Interview Questions in Core Java - Index of

You create an abstract class when you want to manipulate a set of classes through a common interface. All
derived-class methods that match the signature of the base-class declaration will be called using the
dynamic binding mechanism.
An abstract method is an incomplete method. It has only a declaration and no method body. Here is the
syntax for an abstract method declaration: abstract void f();
20) How can you force all derived classes to implement a method present in the base class?
Creating and implementing an interface would be the best way for this situation. Just create an interface
with empty methods which forces a programmer to implement all the methods present under it.
Another way of achieving this task is to declare a class as abstract with all its methods abstract.
21) What is the difference between an Applet and an Application?
1. Applets can be embedded in HTML pages and downloaded over the Internet whereas Applications have
no special support in HTML for embedding or downloading.
2. Applets can only be executed inside a java compatible container, such as a browser or appletviewer
whereas Applications are executed at command line by java.exe or jview.exe.
3. Applets execute under strict security limitations that disallow certain operations(sandbox model security)
whereas Applications have no inherent security restrictions.
4. Applets don't have the main() method as in applications. Instead they operate on an entirely different
mechanism where they are initialized by init(),started by start(),stopped by stop() or destroyed by destroy().
22) Java says "write once, run anywhere". What are some ways this isn't quite true?
Any time you use system calls specific to one operating system and do not create alternative calls for
another operating system, your program will not function correctly.
Solaris systems and Intel systems order the bits of an integer differently. (You may have heard of little
endian vs. big endian)
If your code uses bit shifting, or other binary operators, they will not work on systems that have opposide
endianism.
23) Describe java's security model.
Java's security model is one of the most interesting and unique aspects of the language. For the most part
it's broken into two pieces: the user adjustable security manager that checks various API operations like file
access, and the byte code verifier that asserts the validity of compiled byte code.
public abstract class SecurityManager java.lang.SecurityManager is an abstract class which different
applications subclass to implement a particular security policy. It allows an application to determine
whether or not a particular operation will generate a security exception.
24) What is the difference between a Vector and an Array. Discuss the advantages and disadvantages
of both?
The vector container class generalizes the concept of an ordinary C array. Like an array, a vector is an
indexed data structure, with index values that range from 0 to one less than the number of elements
contained in the structure. Also like an array, values are most commonly assigned to and extracted from the
vector using the subscript operator. However, the vector differs from an array in the following important
respects:
The size of the vector can change dynamically. New elements can be inserted on to the end of a vector, or
into the middle. It is important to note, however, that while these abilities are provided, insertion into the
middle of a vector is not as efficient as insertion into the middle of a list.