J2SE Interview Questions and Answers 15

What is URN ?
Uniform Resource Name. A unique identifier that identifies an entity, but doesn't tell where it is located. A system can use a URN to look up an entity locally before trying to find it on the Web. It also allows the Web location to change, while still allowing the entity to be found.

What is variable ?
An item of data named by an identifier. Each variable has a type, such as int or Object, and a scope. See also class variable, instance variable, local variable.

What is virtual machine ?
An abstract specification for a computing device that can be implemented in different ways, in software or hardware. You compile to the instruction set of a virtual machine much like you'd compile to the instruction set of a microprocessor. The Java virtual machine consists of a byte code instruction set, a set of registers, a stack, a garbage-collected heap, and an area for storing methods.

What is void ?
A Java keyword used in method declarations to specify that the method does not return any value. void can also be used as a nonfunctional statement.

What is volatile ?
A Java keyword used in variable declarations that specifies that the variable is modified asynchronously by concurrently running threads.

What is Web server ?
Software that provides services to access the Internet, an intranet, or an extranet. A Web server hosts Web sites, provides support for HTTP and other protocols, and executes server-side programs (such as CGI scripts or servlets) that perform certain functions.

What is while ?
A Java keyword used to declare a loop that iterates a block of statements. The loop's exit condition is specified as part of the while statement.

What is world readable files ?
Files on a file system that can be viewed (read) by any user. For example: files residing on Web servers can only be viewed by Internet users if their permissions have been set to world readable.

What is wrapper ?
An object that encapsulates and delegates to another object to alter its interface or behavior in some way.

What is XML ?
Extensible Markup Language. A markup language that allows you to define the tags (markup) needed to identify the data and text in XML documents.

What is ACID ?
The acronym for the four properties guaranteed by transactions: atomicity, consistency, isolation, and durability.

What is Embedded Java Technology ?
The availability of Java 2 Platform, Micro Edition technology under a restrictive license agreement that allows a licensee to leverage certain Java technologies to create and deploy a closed-box application that exposes no APIs.

What is class method ?
A method that is invoked without reference to a particular object. Class methods affect the class as a whole, not a particular instance of the class. Also called a static method. See also instance method.

What is class variable ?
A data item associated with a particular class as a whole--not with particular instances of the class. Class variables are defined in class definitions. Also called a static field. See also instance variable.

What is constructor ?
A pseudo-method that creates an object. In the Java programming language, constructors are instance methods with the same name as their class. Constructors are invoked using the new keyword.

What is core packages ?
The required set of APIs in a Java platform edition which must be supported in any and all compatible implementations.