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 Q201. When were Enums introduced in Java ?
Ans. Enums were introduced with java 5.

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 Q202. What is an API ( Application Programming Interface ) ?
Ans. An API is a kind of technical contract which defines functionality that two parties must provide: a service provider (often called an implementation) and an application. an API simply defines services that a service provider (i.e., the implementation) makes available to applications.

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 Q203. What is URL?
Java EE
Ans. URL is Uniform Resource Locator which is representation of HTTP address.

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 Q204. What is HTTP ?Java EE
Ans. HTTP or Hypertext Transfer Protocol is internet protocol for tranmission of hypertext ( text with meta data ) over internet.

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 Q205. What are the default or implicitly assigned values for data types in java ?
Ans. boolean ---> false
byte ----> 0
short ----> 0
int -----> 0
long ------> 0l
char -----> /u0000
float ------> 0.0f
double ----> 0.0d
any object reference ----> null

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 Q206. Can I import same package/class twice? Will the JVM load the package twice at runtime?Core Java
Ans. One can import the same package or same class multiple times. Neither compiler nor JVM complains wil complain about it. And the JVM will internally load the class only once no matter how many times you import the same class.

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 Q207. Which access specifier can be used with Class ?Core Java
Ans. For top level class we can only use "public" and "default". We can use private with inner class.

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 Q208. Give an Example of Annotations ?
Ans. Suppose that a software group traditionally starts the body of every class with comments providing important information:
public class Generation3List extends Generation2List {

// Author: John Doe
// Date: 3/17/2002
// Current revision: 6
// Last modified: 4/12/2004
// By: Jane Doe
// Reviewers: Alice, Bill, Cindy

// class code goes here

}
To add this same metadata with an annotation, you must first define the annotation type. The syntax for doing this is:
@interface ClassPreamble {
String author();
String date();
int currentRevision() default 1;
String lastModified() default "N/A";
String lastModifiedBy() default "N/A";
// Note use of array
String[] reviewers();
}
The annotation type definition looks similar to an interface definition where the keyword interface is preceded by the at sign (@) (@ = AT, as in annotation type). Annotation types are a form of interface, which will be covered in a later lesson. For the moment, you do not need to understand interfaces.
The body of the previous annotation definition contains annotation type element declarations, which look a lot like methods. Note that they can define optional default values.
After the annotation type is defined, you can use annotations of that type, with the values filled in, like this:
@ClassPreamble (
author = "John Doe",
date = "3/17/2002",
currentRevision = 6,
lastModified = "4/12/2004",
lastModifiedBy = "Jane Doe",
// Note array notation
reviewers = {"Alice", "Bob", "Cindy"}
)
public class Generation3List extends Generation2List {

// class code goes here

}

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 Q209. What are few of the Annotations pre defined by Java?
Ans. @Deprecated annotation indicates that the marked element is deprecated and should no longer be used. The compiler generates a warning whenever a program uses a method, class, or field with the @Deprecated annotation.

@Override annotation informs the compiler that the element is meant to override an element declared in a superclass.

@SuppressWarnings annotation tells the compiler to suppress specific warnings that it would otherwise generate.

@SafeVarargs annotation, when applied to a method or constructor, asserts that the code does not perform potentially unsafe operations on its varargsparameter. When this annotation type is used, unchecked warnings relating to varargs usage are suppressed.

@FunctionalInterface annotation, introduced in Java SE 8, indicates that the type declaration is intended to be a functional interface, as defined by the Java Language Specification.

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 Q210. What are meta Annotations ?
Ans. Annotations that apply to other annotations are called meta-annotations.

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 Q211. Name few meta-annotations ?
Ans. @Retention annotation specifies how the marked annotation is stored:

@Documented annotation indicates that whenever the specified annotation is used those elements should be documented using the Javadoc tool. (By default, annotations are not included in Javadoc.)

@Target annotation marks another annotation to restrict what kind of Java elements the annotation can be applied to.

@Inherited annotation indicates that the annotation type can be inherited from the super class. (This is not true by default.) When the user queries the annotation type and the class has no annotation for this type, the class' superclass is queried for the annotation type. This annotation applies only to class declarations.

@Repeatable annotation, introduced in Java SE 8, indicates that the marked annotation can be applied more than once to the same declaration or type use. For more information, see Repeating Annotations.

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 Q212. Different ways of implementing Threads in Java ?Core Java
Ans. Threads in Java can be implement either by Extending Thread class or implementing runnable interface.

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 Q213. What is a Final Method ?Core Java
Ans. A Method that cannot be overriden in the sub class.

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Very Frequently asked.Favorite question in Walk in drive for many Indian service companies.
 Q214. What is a Final Class ?
Ans. A Class that cannot be sub classed.

  Sample Code for final class

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Very frequently asked. Usually asked with questions related to String. Frequently asked at CTS / Cognizant
  Q215. What is an Immutable Object ?Core Java
Ans. Object that can't be changed after instantiation.

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 Q216. Does Declaring an object "final" makes it immutable ?Core Java
Ans. Only declaring variables as final makes them immutable. Making objects final means that the object handler cannot be used to target some other object but the object is still mutable.

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 Q217. Can we override static methods ? Why ?Core Java
Ans. No.

Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior.

A static method is not associated with any instance of a class so the concept of overriding for runtime polymorphism using static methods is not applicable.

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 Q218. Can we reduce the visibility of the inherited or overridden method ?Core Java
Ans. No.

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 Q219. Name few Java Exceptions ?Core Java
Ans. IndexOutofBoundException ,
NoClassDefFoundException ,
OutOfMemoryException ,
IllegalArgumentException,
NullPointerException,
IllegalStateException

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 Q220. How can we make sure that a code segment gets executed even in case of uncatched exceptions ?
Ans. By putting it within finally.

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 Q221. What is "super" used for ?
Ans. Used to access members of the base class.

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 Q222. What is "this" keyword used for ?
Ans. Used to represent an instance of the class in which it appears.

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 Q223. Difference between boolean and Boolean ?Core Java
Ans. boolean is a primitive type whereas Boolean is a class.

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 Q224. Difference between Serialization and Deserialization ?Core Java
Ans. Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

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 Q225. What is an Enum type ?Core Java
Ans. An enum type is a special data type that enables for a variable to be a set of predefined constants

  Sample Code for enum

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 Q226. What are Wrapper Classes ? What are Primitive Wrapper Classes ?
Core Java
Ans. A wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component. A Wrapper Class that wraps or encapsulates the primitive data type is called Primitive Wrapper Class.

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 Q227. What Design pattern Wrapper Classes implement ?Design
Ans. Adapter.

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 Q228. What is "Import" used for ?Core Java
Ans. Enables the programmer to abbreviate the names of classes defined in a package.

  Sample Code for import

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 Q229. Different types of memory used by JVM ?Core Java
Ans. Class , Heap , Stack , Register , Native Method Stack.

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 Q230. What is a class loader ? What are the different class loaders used by JVM ?Core Java
Ans. Part of JVM which is used to load classes and interfaces.

Bootstrap , Extension and System are the class loaders used by JVM.

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