Understanding Exceptions and Errors

This post will cover very basic meaning and understanding of Exception and Error in java.

What is Exception?

Sometimes the program(code) we write does not behave the way we intent to. Java is designed to be simple. Hence Java developers provided exception handling from the very beginning. By Exception handling it is made sure that all the possible cases in your code which may led to unexpected behavior are handles.

How is Exception different from Error?

Exception as I mentioned previously is  unexpected behavior of your code. This statement is also applicable to Error but the difference being we can recover from Exception but we cannot recover from an Error. Error is fatal and will directly terminate your program and on the other hand Exceptions are situations that can be taken care of . They can be handled so that program can carry on it's execution.

 Example

   Consider a program in which you read some data from user store it in an array and then when user is finished entering data you write this data in a file. Now we know that when we have to write some data in a file it is very much possible that the files may not be present. This results in an Exception. But note that this situation is recoverable. We can specify a construct(we will get to it's syntax and technical aspect a bit later) that will help us catch this type of situation and figure out a way out of it. Possible way out would be inform user that the file is not present and ask him to provide a path for the same.

How decides whether some Exception can occur in curtain situation? Can we create our own custom Exception?

Java is an Object oriented programming language. There are objects and they have methods that are used to interact with other objects.  Exception and Error are also objects. We know Exceptions can occur while executing a particular method. So when we declare this method which we know can throw a known exception we simply state(in the methods signature) that whoever uses this method must handle the corresponding Exception. In fact in java your code will not compile unless you handle all the exceptions associated with the methods in the code you are compiling. So as far as our first question in concerned developers who write(code) a method also indicate the corresponding exceptions in the methods signature. Yes! we can create our own custom Exception. We can create our own function/methods that throw these Exception. Of course there are rules as of how can you achieve this in Java but none the less it can be very much done. 

Exception and Error Structure in Java

This is just to give a structural overview of Exception and Error in java. We will get to it's technicality is future posts.

Understanding differences between dpkg and apt-get/aptitude tools.

dpkg only installs a package, so doing dpkg -i packageName.deb will only install this Deb package, and will notify you of any dependencies that need to be installed, but it will not install them, and it will not configure the packageName.deb because well...the dependencies are not there.

apt-get is a Package Management System that handles the installation of Deb packages on Debian-based Linux distributions. A Package Management System is a set of tools that will help you install, remove, and change packages easily. So apt-get is like a clever dpkg.

I like to think of the timeline this way:

• They came up with a way to "store" the files of an application in a "package" so that it can be installed easily. So the Deb package (.deb extension file) was born.
• They needed a tool to install these .deb files, so they came up with the dpkg tool. This tool, however, will just install the .deb file, but will not install its dependencies because it doesn't have those files and it does not have access to "repositories" to go pull the dependencies from.
• Then, they came up with apt-get, which automates the problems in the previous point. Underneath the hood, apt-get is basically dpkg (so apt-get is a front-end for dpkg), but a clever one that will look for the dependencies and install them. It even looks at the currently installed dependencies and determines ones that are not being used by any other packages, and will inform you that you can remove them.

aptitude then came along, and it's nothing but a front-end for apt-get [so, it's a front-end of a front-end]. aptitude is a UI (user interface) for apt-get. If you want to see this UI, simply type aptitude in the terminal; that's aptitude, that's what it was originally created for. aptitude leverages the apt tools to provide more options and perks than apt-get. For example, aptitude will automatically remove eligible packages, while apt-get needs a separate command to do so. But, in the end, doing sudo aptitude install packageName.deb is the same as sudo apt-get install packageName.deb. There might be subtle differences here and there that I do not know about, but they will both look for the dependencies and do all that stuff.
Also, aptitude does not have Super Cow Powers.

How do I install already downloaded packages in Ubuntu?

You may wish to install a package you have downloaded from a website, rather than from a software repository. These packages are called .deb files. Because they may have been created for a different Linux distribution, you might find that there's dependency issues with Ubuntu, meaning that they may be uninstallable. 

To find a package which you have previously downloaded using Synaptic, aptitude or apt-get, look in /var/cache/apt/archives 

You can then install the package using apt-get or aptitude.  You can also use dpkg.

Simply type the following command to install a package using dpkg.

dpkg -i <packageName>

Note :  Though dpkg will resolve the dependencies and show you if they are not met, it will not automatically fetch and install them from repositories. In fact apt-get internally calls dpkg to install packages after resolving and downloading dependent packages.

How to install packages in Ubuntu?

This post will explain how can you install packages in Ubuntu

1. Vis Basic Graphical Method

   
   You can install your softwares using Ubuntu Software center which a basic GUI tool to install softwares. You can access it simply by typing Ubuntu software center in your dash board.
   
   
   
    
   You have softwares divided into various categories on the left panel like Games, System etc. You can directly search for your software on the top right corner search area.

2. Via Advanced Graphical Method

   
   The Synaptic Package Manager offers a more advanced way of installing packages. If you have problems finding a suitable package with the Add/Remove tool, try using the search in Synaptic. This searches all of the packages in the available repositories, even the ones which don't contain programs. You can access it simply by typing synaptic package manager in your dashboard. Note you need to have root access(authentication) for this.
   
   
   
   
   This will also resolve and install dependencies (if any exists) for the software you are installing.

3. By using the CLI(Command line interface)

   
   The apt-get program is a command-line package manager, which should be used if the Add/Remove tool and Synaptic ever run into problems. It provides an advanced interface to APT, the underlying package management system which Ubuntu uses, but is reasonably easy to operate. Power users may find that apt-get is quicker to use and more powerful than the graphical options above.
   
   Some commonly used commands are
   
   

   • apt-get install <package_name> :  This command installs a new package 

   • apt-get build-dep <package_name>  :  This command searches the repositories and installs the build 
     dependencies for <package_name>. If the package is not in the 
     repositories it will return an error. 

   • apt-get install <package1_name> <package2_name> <package3_name> : APT will accept multiple package names as a space delimited list. 

   • apt-get update : This is the equivalent of "Reload" in Synaptic or "Fetch updates" in Adept.

   • apt-get upgrade : This command upgrades all installed packages.  This is the equivalent of "Mark all upgrades" in Synaptic.

   • apt-get dist-upgrade : The same as the above, except add the "smart upgrade" checkbox. It tells
      APT to use "smart" conflict resolution system, and it will attempt to 
     upgrade the most important packages at the expense of less important 
     ones if necessary. 

Interview Question #13 synchronization on static function/method.

Let me clarify the question with an example. You have the following class.

public class Counter{
private static int count = 0;

public static synchronized int getCount()
{
  return count;
}

public synchronized setCount(int count)
{
   this.count = count;
}

}

You have multi threaded environment. Can two threads simultaneously access

or process getCount() and setCount() methods? 

Even if we have following case.

Counter myCounter = new Counter();

myCounter.setCount(10);

myCounter.getCount();

Answer

First of all you should not access static functions/block using class instance. Though it is perfectly valid to do so. Why is it valid? Each of the class instances(objects) have class information and hence can hence access it. Lets get to our original question on synchronization.




Answer is yes! Two threads can simultaneously operate/process on the two functions getCount() and setCount(). This is because when synchronized method setCount() is called thread acquires an object level lock where as when getCount() method is called it being a static method(owned by class rather than it's instances) acquires a class level lock.

When you use synchronized method or a block lock is obtained on the monitor if the corresponding instance. So in above example for setCount() method lock obtained will be on the myCounter instance where as for method getCount() which is a static or class method lock is obtained on the Class object. You can get this object by using .class keyword or getClass() method. This Class object will be same for all the instances of that class.