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POJava variables changing randomly
 

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    <p>I'm working on a particle engine, in Eclipse on Windows 7, and I have a problem I've never seen before. I have this in my code:</p> <pre><code>float start = startPosition.getY(); p.position.addTo(p.moveVector); float end = startPosition.getY(); if(start != end){ System.out.println("hit"); } </code></pre> <p>And it does print hit quite often. The reason I did this was because startPosition was being changed and it was messing up my program.</p> <p>I also went into the debugger and looked at the values when it printer hit, and I got this for end, all variables in position and moveVector: 1.0E-5 I have no idea what to make of that.</p> <p>Here is my code. </p> <p>Particle Emitter <a href="http://tinyurl.com/9ahwodx" rel="nofollow">http://tinyurl.com/9ahwodx</a></p> <p>Particle <a href="http://tinyurl.com/8rw979d" rel="nofollow">http://tinyurl.com/8rw979d</a></p> <p>Vector 3f <a href="http://tinyurl.com/9do6v2k" rel="nofollow">http://tinyurl.com/9do6v2k</a></p> <p>The code in question is at the bottom of ParticleEmitter.</p> <p>If you want to see or know anything else let me know, but I think that's all the relevant info.</p>
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    1. COI doubt they are "changing randomly" (this would be a serious bug that **would make Java/JVM unsuitable for any serious development**). So, eliminating that line of thought for a minute, what are *other* possibilities for this behavior? Are `startPosition` and `position` *the same object*, perhaps? If not, *do they mutate/update the same variables*? Or, *is there a thread involved*?
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    2. COLet's see now... either java is fundamentally flawed, or you have a bug in your code. Gee, that's a tough one! May I recommend putting breakpoints in the relevant object's setters and examining the call stack to see *who* is setting the values. You may just find out why they are "randomly" changing. Also, sgusc's answer has good info. Try printing the difference between `start` and `end`: You may find it is tiny and therefore an artefact of floating point arithmetic
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    3. COQ: Is it possible that maybe your have a thread (or threads!) that you're not telling us about, that might be updating values behind your back ;)? ALSO: as sgusc correctly pointed out, you *cannot* reliably compare two floating point values for exact equality. This is true for *any* floating point implementation in *any* language: but especially true for (single-precision) "float" values. 'Hope that helps .. PSM
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