String comparisons : == vs. equals()

It is a well known that you cannot compare two objects obj1 and obj2 using == operator. == operator checks for the equality of the references not the equality of the objects. Just a little snippet is shown below

Point p1 = new Point(100,100);
Point p2 = new Point(100,100);
System.out.println(p1==p2); // ??

The above code snippet, when run, displays "false" in the output console. But still both p1 and p2 refers to a coordinate (100,100). So if you wish to get your equality to be true for such cases (where the object content! matches), then you should be using "equals()" method. This works for almost all the classes shipped with JDK. For this to work on the objects of the classes that you write (eg: Employee class or Chair or Bench.java), then you should override the method "public boolean equals(Object two)" and implement your meaning of equality of the objects of your class. Also note that, it is highly recommended and important that you override the "int hashCode()" method when you override the "equals" method.

Anyway, now when we deal with String comparisons, one should remember that "Strings" are not value types but are reference types (I mean String is a class). So Strings have to compared for equality using equals method instead of == operator. But the way the JVM is implemented, there are cases under which the == returns true for matching strings. So what are the cases?

Case 1: Where == returns true

String s1 = "Krishna";
String s2 = "Krishna";
if(s1==s2)
System.out.println("Strings are equal!!");
else
System.out.println("You cannot use == here");

Trying running the code as it is and you see the "Strings are equal" on the console. Well can you guess why?

The reason for the == operator to work in this case is that the Java compiler optimizes the strings s1 and s2. Since they are both "initialized" to "Krishna", instead of having two different string objects, optimization is done by having only one object and both s1 and s2 refer to the same "Krishna" object. Remember that the == operator only works for "static" kind of initialization. So if it was the case where s1 = new String("Krishna"); then s1 == s2 would return false instead of true, though it appears both are being initialized to the same "Krishna". Let us see another case where s1 == s2 fails.

Case 2: Where == returns false and equals() return true

String s1 = "Krishna";
String s2 = new String(s1);
if(!(s1==s2))
System.out.println("You cannot use == here");
else if(s1.equals(s2))
System.out.println("You should use 'equals' method in all cases");

Anyway the moral of the story is that you should always use "equals" to compare strings and should never use == even though it works in few cases.

More C++ Pointers

When you perform pointer arithmetic, you move the pointer to the next or previous element of the array. When you say ptr++ where ptr is an integer pointer, it is equivalent to ptr = ptr + 4 (mathematically) assuming integer is of 4 bytes.

Do not do math on pointers that does not point to array. If one keeps this in mind, then you would probably prevent most of the problems.

The following code snippet is perfectly legal:

int *ptr = array;
ptr = ptr+2;
ptr--;
ptr++;

Consider the following declaration:

int numbers[10];

In the above declaration, numbers is of type int const*. This implies that numbers even though is a pointer, is a constant pointer and it cannot appear on the left side of an expression(as a pointer).

int *numbers;

In the above shown declaration,numbers is of type int*. It can still point to an array of itegers and you can actually do a math and can appear on the left side of an expression. So the statement numbers++ in this case is perfectly legal while in the previous case; it is not legal.

Technorati Tags: C++,Dave

.NET 3.5 : VB and C#

[The code in this post appears weird for some reasons, I used LiveWriter and few good but buggy plugins to attach code]

I recently made a list of ASP.NET Videos that are out there online. Excluding the MSDN Nuggets and Microsoft Webcast events, the listings spanned over 4 pages!!! I am really concerned and am cursing myself for not having watched any of those for years. I have actually been a close follower of videocasts/screencasts right from those days when there weren't that many online for free. Now Microsoft, I should say is the largest provider of screencasts, of course only about MS technologies and yet there are tons of their videos that I should be watching. (Of course, lets not forget about Channel 9 where there are quiet a lot more screencasts about everything from MS)

Anyway there is also this dnrTV.com which is run by .NET Rocks guys and provides .NET TV Sessions. The most recent two shows are really something that everyone who is learning .NET should watch. It is by Kathleen Dollard and gives a first hand explanation about .NET 3.5 and shows VB 9.0 and C# 3.0 side by side which is really something! The actual show can be watched here.

I am writing down some important points for my own use and others might find it useful. I hope I am not violating any rights(hopefully).The disclaimer is that the images shown below is taken from the show using Snipping Tool which is shipped along with Windows Vista. This is not my work and I hope not to make any violations. In case of any, please let me know so that I can remove this post online.
Anyway my notes follows ....

.NET 3.5 Assemblies:

The important aspect is that .NET 3.5 still has the same Core - .NET 2.0. Simply put .NET 3.5 is nothing but .NET 3.0 plus additional features and we know that .NET 3.0 is nothing but .NET 2.0 Core plus few amazing libraries like the WPF, WCF, WF, CardSpace. It is also mentioned by Kathleen that Speech library is shipped in the .NET 3.0 and it is not that well known.

.NET Versions

There have been five versions of .NET till date. While VB 7 and C# was for .NET 1.0 and 1.1; VB 8 and C# 2.0 was for .NET 2.0 and .NET 3.0. Now the brand new C# 3.0 and VB 9.0 adds a lot of new features makes programming in these languages very exciting. Anyway the slides look amazing, you should watch the show without fail if you are interested!

Nullable in VB vs. C#

Declaring Nullable in VB

   1: Dim x as Nullable(Of Boolean) = False

   2: Dim y as Boolean? = False   

C# Nullables and bit more code to help you understand what Nullables are!

   1: bool? x = null;

   2: if(x.HasValue)

   3:  Console.WriteLine("x does have a value!");

   4: else

   5:  Console.WriteLine("x do not have a value and is null!");

   6: bool? y = x.GetValueOrDefault(); //y is initialized to false!

   7: Console.WriteLine("y is "+y);

Differences:

Suppose x and y are nullable booleans and x is set to null/nothing while y is set to false. Look at these:
x == y => False in C# and Nothing is VB (since one of them is nothing!). So can you guess the output of the following VB code snippet!

   1: Dim x As Nullable(Of Boolean) = Nothing    

   2: Dim y As Boolean = Nothing    

   3: Console.WriteLine("x = " + ((x = y) Is Nothing).ToString)

Another note for beginners! Consider the C# code:

   1: Employee emp = null;    

   2: if(emp==null)    

   3:  Console.WriteLine("Emp object is set to null!");

The equivalent VB code is:

   1: Dim emp as Employee = Nothing    

   2: If(emp = Nothing) 'This does not return true, = Nothing is not equivalent to == null    

   3:  Console.WriteLine("You would not see this line in the output!")    

   4: If emp Is Nothing ' This is how you check for null!    

   5:  Console.WriteLine("emp Object is set to null!")

If you read the above code snippet, I have conveyed the message in the comment on Line 2.

Frankly there are lot more differences with respect to how operators work differently in C# than in VB. Simply put the key point to remember is that

In Visual Basic, comparison between anything that is null(Nothing) to anything else returns Null/Nothing!!

In C#, you cannot use any operator but == on boolean

Note: By the way a great tool for .NET Developers in Snippet Compiler!

Operators in VB vs C#

Ternary Operator in C#:

   1: bool buddi = false;    

   2: string nameOfBuddi = buddi?"Bhargav":"Someone";    

   3: Console.WriteLine("Buddi is "+nameOfBuddi); //Displays "Someone"    

   4: buddi = true;    

   5: Console.WriteLine("Buddi is "+buddi?"Bhargav":"Someone"); //Displays "Bhargav"

Ternary Operator in VB:

IIf was used as ternary operator in VB but it is actually a function! But in VB 9.0, IIF is an operator! The actual difference would be understood when you look at this snippet:

   1: Shared Sub Main()    

   2:   Console.WriteLine(IIf(true,Method1(),Method2()))    

   3: End Sub    

   4:      

   5: Function Method1 as Integer    

   6:     Console.WriteLine("Method 1")    

   7:     Method1 = 1    

   8: End Function    

   9:      

  10: Function Method2 as Integer    

  11:     Console.WriteLine("Method 2")    

  12:     Return 5    

  13: End Function

The output of the above snippet is

Method 1
Method 2
1

As you can see, Method1() and Method2() are first evaluated and then the appropriate return value is returned. Here since its true, return value of Method1() is returned on the screen.

Another note for beginners, in Visual Basic, you can return a value in two ways. The first way is shown in the above code snippet in Line 7 where you assign FunctionName = returnValue and the second way is to use return keyword, like in other languages. You can use which ever makes more sense to you, well based on your choice.

The next few features were actually present in earlier versions of VB and unavailable in C# ( the above talked features were in C# only until VB 9.0)

Contd...

Storage Classes in C++

Courtesy: Dave, UGA

Storage Classes

Storage class specifies where the object exists in memory. Also the lifetime of the variable's storage determines the storage class of the variable/object.

Automatic Storage

1. Default for all the variables
2. Object is created and destroyed within the block
3. eg: auto float x,y;
4. register storage classes is same as auto storage class. Just that it tells the compiler to place the variable to be put in registers(for better performance)

Static Storage

1. Lifetime of variable is entire program execution
2. static can be applied to local variables defined in functions or to global variables
3. Keeps the value even after the function ends, if its a local variable

Scopes

1. Global Scope - visible to any other file - extern
   
2. File Scope - visible to only that file - static
   
3. Block Scope - visible only within the block.

Some funda

1. extern int globalVariable; This is a variable declaration and this can be specified any number of times. No memory is allocated to this variable yet and thus multiple declarations are allowed. If you have an extern variable and try to access it you get a linker error. To prevent this, you should define a variable of the same name. Then you have actually defined a reference.
   Ex: extern int gv; int gv; //then you can access gv.
   
2. int anotherVar; This is a variable definition and this can be done only once. This variable has been allocated memory and falls under the auto storage class.
   
3. static int getVar() If this function is present in File1.cc and if you try to invoke getVar() in File2.cc, then this is a compiler error since the scope of this function is limited to the File1.cc only.
4. static has another purpose in C/C++. You have a static variable in a function foo(), then within foo(), increment the variable and print out the variable. Now within main call foo() three times. Then each time you get a different(incremented) value. That is the lifetime of the variable is "complete" program. As long as the program runs, the variable value would be stored. The value is restored even after the function returns.
5. Global variables in C/C++ default to zero, while local variables are garbage!
   

What is the output of this C++/C Program?

When compiled using GCC compiler, can you tell what is the output of the following program?

#include
int main()
{
int y=10,p=20;
int num[10]={0};
int x=10,z=4;

num[-2] = -2;
num[10] = 10;
printf("%d\n%d\n",p,z);
}

This program is supposed to be compiled on SunOS, using GCC compiler. Running it on different machine might give a totally different output.

If you run this program, you would understand how dangerous is C++ arrays, if not properly coded. Ofcourse this program does not actually corrupt your system but it does corrupt one of the variables and at times this might be dangerous.

Courtesy: Got this snippet from Dr. Dave's class. But a different example. I really liked this simple program that conveys a very big idea.

My work till date ...

Below is a list of projects that I have developed/worked on. This is to let me have a good idea on what I have worked on, for the upcoming interviews.

Software Associate
Online banking system using CICS
Legacy Decommissioning of 21C Systems

Freelance Developer
VB.NET Scheduler Program(with GUI and Win Service)
FTP Uploader/Downloader(VB.NET)
IP and System Info(ASP.NET,C# Win Service, Binary Serialization, Set up projects in VS 2005)
C# to VB.NET Conversion(ASP.NET Project)
URL Ping Service(C# Win Forms, Set up package, SMTP Coding)
Localization in .NET(ASP.NET and WinForms examples how-to)
Social Networking (Java Servlets! Orkut clone like Software)
ASP.NET Content Management System(ASP.NET VB.NET)
Postscript Expression Evaluation in C#
ASP.NET GDI+ Cropping(ASP.NET)
Small Various VB.NET Projects
XML Serialization Tutorial and Sample project
Distributed Error Reporting Tool(C#)
Whois Lookup in VB.NET
various book reviews at java-tips.org
Personal Information Manager(VB.NET)
Java Servlets Project(my first work!!)
And few other small help projects.

School/Personal Work
Windows Registry Tweaker in C#
Windows Remote Desktop in C#
Bulk Mail Sender
Mail GUI and Proxy Server
Bulk Image Compressor
Mean Shift Tracker in Java
Blob Library in Java
URL Redirection Detection
Loads of Web Crawlers
Grails - BlockBusted Movie Website
and a few others which i cannot recall at this moment!

Configuring SWT Browser Preferences (2)

In one of my previous posts, I blogged about the way one could set SWT Browser Widget Preferences and there has been a comment on my blog!!! (well apart from the comments that my friend Suri makes)
The comment has rightly pointed out to a way that one could programatically set the preferences of SWT Browser widget. The way it could be done is shown in the following code snippet.

Browser browser = MozBrowserUtil.openMozillaBrowser("about:blank");
nsIServiceManager servMgr = Mozilla.getInstance().getServiceManager();
nsIPrefBranch prefs = ( nsIPrefBranch )servMgr.
getServiceByContractID("@mozilla.org/preferences-service;1",
nsIPrefBranch.NS_IPREFBRANCH_IID);

prefs.setIntPref("network.proxy.type", 1);
prefs.setCharPref("network.proxy.http", host);
prefs.setIntPref("network.proxy.http_port", port);

browser.setUrl(url);

Again, I thank the anonymous commentor for bringing this to my notice. I appreciate all your comments and would be better if you leave me atleast an email or a URL to your page/blog so that I can get back to you.

Image Processing with Java

Small Background

This article is to help readers understand how to process images with Java. During my course work in Image Processing(8810) under Dr. Hamid R. Arabnia, I have developed this class that helps one to work on images, apply filters and other such operations. It lets you work with images at pixel level and is implemented well enough. Though my assignments were implemented in .NET for which I used Bitmap class, I wrote this utility class to help out my friends and surprisingly it did work very well. Well, cut the crap and lets go into some theory.

Practical Image

An image could be considered as a 2D array of Pixel. A pixel has a position (x,y) and intensity values (R,G,B). You have a color image and wish to covert the image into a gray scale image, then you extract these R,G,B values and then the gray level of that pixel in the simplest form is (R+G+B)/3 and replace each R,G,B with this average. Note that in image processing terminology, a gray scale image is different from a Black and white image. A Black and White image is also known as a Binary Image where the intensity of each pixel is either 0(Black) or 1(actually 255, white). A Gray Scale image has all R,G,B values equal.

Java Image Processing

There are several options in Java to develop image processing applications. The very well known library is the Java Advanced Imaging(JAI) Api. It is a very sophisticated API and does great work, but again, to understand JAI properly, one should really have a strong Image Processing fundamentals. Simpler options include the ImageIO API(again by Sun), ImageJ library and one very good library is the Hiof's Image Processing API. It can be found here. It is a very decent library and relatively old but does great job. And I have to market my own class. I do not call it a library but it has excellent methods that helps you work with raw pixel data. I call this library as ImageUtility and is available upon request. The following are the methods that are available in the ImageUtility:

1.LoadImage(String path) loads the image and returns the BufferedImage object

2.GetImageMatrix(BufferedImage img) returns the image passed as 2d array of integers. A pixel at (0,0) is accessed as returnedArray[0][0] and the value is between 0 to 255.

3.GetHistogram(int[][] matrix) - returns the int[] histogram of the passed matrix. A histogram is the distribution of intensities. For example, histogram[255] gives you the number of pixels whose intensity is White and histogram[0] gives the number of pixels whose intensity is Black.

4.GetImageFromMatrix(int[][] matrix) a function reverse to GetImageMatrix() method. Pass your matrix and get its bufferedImage object.

5.SaveImageToDisk(BufferedImage,String saveFileAs,String type) saves the image to the disk at the specified path(saveFileAs) as a image of type("jpg","png","gif")

6.GetBufferedImage(Image) Image is the super class of BufferedImage and yet is not straight forward to get a BufferedImage from an Image. This function does that exactly and trust me it is very useful.

7.GetHistogramImage(int[] histogram) returns the histogram as an Image, a graph that is more visual than an integer array.

8.ApplyFilter(int[][] filter,int[][] matrix) I should say this is the heart of the library. A Kernel/Filter is a 2D Array that does magic on an image. You can sharpen the image, do noise reduction(smoothing) and different other operations with this amazing filter. Consider the filter as a square piece of paper and your image as bigger sheet of paper. First put the smaller paper on the image paper such that both their (0,0) are on top of each other. Also both the horizontal and vertical borders of each should be aligned. I hope this gives and idea. Now you multiple each pixel positions and add the total value. For example, if you have a 3x3 kernel, first of all, you do a

   1: val = k[0][0]*i[0][0]+k[0][1]*i[0][1]+k[0][1]*i[0][1] +

   2: k[1][0]*i[1][0]+k[1][1]*i[1][1]+k[1][1]*i[1][1] +

   3: k[2][0]*i[2][0]+k[2][1]*i[2][1]+k[2][1]*i[2][1]

where k is the kernel array and i[][] is the image matrix.

Now you put the value at i[1][1] (the center of image where kernel is applied). Once the first position is applied, you just move the kernel by one pixel such that k[0][0] is now at i[0][1]. That way you move the kernel all the way till the row ends. Then you move by one pixel down and reapply the same way. That way you cover the entire image. This infact is a time consuming operation and thus it should be now obvious on why image processing suites such as Photoshop,Paint.NET and GiMP are CPU intesive.

Understanding kernels is very important. Most frequent Image processing operations could be done with Kernels. I recommend you take a look at some Image Processing book to get a list of some filters, I dont remember them now.

9. GetFilesList(dir) Just an out of field method. Given a directory path, it returns the list of files in that directory.

10.SaveImageListToDisk(ArrayList list,folderPath) Saves the images in the arraylist in the specified folder.

Well now it is time for me to explain some code. Some of you might not be interested in my code and might just want to know how to load images from disk, how to extract pixel information from the image, etc. The subsequent sections explains you this.

Reading an Image from Disk

We use ImageIO class to load the image. Shown below is the code that loads the image. It is exactly as in ImageUtility.LoadImage() method.

   1: public static BufferedImage LoadImage(String path) throws IOException

   2: {

   3: //Load the image

   4: BufferedImage img = ImageIO.read(new File(path));

   5: //If it is not a gray scale image

   6: if (img.getType() != BufferedImage.TYPE_BYTE_GRAY)

   7: {

   8: //Let us convert to Gray Scale.

   9: ColorConvertOp conv = new ColorConvertOp(ColorSpace

  10: .getInstance(ColorSpace.CS_GRAY), null);

  11: conv.filter(img, img);

  12: }

  13: return img;

  14: }

Saving the image to disk

Again we use ImageIO class. The following method is in my ImageUtility.java

   1: public static void SaveImageToDisk(BufferedImage img, String filename,

   2: String type) throws IOException

   3: {

   4: //very simple, use ImageIO.write()!!!!

   5: ImageIO.write(img, type, new File(filename));

   6: }

   7:  

   8:  

   9:  

  10:  

Extracting the image matrix, the raw pixel data

Get the WritableRaster using img.getRaster(). Then use getSample method from the raster to get the pixel intensity at the specified location(x,y). It is as simple as that.

   1: public static int[][] GetImageMatrix(BufferedImage img)

   2: {

   3:     int height = img.getHeight();

   4:     int width = img.getWidth();

   5:     int imageMatrix[][] = new int[height][width];

   6:     WritableRaster wr = img.getRaster();

   7:     for(int y = 0; y < height; y++)

   8:       for(int x = 0; x < width; x++)

   9:         imageMatrix[y][x] = (wr.getSample(x,y,0)

  10:                             +wr.getSample(x,y,1)

  11:                             +wr.getSample(x,y,2))/2;

  12:     return imageMatrix;

  13: }

Getting image from the matrix passed

Again, pretty straight forward operation like the GetImageMatrix() method. Get the WritableRaster and use wr.setPixel(x,y,pixel) where pixel is an integer array where pixel[0] is R, pixel[1] is G, pixel[2] is B. Look at the method.

   1: public static BufferedImage GetImageFromMatrix(int[][] matrix)

   2: {

   3:     int height = matrix.length;

   4:     int width = matrix[0].length;

   5:     BufferedImage img = new BufferedImage(width, height,

   6:                                           BufferedImage.TYPE_BYTE_GRAY);

   7:     WritableRaster wr = img.getRaster();

   8:     for(int y = 0; y < height; y++)

   9:         for(int x = 0; x< width; x++)

  10:         {

  11:             int[] pixel = new int[3];

  12:             for(int i =0; i<3; i++) pixel[0] = matrix[y][x];

  13:             wr.setPixel(x,y,pixel);

  14:         }

  15:     return img;

  16: }

How to apply filter?

The signature for the apply filter method is shown below. It returns the new matrix after the filter has been applied.

   1: public static int[][] ApplyFilter(double[][] filter, int[][] matrix)

Typical Image Processing Method

We have looked at the most fundamental operations that could be used to do amazing IP operations in a simple fashion. A Typical image processing operation could look something like this.

   1:  

   2: void Sharpen(String imagePath,String outputPath,int[][] sharpenFilter)

   3: {

   4:     BufferedImage actualImage = ImageUtility.LoadImage(imagePath);

   5:     int[][] actualImage_matrix = ImageUtility.GetImageMatrix(actualImage);

   6:     int[][] sharpenedImage_matrix = ImageUtility.ApplyFilter(sharpenFilter,actualImage_matrix);

   7:     BufferedImage sharpenedImage = ImageUtility.GetImageFromMatrix(sharpenedImage_matrix);

   8:     ImageUtility.SaveImageToDisk(sharpenedImage,outputPath,"jpg");

   9: }

The above image could actually be placed in the ImageUtility.java but sometimes you might want to repeatedly apply certain filters in a sequence. Anyway the above method gives the basic idea on how to do image processing operations. The sharpenFilter, like I said earlier, I dont remember, anyway it is just a 2D integer array. Actually a double[][] filter makes more sense. Anyway my idea behind this article is to give fundamental understanding on how to do image processing operations on images with ease. Not many internet resources has this simple a code to write an image processing java code. I hope this helps atleast a few people.

Email me at krishnabhargav@yahoo.com in case you need the complete ImageUtility or some guidance on Java Programming or Image Processing.