shift operator use in Java API

[Hitesh Djoshi]

I have a simple question..
it can be moved to other forum if not appropriate here..Here it goes...

What is shift operator doing in the ListIterator in java.util.LinkedList.

if (index < (size >> 1)) {
 next = header.next;
// more code...

The code is from Java api.. in the constructor of ListItr. Just I am not able to figure out what it is doing.. May be am not clear on shift operators overall. I know the bit shifting and padding. But how does it help here.

[Jesper de Jong]

Welcome to the Ranch.

When you shift the bits of an integer one place to the right, which is what size >> 1 does, that's the same as dividing the number by 2. So, this is the same as: if (index < (size / 2)).

People use >> 1 instead of / 2 because they think that bit shifting is a faster operation than division. So, this is a low-level micro-optimization. In general, it's a bad idea to micro-optimize like this, because it makes the code harder to understand, and the assumption that shifting is faster than division isn't even always true. You should leave optimizations like this to the compiler.

[Ivan Jozsef Balazs]

The code is from Java api

some nitpicking: this code snippet might be from a given implementation rather than from the AP Interface.

[Hitesh Djoshi]

Thanks Jesper de Jong for the welcome as well as the great and prompt answer.
I remember reading that shift operator is equal to divide by 2. I was not able to connect the dots on this optimzation for traversing the Doubly Linked list they have there.

Now I totally agree with you that it makes the code difficult to understand.
Thanks a lot .. have a good day.

[Tony Docherty]

Hitesh Djoshi wrote:
I remember reading that shift operator is equal to divide by 2.

To be precise shift right 1 place (ie >> 1) is the same as divide by 2.
Shift right 2 places is the same as divide by 4, shift right 3 places is the same as divide by 8. Do you understand why this is so?

[Hitesh Djoshi]

Tony Docherty wrote:
To be precise shift right 1 place (ie >> 1) is the same as divide by 2.
Shift right 2 places is the same as divide by 4, shift right 3 places is the same as divide by 8. Do you understand why this is so?

Thanks for clearing that up Tony. Well caught. I was wrong in saying that it is equal to divide by 2.

I think I do understand... here it goes.
>>n means division by 2^n.
<< n means multiplication by 2^n.

Lets take an example,

16
In binary=10000
let's say 16>>1, now 10000, removing the Right most bit and appending a zero on the left, it becomes 01000, which in decimal is 8. Evidently 16/2^1=8

Now 16<<1, now 10000, lets add one significant bit to the left side of 16's binary representation.It becomes, 100000, which is in decimal 32.Evidently 16*2^1=32.