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POIs this an off-by-one bug in Java 7?
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2013-12-01T05:48:24.127
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I don't know where to seek clarifications and confirmations on Java API documentation and Java code, so I'm doing it here. In the API documentation for <a href="http://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html"><code>FileChannel</code></a>, I'm finding off-by-one errors w.r.t. to file <code>position</code> and file <code>size</code> in more places than one. Here's just one example. The API documenation for <a href="http://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html#transferFrom%28java.nio.channels.ReadableByteChannel,%20long,%20long%29"><code>transferFrom(...)</code></a> states: <blockquote> "If the given position is greater than the file's current size then no bytes are transferred." </blockquote> I confirmed that the OpenJDK code too contains this code... <pre><code>public long transferFrom(ReadableByteChannel src, long position, long count) throws IOException { // ... if (position > size()) return 0; // ... } </code></pre> ... in file <code>FileChannelImpl.java</code> consistent with the documentation. Now, while the above code snippet and the API documentation appear mutually consistent, I 'feel' that the above should be 'greater than or equal to' and not merely 'greater than' because <code>position</code> being a 0-based index into the file's data, reading at <code>position == size()</code> will have no data to return to the caller! (At <code>position == size() - 1</code>, at least 1 byte -- the last byte of the file -- could be returned to the caller.) Here are some other similar instances in the same API documentation page: <ol> <li><a href="http://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html#position%28long%29"><code>position(...)</code></a>: "Setting the position to a value that is greater than the file's current size is legal but does not change the size of the file." (Should have been 'greater than or equal to'.)</li> <li><a href="http://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html#transferTo%28long,%20long,%20java.nio.channels.WritableByteChannel%29"><code>transferTo(...)</code></a>: " If the given position is greater than the file's current size then no bytes are transferred." (Should have been 'greater than or equal to'.)</li> <li><a href="http://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html#read%28java.nio.ByteBuffer,%20long%29"><code>read(...)</code></a>: "If the given position is greater than the file's current size then no bytes are read." (Should have been 'greater than or equal to'.)</li> </ol> Lastly, the documentation section for the return value of <a href="http://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html#read%28java.nio.ByteBuffer,%20long%29"><code>read(...)</code></a> fails to remain even self-consistent with the rest of the documentation. Here's what it states: <blockquote> <a href="http://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html#read%28java.nio.ByteBuffer,%20long%29"><code>read(...)</code></a> Returns: The number of bytes read, possibly zero, or -1 if the given position is greater than or equal to the file's current size </blockquote> So, in this lone instance, I do see them mention the right thing. Overall, I don't know what to make of all of this. If I write my code today matching this documentation, then a future bug fix in Java (code or documentation) will render my code buggy, requiring a fix from my side as well. If I do the right thing myself today with things as they stand today, then my code becomes buggy to start with!
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<java><code-documentation><filechannel><nio2>
Title
Is this an off-by-one bug in Java 7?
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1. USHarry
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