[Java] Allocator `buffer` method no setting the capacity correctly.

[FiV0]

Describe the bug, including details regarding any error messages, version, and platform.

The following snippet

package org.apache.arrow.vector;

import org.apache.arrow.memory.ArrowBuf;
import org.apache.arrow.memory.RootAllocator;
import org.junit.jupiter.api.Test;


public class BufferAllocatorOverAllocation {

    @Test
    void overallocation()  {
        RootAllocator allocator = new RootAllocator();
        ArrowBuf buf = allocator.buffer(9946);

        System.out.println(buf.capacity());

        buf.capacity(9946);

        System.out.println(buf.capacity());

    }

}

prints

16384
9946

So without setting the capacity explicitly the allocator overallocates memory and also seems to set the capacity of the buffer to that. That is contrary to the docstring of the buffer method:

Note that the buffer may technically be larger than the requested size for rounding purposes. However, the buffer's capacity will be set to the configured size.

Tested against the 14.0.2 tag.

Component(s)

Java

[vibhatha]

@FiV0 Did you look into the definition of capacity function here?

[FiV0]

@vibhatha The point of this ticket was for the buffer method for which (IMO) the docstring or implementation is wrong.

[davisusanibar]

Thanks for opening this issue, @FiV0.

The sentences are separated by dots, maybe this will be improved, but how I understand this is:

Note that the buffer may technically be larger than the requested size for rounding purposes. However, the buffer's capacity will be adjusted based on the configured size using the method properly (buffer.capacity).

[FiV0]

@davisusanibar Then it's a misunderstanding on my part. Your docstring improvement than solves this ticket for me.