Skip to main content

Bit manipulation in Java

I have always had a hard time remembering rules of bit manipulation in Java.

So, when someone asked this question on Stackoverflow.com, I knew he had to do masking, but I forgot why.

I decided to look up bit wise operations on Wikipedia to refresh my memory. However, this time I am also blogging the answer so I don't forget (yet another use of blogging :-) ).

There are two types of bit shift operations: arithmetic shift, and logical shift. An arithmetic right shift preserves the sign bit, while logical shifts always insert a zero.

Representing -1 as a signed byte we get: 11111111

-1 >> 1 gives us: 11111111
-1 >>> 1 gives us: 01111111


so by this logic (-1 >>> 8) should give us 00000000 which is 0

Well not so:

byte b = -1;
System.out.println("-1 >>> 8 = " + (b >>> 8));

The output I get is:

-1 >> 8 = 16777215

Hmmm. what just happened? Java converted the signed byte into a signed int, and then did an arithmetic right shift of 8 bits.

So Java converted -1 to a signed 32 bit integer:
1111111111111111111111111111111

Then did a logical right shift of 8 bits getting
00000000111111111111111111111111

which is: 16777215 in base 10

ok, let's try casting the output back into a byte

byte b = -1;
System.out.println("-1 >>> 8 = " + (byte)(b >>> 8));

The output is:

-1 >> 8 = -1

That did not help because the lower 8 bits of the answer are still all 1's, so casting to a byte gives us -1.

ok, so how do I get the 0 I was after? We can get it by bit masking the byte.

byte x = -1;
System.out.println("-1 >>> 8 = " + ((x&0xFF) >>> 8));


Running the above code gives us an output of:

-1 >>> 8 = 0

So what just happened? b is still converted into a signed 32 bit integer, giving us:
1111111111111111111111111111111


but then we mask it with 0xFF, which gives us
0000000000000000000000011111111


Now when we do a logical right shift of 8 bits, we get
0000000000000000000000000000000


I hope this post helps me remember bit wise operations in Java, and I hope it helps you too. If you have a problem remembering things like I do, you may want to create a little example code and blog about it yourself.
Labels:

Comments

Unknown said…
Build yourself a class that knows how to peruse and compose bits to a stream instead of the usual Java input and output streams that thoroughly understand perusing and composing bytes.You'll see it accommodating to discrete out the operations of "give me the next N bits" and "advance the cursor by M bits."For instance,this would permit you to peruse enough data to cover the longest possible Huffman code.When you discover the true length of the Huffman code you simply read,then you advance the cursor by just that numerous bits.A class like that additionally gives you a chance to attempt to compartmentalize the uglier aspects of bit manipulation into a fairly small chunk of code.

,,,,,,,,,,,,,,,,,,,,,,,,,
hidden object game
1:41 PM
Post a Comment

Popular posts from this blog

Commenting your code

Comments are an integral part of any program, even though they do not contribute to the logic. Appropriate comments add to the maintainability of a software. I have heard developers complain about not remembering the logic of some code they wrote a few months back. Can you imagine how difficult it can be to understand programs written by others, when we sometimes find it hard to understand our own code. It is a nightmare to maintain programs that are not appropriately commented. Java classes should contain comments at various levels. There are two types of comments; implementation comments and documentation comments. Implementation comments usually explain design desicisions, or a particularly intricate peice of code. If you find the need to make a lot of implementation comments, then it may signal overly complex code. Documentation comments usually describe the API of a program, they are meant for developers who are going to use your classes. All classes, methods and variables ...
Post a Comment
Read more

Inheritance vs. composition depending on how much is same and how much differs

I am reading the excellent Django book right now. In the 4th chapter on Django templates , there is an example of includes and inheritance in Django templates. Without going into details about Django templates, the include is very similar to composition where we can include the text of another template for evaluation. Inheritance in Django templates works in a way similar to object inheritance. Django templates can specify certain blocks which can be redefined in subtemplates. The subtemplates use the rest of the parent template as is. Now we have all learned that inheritance is used when we have a is-a relationship between classes, and composition is used when we have a contains-a relationship. This is absolutely right, but while reading about Django templates, I just realized another pattern in these relationships. This is really simple and perhaps many of you may have already have had this insight... We use inheritance when we want to allow reuse of the bulk of one object in other ...
3 comments
Read more

Planning a User Guide - Part 3/5 - Co-ordinate the Team

Photo by  Helloquence  on  Unsplash This is the third post in a series of five posts on how to plan a user guide. In the first post , I wrote about how to conduct an audience analysis and the second post discussed how to define the overall scope of the manual. Once the overall scope of the user guide is defined, the next step is to coordinate the team that will work on creating the manual. A typical team will consist of the following roles. Many of these roles will be fulfilled by freelancers since they are one-off or intermittent work engagements. At the end of the article, I have provided a list of websites where you can find good freelancers. Creative Artist You'll need to work with a creative artist to design the cover page and any other images for the user guide. Most small to mid-sized companies don't have a dedicated creative artist on their rolls. But that's not a problem. There are several freelancing websites where you can work with great creative ...
Post a Comment
Read more