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From: Travers Naran <tnaran@gmail.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: Java left shift and right shift operators.
Date: Tue, 26 Apr 2011 07:30:38 -0700
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On 26/04/2011 2:04 AM, Sanny wrote:
>> Is there any way to avoid the if condition and do right shift and left
>> shift using operators depending on shiftby is +ve or -ve?
>
> I used below technique but it gives error.
>
> divider=Math.pow(2,shiftby);
> then
> output = (long) N / divider;

My advice:
if (shift>0) output = N >> shiftby; else output = N << (-shiftby);

The above is pretty efficient, and once the hotspot compiler gets a hold 
of it, it will be even faster.

There is no assembly operator that shifts based on +/- bits; there are 
only shift-left and shift-right operators.  If there was a Java 
operator/function to do what you want, it would still be implemented (in 
machine language) as:

if (shift>0) output = N >> shiftby; else output = N << (-shiftby);

But I am curious why you consider the "if" test is expensive?

The usual rules for optimization:

1. Profile your code
2. Determine where your code is spending most of its time
3. Re-visit your algorithm and decide if you are using the right algorithm.
3a. If so, optimize the methods that are consuming most of the time.
3b. Implement a better algorithm and repeat these processes.