Time required in java
I would like to know how long it takes to execute conditional loops individually. For example, if I could use an "if else", "while", "for" or "foreach" loop, which loop would run faster. I know the difference will be very small and most of them will say that it won't, but if I had a program that could access thousands of data, then this moment would appear in the image.
I would like to know if I am declaring a variable at the beginning in java, and if I were to declare it before using it, it would make some difference. will the total time be reduced? if so what is practically used (the one where the variables are declared at the beginning or where they are just declared b4 they are used)?
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There is no point in trying to manually optimize your code - the JIT compiler will most likely do much better than you. Not least because it will have a lot more information about a particular environment at runtime than you might ever have before compiling.
Also, since the optimizers are tuned for the "general case", introducing unusual optimizations into your code can actually make it less optimized by the JIT compiler, thus slower in the end.
However, if you still want to measure something, you can do it this way:
long before = System.currentTimeMillis();
// execute your code e.g. a million times
long after = System.currentTimeMillis();
System.out.println("Execution took " after - before " milliseconds");
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Impossible to answer in general because of all the optimizations Hotspot may or may not do for your code.
Importantly, polygenic lubricants claim to write clear, concise code - not only will it be easier for humans to understand (perhaps most importantly!), But it will be easier to optimize compilers to "understand" and turn into efficient code.
If you're having performance issues then it's time to hack the profiler, see where your code is really spending its time and optimize this section. Again, this usually has to do with improving the algorithm, rather than using a slightly different primitive.
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The difference will be very small, if they are relevant at all :-)
Writing clear, maintainable code is much more important these days than thinking about the difference between foreach or while loops.
The javac compiler will optimize the resulting bytecode so there shouldn't be any difference. Also its very specific algorithms that you use.
If you declare your variable at the beginning (? Whats "start"), it may affect visibility, but most likely not the performance of your application.
Anyway, if you are interested in such performance optimization, take a look at the Java VM publications from Sun: http://java.sun.com/docs/performance/ p>
Also, you might want to take a look at "profiling" your java applications so you can see the difference yourself. Below is a list of open source Java profiling tools: http://java-source.net/open-source/profilers
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Let it be perfectly clear. Here you are micromanaged. It's a waste of time. Here are the optimization rules:
- Do not do this.
- (For experts only!) Don't do it yet.
Write code to use good algorithms. Write the code to be clear. Check if you have problems. Measure to find out exactly where the problem exists. Only optimize, and only if you need to. (And keep in mind that it is very difficult to get good temp data on modern computers, especially if you have non-trivial other things in memory like an IDE.)
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