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POScala - how to explicitly choose which overloaded method to use when one arg must be null?
 

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    <p>All,</p> <p>I'm doing some image manipulation in Scala by making use of BufferedImages and Raster objects. I am attempting to get all the pixels in the buffered image with the following code.</p> <pre><code>val raster = f.getRaster() // Preallocating the array causes ArrayIndexOutOfBoundsException .. http://forums.sun.com/thread.jspa?threadID=5297789 // RGB channels; val pixelBuffer = new Array[Int](width*height*3) val pixels = raster.getPixels(0,0,width,height,pixelBuffer) </code></pre> <p>Now, when I read in relatively large files, this works fine. When I read in 20x20 PNG files, I get an ArrayIndexOutOfBoundsException:</p> <pre><code>java.lang.ArrayIndexOutOfBoundsException: 1200 at sun.awt.image.ByteInterleavedRaster.getPixels(ByteInterleavedRaster.java:1050) </code></pre> <p>I've read <a href="http://forums.sun.com/thread.jspa?threadID=5297789" rel="noreferrer">online</a> that the way around this problem is to NOT preallocate the pixelBuffer, but instead to pass in a null value and use the one returned by the Raster.getPixels method.</p> <p>Here's my problem. When I do the naive approach and just pass Nil as the last argument:</p> <pre><code>val pixels = raster.getPixels(0,0,width,height,Nil) </code></pre> <p>I get the error</p> <pre><code>error: overloaded method value getPixels with alternatives (Int,Int,Int,Int,Array[Double])Array[Double] &lt;and&gt; (Int,Int,Int,Int,Array[Float])Array[Float] &lt;and&gt; (Int,Int,Int,Int,Array[Int])Array[Int] cannot be applied to (Int,Int,Int,Int,Nil.type) val pixels = raster.getPixels(0,0,width,height,Nil) </code></pre> <p>So obviously the compiler cannot determine which of the two methods I'm trying to call; it's ambiguous. If I were using Java, I would cast the null to make my intent explicit. I can't quite figure out how to get the same effect in Scala. Things I've tried:</p> <pre><code> val pixelBuffer:Array[Int] = Nil // Cannot instantiate an Array to Nil for some reason Nil.asInstanceOf(Array[Int]) // asInstanceOf is not a member of Nil </code></pre> <p>Any idea how to tell the compiler explicitly that I want the method with the Int array as last parameter rather than a Float array?</p> <p>EDIT: As an answer points out, I was getting Nil mixed up with null. Nil is an empty list. See the following <a href="http://javaforyou.wordpress.com/2009/07/12/nothingness/" rel="noreferrer">blog post</a></p> <p>Also, I should point out that the array out of bounds exception was my fault (as these things often are). The problem was I was assuming that the raster had 3 channels, but my image had 4 channels, since I had created it that way. I instead preallocate the array as follows:</p> <pre><code>val numChannels = raster.getNumBands() val pixelBuffer = new Array[Int](width*height*numChannels) val pixels = raster.getPixels(minX,minY,width,height,pixelBuffer) </code></pre> <p>Thanks for the help</p>
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