Some interview questions

What is the default value of byte datatype in Java

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How can a dead thread be started?

A dead thread cannot be restarted

what is an applet

A java program that runs in a Web browser, can be fully functional Java app because it has the entire Java API at its disposal

Difference between Swing and AWT components

AWT is heavy weight, Swing is light weight. Heavy weight depends on the local windowing toolkit (ex. java.awt.Button) when running on Java platform it maps to a real Motif button

what is polymorphism

Ability of object to take on many forms. most common use is when parent class reference is used to refer to child class object

difference between an interface and an abstract class

Abstract class can have instance methods that implement a default behavior Interface can only declare constans and instance methods BUT cannot implement default behavior, and all methods are implicitly abstract. Interface has all public members and no implementation

When a thread is created and started what is its initial state

After created and started a thread is in the ready state

What inokes a thread's run() method

After the thread it started via its start() method of Thread class the JVM invokes the threads run() method when the thread is inititally executed

What needs to be kept in mind while creating your own expections in java

All exceptions must be a child of throwable if you want to write a checked exception that is automaticaally enforced by Handle or Declare rule you need to extend the Exception class if you write a runtime exception you need to extend the runtimeException class

What is a Runtime Exception

Are ignored at the time of compilation, will compile

Difference between a break statement and continue statment

Break results in termination of statement to which it applies(switch, for, do, or while) a continue statement is used to end the current loop iteration and return control to the loop statement

What are advantages of arrayList over arrays

Can grow dynamically and provides more powerful insertion and search mechanisms than arrays

What is meant by checked exception

Cannot be ignored at the time of compilation, won't compile

What kind of variables can be in a class?

Class can consists of local, instance or class variables

What is a static variable

Class variable, declared with stataic keyword but outside a method, constructor or a block

What are Wrapper classes

Classes that allow primitive types to be accessed as objects EX: Integer, Character, Double, Boolean, etc.

Why is Java Architectural Neutral

Compiler generates achitecture-neutral object file format, makes compiled code executable on many processors as long as Java runtime is present

which class is immediate superclass of Container class

Component class is immediate super class

difference between ocnstructors and other methods

Constructors must have same name as class and can not return a value, only called once Regular methods could be called many times

Singleton class

Control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes

Three steps for creating an Object for a class

Declared Instantiated Initialized

What is the difference between error and an exception

Error is irrecoverable condition at runtime(ex. OutOfMemory) Exceptions are conditinos that occur because of bad input (ex. FileNotFound)

What is final class

Final classes are creataed so the methods implemented by that class cannot be overridden can't be inherited

Difference between Window and a Frame

Frame class extends Window to define a main app window that can have a menu bar

Define packages in Java

Group of related types(classes, interfaces, enumeraations and annotations) providing access protection and name space management

Define composition

Holding the reference of the other classes within some other class is known as composition

What are two subclasses under exception class?

IOException class and RuntimeException class

When is throws keyword used

If a method doesn't handle a checked exception, the method must declare it using the throws keyword, appears at end of methods signature.

What are two ways in which thread can be created

Implementing Runnable interface, extending the thread class

Which arithmetic operations can result in the throwing of an ArithemticException

Integer / and %

List IDE's for Java

Intelisence, Netbeans, Eclipse

Features of interface

Interface -cannot be instantiated -does not contain any constructors -all the methods are abstract

What do mean by Constructor

Invoked when a new object is created. Every claass has one, if you don't make one then the compiler creates a default one for you

Is it necessary that each try block must be followed by a catch block

It is not necessary that each try block be followed by a catch block, it should be followed by a catch block or a finally block

Define Jave Runtime Element

JRE: an implementation of the JVM which executes Java programs. provides min requirements for executing a Java app

What is JAR file

Java Archive File, it aggregates many files into one. Holds Java classes in a library, JAR files are build on ZIP file formate and have .jar file extension

What do you know about Java?

Java is a high-level programing language. Runs on variety of platforms

What makes Java High Performance

Java uses Just-In-Time compiler, turns Java bytecode which is a program that must be interpreted into instructions that are sent directly to processor

purpose of default constructor

Javaa compiler creates a default constructor only if there is no constructor in the class

What is a method

Methods operate on an objects internal state and serve as the primary mechanism for object-to-object communication

Can a top level class be private or protected?

No, a top level class is either public or has no modifier

Can constructor be inherited

No, constructor cannot be inherited

Does garbage collection guarantee that a program will not run out of memory

No, it is possible for programs to use up memory faster than garbage is collected, also possible to create objects not subject to garbage collection

Caan a constructor be made final

No, that is not possible

What whill happen if a static modifier is removed from signature of main method?

NoSuchMethodError is thrown at runtime

default value of ano object reference delcared as an instance variable

Null, unless it is defined explicitly

List any five features of Java

Object Oriented Platform Independent Robust Multi-threaded Interpreted

What is difference between object oriented programming language and object based programming language

Object based programming langauges follow all features of OOPs except inheritance. JavaScript is an example of object based programming language

What does Object mean

Object is an instance of a class, its state is stored in fields and its behavior is shown in methods.

Define Immutable

Once created it cannot be changed

What is String considered immutable

Once it is created the object cannot be changed. It can safely be shared between many threads which is important for multi-threading programming

If a variable is private, where may the variable be accessed?

Only within the class in which it is declared

Immediate superclass of Applet class?

Panel is the immediate superclass. provides space in which an app can attach any other component including other panels

java.util.regex consists of which classes?

Pattern class Matcher class PatternSyntaxException class

According to Java Operator precedence, which operator is considered to have the highest precedence?

Postfix operators () [] have the highest precedence

Define Inheritance

Process where one object acquires the properties of another. With use of inheritance information is managed in a hierarchical order

Why is Vector class used?

Provides capability to implement a growable array of objects. Vector is useful if don't know the size of array in advance, or if you need one that can change sizes

Difference between Reader/Writer class heirarchy and InputSteam/OutputStream class heirarchy

Reader/Writer class is character oriented, Input/Output class is byte oriented

What is runtime polymorphism or dynamic method dispatch

Runtimme polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process an overrident method is called through the reference variable of a superclass

Serialization and deserialization

Serialization is process of writing the state of an object to a byte stream, deserialization is process of restoring these objects

Why is StringBuffer called mutable

String class is immutable, so once created it cannot be chaanged, You would use StringBuffer to make modifications to the String of characters. So since it can modify/change the Buffer is considered mutable

Use of subclass in a Java program

Sub class inherits all the public and protected methods and implementation. also inherits the default modifier methods and their implementation

What are synchronized methods and synchronized statements

Synchronized methods are used to control access to an object Syncrhonized statements can only be executed after a thread has acquired the lock for the object class or reference in the sychronized statement

Which Java operator is right associative

The = operator is right associative

What does access modifier mean/refer to

They set access levels for claasses, variables, methods and constructors members have package or default accessibility if one isn't declared

difference between throw and throws

Throw: used to trigger an exception where as Throws: is used in declaration of exception w/out throws checked exception cannot be handled, where as a checked excpetion caan be propagaated with throws

Whata is nullPointerException

Thrown when calling the instance method of a null object, accessing, or modifying the filed of a null object

When can pasreInt() method be used?

To get the primitive data type of a specific String

when should you use the byte datatype

To save space in a large array, mainly in place of integers

How many bits are in Unicode ASCII UTF-16 UTF-8

Unicode requires 16 ASCII requires 7, represented as 8 UTF-8 represents usins 8, 16 and 18 bit patters UTF-16 uses 16 bit and larger

Explain garbage collection in Java

Used to free memory, cleans objects that are no longer reference by any of the program

Class variables

Variables declared within a class, outside any method with the static keyword

What is a local variable

Variables defined inside methods, constructors or blocks. variable is declared and initialized within the method and it will be destroyed once the method is completed

Instance variable

Variables witin a class but outside any method. They are instantiated when the class is loaded

Why do we need wrapper classes?

We can pass them around as method parameters where a method expects and object. it also provides utility methods

What is a WAR file

Web Archive file, used to store XML, java classes, and JavaServer pages. used to distribute a collection of JavaServer Pages, Java Servlets, Java classes, XML files, static web pages etc.

What is function overloading

When a class has multiple functions of the same name but with different parameters. Same name, different signature

What is Java Virtual Machine

When compiled Java is not platform specific, it is platform independent, the JVM interprets the code on whichever platform Java is running on

When is ArrayStoreException thrown

When copying elements between diff arrays if the source or destination argument types are not compatible

What is difference between yielding and sleeping

When invoking yield() method it returns to ready state When invoking sleep() method it returns to waiting state

What platforms are supported by Java?

Windows, Mac OS, Unix/Linus

Limitations of using inheritance?

Yes, since inheritance inherits everything from superclass and its interface it may make subclass too clusterd and error-prone when dynamic overloading or overriding

Can try statements be nested?

Yes, try statements can be nested

Can you call one constructor from another if a class has multiple constructors?

Yes, use this() syntax

Does it matter what order catch statements for FileNotFoundException and IOException are written

Yes. FileNotFoundException is inherited from the IOException. Exception's subclasses have to be caught first

Can you write a Java class that could be used both as an applet as well as an application?

Yes. just add a main() method to the applet

Define class

a blue print from what objects are created. Can contain fields and methods to describe the behavior of an object

What are ClassLoaders

a class loader is an object that is responsible for loading classes, ClassLoader is an Abstract class

What is an Exception

a problem that arises during the execution of a program. caught by hanglers positioned along the threads method invocation stack

Variable that boolean is automatically initialized as

booleans are automatically false

What datatypes can be used in in switch statments?

byte, short, int or char

what is Abstract class

cannot be instantiated and are either partially implemented or not at all implemented. Class contains one or more abstract methods which are simply method declarations without a body

synchronization

capability to control the access of mulitple threads to sharaed resources, provids locking which ensures mutuaal exclusive access of shared resources and prevent data race

what is an interface

collection of abstract methods. A class implements an interface thereby inheriting the abstract methods of interface

Explain Set interface

collection of element which cannot contain duplicaate elements. contains only methods inherited from Collection aand adds the restriction that duplicate elements are prohibited

What does multithreaded program mean?

contains two or more parts that can run concurrently. Each part of such a program is called a thread, each thread defines a seperate path of execution

Why is java considered dynamic

designed to adapt to an evolving environment can carry extensive amount of run-time information that can be used to verify and resolve accesses to objects on run-time

what is default value of floaat or double datatype

float = 0.0f double = 0.0d

what conditions is an objects finale() invoked by garbage collector

garbage collector invokes an objects finalize() method when it detects that the object is unreachable

what class exceptions are generated by Java run-time system

generates RuntimeException and Error exceptions

When is super keyword used?

if child method overrides one of its superclass's methods. overridden methods can be invoked through use of super. can also refer to a hidden field

When is an Abstract method used

if you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes

List keywords unique to Java

import, super, finally

JIT compiler?

improves the runtime performance of computer programs based on bytecode

List primitive java types

int, boolean, char, double, float, short, long, byte

Extends which class

java.applect.Applet

Which class should you use to obtain design information about an object

java.lang.Class class instance represents classes, interfaces in a running Java app

Which package is used for pattern matchine with regular expressions

java.util.regex

Which package has light weight components

javax.Swing.package, most components of Swing are light weight

how does multi-threading take place on a computer with a single CPU

operating systems task schedulr aallocates execution time to multiple tasks. By quickly switching between executing tasks it creates the impression that the task execute sequentially

Overloading vs. overriding

overloading increases readability of the prograam. overriding provides specific implementation of the method that is already provided by its super class

what does protected accesss modifier mean

package level, used in inheritence when a class is extended from the superclass

paint() vs repaint()

paint() supports painting via a Graphics object. repaint() used to cause paint() to be invoked by the AWT painting thread

What are use cases?

part of the analysis of a program and describes a situation that a program might encounter and what behavior the program should exhibit in that circumstance

snychronized Non Access Modifier

provide functionalities other than Access Modifiers, syncrhonized is used to indicate that a method can be accessed by only one thread at a time

Explain public static void main (String args[])

public: access specifier static: allows main() to be called without instantiating a particular instance of a class void: tells compiler that no value is returned by main main(): called at beginning of Java program String args[]: args parameter is an instance arry of class String

what is abstraction

reduces complexity and improves maintainabiltiy of the system.

What is Dynamic binding(late binding)

referes to linking of a procedure call to the code to be executed in response to the call. Dynamic binding means that the code associated with a given procedure call is not known unitl time of the call at run-time

Explain treeset

set implemented when we wanat elements in a sorted order

What is Collections API

set of classes and interfaces that support operations on collections of objects

Diff between sleep() and wai()

sleep() puts thread aside for exactly 2 sec, wait() causes a wait up to 2 seconds

Difference between static and non-static variables

static variable is associated with the class as a whole, rather than a specific instance of the class. Non-static variables take on unique values with each object instance

what is encapsulation

technique of making fields in a class private and providing access to the fields via a public method. if field is declared private it cannot be accessed by anyone outside the class, also called data hiding

benefit of encapsulation

the main benefit of encapsulation is ability to modify our implemented code w/out breaking the code of oethers who use our code. gives maintaiablity, flexibilty and extensibility of our code

What are the ways in which a thread can enter the waiting state

thread can enter the waiting state by invoking its sleep() method by blocking on IO, by unsuccessfully attempting to acquire an objects lock or by invoking an objects wait() method. it can also enter the waiting state by invoking its (deprecated) suspend() method

why are packages used?

to prevent naming conflicts, control access, and organization

Purpose of the System class

to provide access to system resources

Difference between StringBuffer and StringBuilder

use StringBuilder whenever possible because its faster, but if thread safety is necessary then use StringBuffer objects

How is finally used under exception handling?

used to create a block of code that follows a try block, always executes whether or not an exception has occured

Purpose of File class?

used to create objects that provide access to the files and directorires of local file system

what is finalize() method

used to ensure that object terminates cleanly, defined just before aan objects final destruction by the garbage collector

Whata is static block

used to initialize the static data member, executed before main method at the time of classloading

comparable interface

used to sort collections and arraays of objects using the collections.sort() and java.utils the objects of that class implementing Comparable interface can be ordered

What is the Locale class

used to tailor program output to the conventions of a particular geographic, polirical or cultural region

explain use of () with regard to constructors

used with variables or methods, used to call constructor of same class

What is a transient variable

variable that may not be serialized during Serialization and which is initilized by its default value during de-serialization

What is function overriding

when a subclass provides a specific implementation of a method that is already provided by its paarent class

relationship between clipping and repaining under AWT

when a window is repainted by AWT it sets clipping regions to the area of the window that requires repainting

When is ArithmeticException thrown

when integer is divided by zero or taking remainder of a number by zero, never thrown in floating-point operations