Answers Wanted to these QUESTIONS ?

[aparna3032]

Q.)Why do you Canvas ?

Q.) What are ClassLoaders?


Q.) Can we implement an interface in a JSP? If yes how? and No then why not?


Q.) What is the difference between superclass & subclass?



Q.)Dear Friends, in this sentence "Abstract classes are those for which instances can’t be created",What is the meaning of instances ?











Aparna."Always aim for the moon, Never mind if you fail, You will fall among the Stars”

[Parash]

Q.)Dear Friends, in this sentence "Abstract classes are those for which instances can’t be created",What is the meaning of instances ?


I hope you are quite familiar with the object creation in Java.


Lets take a class A,


Class A{


/*Constructor */


A(){


System.out.println("I am class A");}


public static void main(String args[]){


/* Lemme create my instance*/


A myInstanceA = new A();


}


}


myInstanceA is the instance of class A. If the class A abstract you can not apply the new on that. That means instance can not be created.


Hope you it helps you.


Lemme know if you need clarification


Cheers


arash

[Parash]

Q.) What are ClassLoaders?


http://www.javaworld.com/javaworld/j.../jw-10-indepth. html


Go through above link to know about the class loaders


Q.) What is the difference between superclass & subclass?


Superclass and the subclassare the terms used in the inheritance. The parent class is called the superclass and the class wich extends the parent class is called the subclass.


E.g : A extends B, here is A is super class and the B subclass.

[sowmya571]

hello



Q.)Why do you need Canvas ?





A Canvas component represents a blank rectangular
area of the screen onto which the application can draw or from
which the application can trap input events from the user.




An application must subclass the Canvas class in
order to get useful functionality such as creating a custom
component. The paint method must be overridden
in order to perform custom graphics on the canvas.




Q.) What are ClassLoaders?




The
class loader concept, one of the cornerstones of the Java virtual
machine, describes the behavior of converting a named class into the
bits responsible for implementing that class. Because class loaders
exist, the Java run time does not need to know anything about files and
file systems when running Java programs





Classes are introduced into the Java environment when they are
referenced by name in a class that is already running. There is a bit
of magic that goes on to get the first class running (which is why you
have to declare the main()
method as static, taking a string array as an argument), but once that
class is running, future attempts at loading classes are done by the
class loader.




At its simplest, a
class loader creates a flat name space of class bodies that are
referenced by a string name. The method definition is:











Class r = loadClass(String className, boolean resolveIt);





The variable className
contains a string that is understood by the class loader and is used to
uniquely identify a class implementation. The variable resolveIt
is a flag to tell the class loader that classes referenced by this
class name should be resolved (that is, any referenced class should be
loaded as well).




All Java virtual
machines include one class loader that is embedded in the virtual
machine. This embedded loader is called the primordial class loader. It
is somewhat special because the virtual machine assumes that it has
access to a repository of trusted classes which can be run by the VM without verification.




The primordial class loader implements the default implementation of loadClass(). Thus, this code understands that the class name java.lang.Object is stored in a file with the prefix java/lang/Object.class
somewhere in the class path. This code also implements both class path
searching and looking into zip files for classes. The really cool thing
about the way this is designed is that Java can change its class
storage model simply by changing the set of functions that implements
the class loader.




Digging around in the
guts of the Java virtual machine, you will discover that the primordial
class loader is implemented primarily in the functions FindClassFromClass and ResolveClass.




So when are classes loaded? There are exactly two cases: when the new bytecode is executed (for example, FooClass f = new FooClass();) and when the bytecodes make a static reference to a class (for example, System.out).




Q.) Can we implement an interface in a JSP? If yes how? and No then why not?




i think only a classor interface can
implement an interface and should implements all the methods defined in
that interface but incase of jsp the webcontainer is creating the
instances and call the respective methodsthat is defined in that
container.So we cant implement interfaces in jsp.





Q.) What is the difference between superclass & subclass?



Super class is the one which is inherited

Sub class is the one whicch inherits




class A-------------SUPERCLASS

{




............




}




class B extends A ----------------- SUBCLASS




{

.......




}






Q.)Dear Friends, in this sentence "Abstract classes are those for which instances can’t be created",What is the meaning of instances ?
objects are the instances of the class



instances are nothing but the copy of data members or function which can be accessible for every call on the object