How to get the size of an empty "unwritten" stream?

Question

I have an stream that is created with a fixed size. The buffer is empty. I want to be able to get the size of this buffer. What is an efficient way of doing this?

Here is what I have done.

static OutputStream CreateBuffer()
{
    return (OutputStream) new ByteArrayOutputStream(100);
}

static int GetBufferSize(OutputStream out)
{
    return ( (ByteArrayOutputStream) out).size();
}


public static void main(String[] args)
{
    System.out.println(GetBufferSize(CreateBuffer()));
}

I expect 100. I know why it is 0 -- the stream was only allocated but nothing has actually been written to it yet. What is a good implementation to get the size of an empty outputstream without actually copying data?

If you're hoping to get out 100, I'm fairly certain what you're asking for is impossible. Not on a vanilla `ByteArrayOutputStream`, not without evil reflection hackery. The API is designed specifically to avoid answering this question. — Louis Wasserman, Jul 13 '16 at 22:14
it was a mistake. I was updating with proper foramtting, ended up adding the wrong version. corrected. thank you. — nikk, Jul 13 '16 at 22:56

shmosel · Accepted Answer · 2016-07-13T22:20:48.540

1

You can get the internal buffer size by subclassing ByteArrayOutputStream:

private static class ByteArrayOutputStreamWithCapacity extends ByteArrayOutputStream {
    public ByteArrayOutputStreamWithCapacity(int size) {
        super(size);
    }

    public synchronized int capacity() {
        return buf.length;
    }
}

void CreateBuffer()
{
    OutputStream out = new ByteArrayOutputStreamWithCapacity(100);
}

int GetBufferSize(OutPutStream out)
{
    return ( (ByteArrayOutputStreamWithCapacity) out). capacity();
}

edited Jul 13 '16 at 22:20

answered Jul 13 '16 at 22:15

shmosel

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Thank you. Just what I needed. Cheers. – nikk Jul 13 '16 at 22:33
1

@nikk Although this *technically* answers your question, it's very questionable whether you actually need this, as explained in EJP's answer. – shmosel Jul 13 '16 at 22:36
thanks. The answer solves my problem. I just needed to get that size for my application. Thank you. – nikk Jul 13 '16 at 22:53
1

@nikk I doubt it. There is nothing useful you can do with that information. – user207421 Jul 13 '16 at 22:57

user207421 · Answer 2 · 2016-07-13T22:25:55.770

I have a java.io.OutputStream that is created with a fixed size.

No you don't. You have a ByteArrayOutputStream that is created with a non-zero initial capacity.

The buffer is empty. I want to be able to get the size of this buffer.

The logical size of the buffer is zero. The initial capacity of the buffer is whatever you specified in the constructor.

What is an efficient way of doing this?

Remember what you specified in the constructor.

I expect 100.

You should expect zero. See the Javadoc.

I know why it is 0 -- the stream was only allocated but nothing has actually been written to it yet.

Exactly. So why you expected 100 is a mystery.

What is a good implementation to get the size of an empty outputstream without actually copying data?

'The size of an empty output stream' is zero. The initial capacity of your ByteArrayOutputStream is whatever you specified when constructing it. Unclear why you think you need this. The capacity will change efficiently as required. You seem to be confused between 'fixed size' and 'initial capacity'. What you specified was the latter.

How to get the size of an empty "unwritten" stream?

2 Answers2