[I] [Java] Allocator `buffer` method no setting the capacity correctly. [arrow-java]

[via GitHub]

FiV0 opened a new issue, #141:
URL: https://github.com/apache/arrow-java/issues/141

   ### Describe the bug, including details regarding any error messages, 
version, and platform.
   
   The following snippet
   ```java
   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


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