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    <p>Input streams aren't really designed for creating copies or re-using, they're specifically for situations where you don't want to read off into a byte array and use array operations on that (this is especially useful when the whole array isn't available, as in, for e.g. socket comunication). You could buffer up into a byte array, which is the process of reading sections from the stream into a byte array buffer until you have enough information.</p> <p>But that's unnecessary for calculating an md5. Notice that <a href="http://docs.oracle.com/javase/6/docs/api/java/io/InputStream.html" rel="nofollow noreferrer">InputStream</a> is abstract, so it needs be implemented in an extended class. It has many implementations- <a href="http://docs.oracle.com/javase/6/docs/api/java/util/zip/GZIPInputStream.html" rel="nofollow noreferrer"><code>GZIPInputStream</code></a>, fileinputstream etc. These are, in design pattern speak, <a href="http://www.javaworld.com/javaworld/jw-12-2001/jw-1214-designpatterns.html" rel="nofollow noreferrer">decorators</a> of the IO stream: they add extra functionality to the abstract base IO classes. For example, <code>GZIPInputStream</code> gzips up the stream.</p> <p>So, what you need is a stream to do this for md5. There is, joyfully, a well documented similar thing: see <a href="https://stackoverflow.com/questions/304268/getting-a-files-md5-checksum-in-java">this</a> answer. So you should just be able to pass your dropbox input stream (as it will be itself an input stream) to create a new <a href="http://docs.oracle.com/javase/6/docs/api/java/security/DigestInputStream.html" rel="nofollow noreferrer"><code>DigestInputStream</code></a>, and then you can both take the md5 and continue to read as before.</p> <p>Worried about type casting? The idea with decorators in Java is that, since the <code>InputStream</code> <em>base class</em> interfaces all the methods and 'beef' you need to do your IO, there's no harm in passing instances of objects inheriting from <code>InputStream</code> in the constructor of each stream implementation, and you can still do the same core IO.</p> <p>Finally, I should probably answer your actual question- say you <em>still</em> want to "cache" or "backup" the stream anyway? Well, you could just write it to a byte array. This is well documented, but can become faff when your streams get more complicated. Alternatively, try looking at a <a href="http://docs.oracle.com/javase/6/docs/api/java/io/PushbackInputStream.html" rel="nofollow noreferrer"><code>PushbackInputStream</code></a>. Here, you can easily write a function to read off n bytes, perform and operation on them, and then restore them to the stream. Generally good to avoid these implementations of streams in Java, as it's bad for memory use, but no worse than buffering everything up which you'd otherwise have to do.</p> <p>Or, of course, I would have a go with <code>DigestInputStream</code>.</p> <p>Hope this helps,</p> <p>Best.</p>
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    1. POAvoid obtaining same InputStream more than once
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    1. COThanks Tom. Please see my edit for more information.
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    2. COGreat- so you're right about /dev/null; but perhaps I haven't understood but why can't you use input streams all the way? How do you end up with an output stream?
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    3. COSince you're getting these files in memory anyway, you may as well stay there. Try http://commons.apache.org/proper/commons-io/javadocs/api-1.4/org/apache/commons/io/IOUtils.html to fill in any gaps.
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