondahl

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5 Comments

daydreamer Over a year ago

true, this doesn't even compiles

Lukas Eder Over a year ago

The OP wants a byte[], not a Byte[]... I guess?

daydreamer Over a year ago

Yes, I wanted byte[] not Byte[] as mentioned in OP

Ali Over a year ago

it is not necessary to pass array size as an argument.

Ted Hopp Over a year ago

So instead of simply iterating over the original ArrayList<Byte> as OP is doing, you propose converting the ArrayList<Byte> to a Byte[] and then iterating. How is this an improvement? (OP is using ArrayList, not some random linked list implementation.)

Lukas Eder · Accepted Answer · 2012-09-06 20:43:38Z

1

You could always use something like TByteList from trove4j, instead of your ArrayList<Byte>. Your algorithm would then become:

TByteList bytes = new TByteArrayList();
try {
    int data = putObjectRequest.getInputStream().read();
    bytes.add((byte) data);
    while (data != -1) {
        data = putObjectRequest.getInputStream().read();
        bytes.add((byte)data);
    }
} catch (IOException e) {
    e.printStackTrace();
}

byte[] byteArray = bytes.toArray();

edited Sep 6, 2012 at 20:43

answered Sep 6, 2012 at 18:49

Lukas Eder

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2 Comments

Ted Hopp Over a year ago

This won't quite work, since TByteList is an interface. It would need to be TByteList bytes = new TByteArrayList();. Then it wouldn't be much different from using the core ByteArrayOutputStream class (which would avoid a jar dependency).

Lukas Eder Over a year ago

@TedHopp: Whoops thanks for the interface hint. You're right, using BAOS is probably optimal in this case. I was kind of throwing this in as an option to the OP's question "is this this the only way?"

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