DBMS Notes (Unit 7- Recovery Systems) (Office-v2003)

Attached complete notes for the 7th Unit (Recovery System)

Prepare well for the Exam.

All the best

click here to Download

External Programs...

This post will be updated frequently....

PROBLEM 15

Problem 14

DBMS record updated

hi we need to append the record to the present record.....

Our HOD demanded......

click here to download

program for exam

/*Applet that displays two TextFields and Calclates Factorial and displays in other Text Field*/
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
public class simpleApplet extends Applet
{

Read more »

How to Group check boxes - Exercise 14

Please see in this example how India, SA and England are in Checkboxgroup A and the rest in B. You can only select one team in a group (the other team becomes automatically unchecked).

import java.awt.*;
import java.awt.event.*;
/**
*
* @author root
*/
public class CheckBoxGroupDemo extends Frame {

public Checkbox india, srilanka, pakistan, southafrica, england;
public CheckboxGroup groupa, groupb;
public CheckboxListener listen;
public TextField message;
public FlowLayout lay;
public String msg;

public CheckBoxGroupDemo() {

Read more »

Attendance Indiscipline not tolerated at IIMs

Dear Friends

Even the IIMs do not tolerate lack of attendance. See this interesting article.

http://timesofindia.indiatimes.com/city/kolkata-/IIMC-denies-awards-to-8-for-indiscipline/articleshow/7854056.cms

Congratulations to team India

Congratulations to Team India for winning it all!

How to Prepare for VIVA

Dear Friends

First get your basics right in terms of refreshing the concepts from lecture notes and prepare the programs.

Once you are clear on the concepts attempt the questions in the questionnaire and compare your answers with what is given. Then you will get clarity on how much you have understood

DO NOT Mug Up these questions. That approach will never work.

Good Luck.

मोरे इम्पोर्तंत Questions

What are Checked Exceptions and Unchecked Exceptions?

The types of exceptions that need not be included in a methods throws list are called Unchecked Exceptions.

* ArithmeticException
* ArrayIndexOutOfBoundsException
* ClassCastException
* IndexOutOfBoundsException
* IllegalStateException
* NullPointerException
* SecurityException

The types of exceptions that must be included in a methods throws list if that method can generate one of these exceptions and does not handle it itself are called Checked Exceptions.

* ClassNotFoundException
* CloneNotSupportedException
* IllegalAccessException
* InstantiationException
* InterruptedException
* NoSuchFieldException
* NoSuchMethodException

What are Chained Exceptions?

The chained exception feature allows you to associate another exception with an exception. This second exception describes the cause of the first exception. Lets take a simple example. You are trying to read a number from the disk and using it to divide a number. Think the method throws an ArithmeticException because of an attempt to divide by zero (number we got). However, the problem was that an I/O error occurred, which caused the divisor to be set improperly (set to zero). Although the method must certainly throw an ArithmeticException, since that is the error that occurred, you might also want to let the calling code know that the underlying cause was an I/O error. This is the place where chained exceptions come in to picture.


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

Or

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

A thread can enter the waiting state by the following ways:
1. Invoking its sleep() method,
2. By blocking on I/O
3. By unsuccessfully attempting to acquire an object's lock
4. By invoking an object's wait() method.
5. It can also enter the waiting state by invoking its (deprecated) suspend() method.


Explain about "finally" block

finally is a block of code that will run anyways if the exception is caught in the try catch or not.The main purpose of this block is to assure that some of the resources that are left dangling after the code is through , are freed. It however depends on the developer to stuff in the code.
The only way to escape the finaly block after try catch is by calling system.exit();

What is the difference between yielding and sleeping?

When a task invokes its yield() method, it returns to the ready state, either from waiting, running or after its creation. When a task invokes its sleep() method, it returns to the waiting state from a running state.

How to create multithreaded program? Explain different ways of using thread? When a thread is created and started, what is its initial state?

Or

Extending Thread class or implementing Runnable Interface. Which is better?

You have two ways to do so. First, making your class "extends" Thread class. The other way is making your class implement "Runnable" interface. The latter is more advantageous, cause when you are going for multiple inheritance, then only interface can help. . If you are already inheriting a different class, then you have to go for Runnable Interface. Otherwise you can extend Thread class. Also, if you are implementing interface, it means you have to implement all methods in the interface. Both Thread class and Runnable interface are provided for convenience and use them as per the requirement. But if you are not extending any class, better extend Thread class as it will save few lines of coding. Otherwise performance wise, there is no distinguishable difference. A thread is in the ready state after it has been created and started.

What is mutual exclusion? How can you take care of mutual exclusion using Java threads?

Mutual exclusion is a phenomenon where no two processes can access critical regions of memory at the same time. Using Java multithreading we can arrive at mutual exclusion. For mutual exclusion, you can simply use the synchronized keyword and explicitly or implicitly provide an Object, any Object, to synchronize on. The synchronized keyword can be applied to a class, to a method, or to a block of code. There are several methods in Java used for communicating mutually exclusive threads such as wait( ), notify( ), or notifyAll( ). For example, the notifyAll( ) method wakes up all threads that are in the wait list of an object.

What is the difference between preemptive scheduling and time slicing?

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then re-enters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

What invokes a thread's run() method?

After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

What is the purpose of the wait(), notify(), and notifyAll() methods?

The wait(), notify() and notifyAll() methods are used to provide an efficient way for thread inter-communication.

What is thread? What are the high-level thread states?

Or

What are the states associated in the thread?

A thread is an independent path of execution in a system. The high-level thread states are ready, running, waiting and dead.

What is deadlock?

When two threads are waiting for each other and can’t proceed until the first thread obtains a lock on the other thread or vice versa, the program is said to be in a deadlock.

How does multithreading take place on a computer with a single CPU?

The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

What are synchronized methods and synchronized statements?

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Can Java object be locked down for exclusive use by a given thread?

Or

What happens when a thread cannot acquire a lock on an object?

Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it. If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

What’s the difference between the methods sleep() and wait()?

The sleep method is used when the thread has to be put aside for a fixed amount of time. Ex: sleep(1000), puts the thread aside for exactly one second. The wait method is used to put the thread aside for up to the specified time. It could wait for much lesser time if it receives a notify() or notifyAll() call. Ex: wait(1000), causes a wait of up to one second. The method wait() is defined in the Object and the method sleep() is defined in the class Thread.

What is the difference between process and thread?

A thread is a separate path of execution in a program. A Process is a program in execution.

What is daemon thread and which method is used to create the daemon thread?

Daemon threads are threads with low priority and runs in the back ground doing the garbage collection operation for the java runtime system. The setDaemon() method is used to create a daemon thread. These threads run without the intervention of the user. To determine if a thread is a daemon thread, use the accessor method isDaemon()

When a standalone application is run then as long as any user threads are active the JVM cannot terminate, otherwise the JVM terminates along with any daemon threads which might be active. Thus a daemon thread is at the mercy of the runtime system. Daemon threads exist only to serve user threads.

What do you understand by Synchronization?

Or

What is synchronization and why is it important?

Or

Describe synchronization in respect to multithreading?

Or

What is synchronization?

With respect to multithreading, Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access a particular resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption which may otherwise lead to dirty reads and significant errors.
E.g. synchronizing a function:
public synchronized void Method1 () {
// method code.
}
E.g. synchronizing a block of code inside a function:
public Method2 (){
synchronized (this) {
// synchronized code here.
}
}

When you will synchronize a piece of your code?

When you expect that your shared code will be accessed by different threads and these threads may change a particular data causing data corruption, then they are placed in a synchronized construct or a synchronized method.

Why would you use a synchronized block vs. synchronized method?

Synchronized blocks place locks for shorter periods than synchronized methods.

What is an object's lock and which objects have locks?

Answer: An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

Can a lock be acquired on a class?

Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

What state does a thread enter when it terminates its processing?

When a thread terminates its processing, it enters the dead state.


# What is the difference between procedural and object-oriented programs?-
a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOP program, unit of program is object, which is nothing but combination of data and code. b) In procedural program, data is exposed to the whole program whereas in OOPs program, it is accessible with in the object and which in turn assures the security of the code.

# What are Encapsulation, Inheritance and Polymorphism?-
Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse. Inheritance is the process by which one object acquires the properties of another object. Polymorphism is the feature that allows one interface to be used for general class actions.

# What is the difference between Assignment and Initialization?-
Assignment can be done as many times as desired whereas initialization can be done only once.

# What is OOPs?-
Object oriented programming organizes a program around its data, i. e. , objects and a set of well defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code.

# What are Class, Constructor and Primitive data types?-
Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform. Constructor is a special kind of method that determines how an object is initialized when created. Primitive data types are 8 types and they are: byte, short, int, long, float, double, boolean, char.

# What is an Object and how do you allocate memory to it?-
Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.

# What is the difference between constructor and method?-
Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.

# What are methods and how are they defined?-
Methods are functions that operate on instances of classes in which they are defined. Objects can communicate with each other using methods and can call methods in other classes. Method definition has four parts. They are name of the method, type of object or primitive type the method returns, a list of parameters and the body of the method. A method’s signature is a combination of the first three parts mentioned above.

# What is the use of bin and lib in JDK?- Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.

# What is casting?- Casting is used to convert the value of one type to another.

# How many ways can an argument be passed to a subroutine and explain them?- An argument can be passed in two ways. They are passing by value and passing by reference. Passing by value: This method copies the value of an argument into the formal parameter of the subroutine. Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.

# What is the difference between an argument and a parameter?- While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.

# What are different types of access modifiers?- public: Any thing declared as public can be accessed from anywhere. private: Any thing declared as private can’t be seen outside of its class. protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages. default modifier : Can be accessed only to classes in the same package.

# What is final, finalize() and finally?- final : final keyword can be used for class, method and variables. A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods. A final method can’t be overridden. A final variable can’t change from its initialized value. finalize() : finalize() method is used just before an object is destroyed and can be called just prior to garbage collection. finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown. For example, if a method opens a file upon exit, then you will not want the code that closes the file to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this contingency.

# What is UNICODE?- Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.

# What is Garbage Collection and how to call it explicitly?- When an object is no longer referred to by any variable, java automatically reclaims memory used by that object. This is known as garbage collection. System. gc() method may be used to call it explicitly.

# What is finalize() method?- finalize () method is used just before an object is destroyed and can be called just prior to garbage collection.

# What are Transient and Volatile Modifiers?- Transient: The transient modifier applies to variables only and it is not stored as part of its object’s Persistent state. Transient variables are not serialized. Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program.

# What is method overloading and method overriding?- Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading. Method overriding : When a method in a class having the same method name with same arguments is said to be method overriding.
# What is difference between overloading and overriding?- a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method. b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass. c) In overloading, separate methods share the same name whereas in overriding, subclass method replaces the superclass. d) Overloading must have different method signatures whereas overriding must have same signature.

# What is meant by Inheritance and what are its advantages?- Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.

# What is the difference between this() and super()?- this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.

# What is the difference between superclass and subclass?- A super class is a class that is inherited whereas sub class is a class that does the inheriting.

# What modifiers may be used with top-level class?- public, abstract and final can be used for top-level class.

# What are inner class and anonymous class?- Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private. Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.

# What is a package?- A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.

# What is a reflection package?- java. lang. reflect package has the ability to analyze itself in runtime.

# What is interface and its use?- Interface is similar to a class which may contain method’s signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it. Interfaces are useful for: a)Declaring methods that one or more classes are expected to implement b)Capturing similarities between unrelated classes without forcing a class relationship. c)Determining an object’s programming interface without revealing the actual body of the class.

# What is an abstract class?- An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.

# What is the difference between Integer and int?- a) Integer is a class defined in the java. lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other. b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.

# What is a cloneable interface and how many methods does it contain?- It is not having any method because it is a TAGGED or MARKER interface.

# What is the difference between abstract class and interface?- a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract. b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods. c) Abstract class must have subclasses whereas interface can’t have subclasses.

# Can you have an inner class inside a method and what variables can you access?- Yes, we can have an inner class inside a method and final variables can be accessed.

# What is the difference between String and String Buffer?- a) String objects are constants and immutable whereas StringBuffer objects are not. b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.

# What is the difference between exception and error?- The exception class defines mild error conditions that your program encounters. Exceptions can occur when trying to open the file, which does not exist, the network connection is disrupted, operands being manipulated are out of prescribed ranges, the class file you are interested in loading is missing. The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered.

# What is the difference between process and thread?- Process is a program in execution whereas thread is a separate path of execution in a program.

# What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?- Multithreading is the mechanism in which more than one thread run independent of each other within the process. wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class. wait() : When a thread executes a call to wait() method, it surrenders the object lock and enters into a waiting state. notify() or notifyAll() : To remove a thread from the waiting state, some other thread must make a call to notify() or notifyAll() method on the same object.

# What is the class and interface in java to create thread and which is the most advantageous method?- Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here.

# What are the states associated in the thread?- Thread contains ready, running, waiting and dead states.

# What is synchronization?- Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.

# When you will synchronize a piece of your code?- When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.

# What is deadlock?- When two threads are waiting each other and can’t precede the program is said to be deadlock.

# What is daemon thread and which method is used to create the daemon thread?- Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.

# Are there any global variables in Java, which can be accessed by other part of your program?- No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.

# What is an applet?- Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.

# What is the difference between applications and applets?- a)Application must be run on local machine whereas applet needs no explicit installation on local machine. b)Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser. d)Application starts execution with its main method whereas applet starts execution with its init method. e)Application can run with or without graphical user interface whereas applet must run within a graphical user interface.

# How does applet recognize the height and width?- Using getParameters() method.

# When do you use codebase in applet?- When the applet class file is not in the same directory, codebase is used.

# What is the lifecycle of an applet?- init() method - Can be called when an applet is first loaded start() method - Can be called each time an applet is started. paint() method - Can be called when the applet is minimized or maximized. stop() method - Can be used when the browser moves off the applet’s page. destroy() method - Can be called when the browser is finished with the applet.

# How do you set security in applets?- using setSecurityManager() method

# What is an event and what are the models available for event handling?- An event is an event object that describes a state of change in a source. In other words, event occurs when an action is generated, like pressing button, clicking mouse, selecting a list, etc. There are two types of models for handling events and they are: a) event-inheritance model and b) event-delegation model

# What are the advantages of the model over the event-inheritance model?- The event-delegation model has two advantages over the event-inheritance model. They are: a)It enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component’s design and its use. b)It performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance.

# What is source and listener?- source : A source is an object that generates an event. This occurs when the internal state of that object changes in some way. listener : A listener is an object that is notified when an event occurs. It has two major requirements. First, it must have been registered with one or more sources to receive notifications about specific types of events. Second, it must implement methods to receive and process these notifications.

# What is adapter class?- An adapter class provides an empty implementation of all methods in an event listener interface. Adapter classes are useful when you want to receive and process only some of the events that are handled by a particular event listener interface. You can define a new class to act listener by extending one of the adapter classes and implementing only those events in which you are interested. For example, the MouseMotionAdapter class has two methods, mouseDragged()and mouseMoved(). The signatures of these empty are exactly as defined in the MouseMotionListener interface. If you are interested in only mouse drag events, then you could simply extend MouseMotionAdapter and implement mouseDragged() .

# What is meant by controls and what are different types of controls in AWT?- Controls are components that allow a user to interact with your application and the AWT supports the following types of controls: Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, Text Components. These controls are subclasses of Component.

# What is the difference between choice and list?- A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices and only one item may be selected from a choice. A List may be displayed in such a way that several list items are visible and it supports the selection of one or more list items.

# What is the difference between scrollbar and scrollpane?- A Scrollbar is a Component, but not a Container whereas Scrollpane is a Conatiner and handles its own events and perform its own scrolling.

# What is a layout manager and what are different types of layout managers available in java AWT?- A layout manager is an object that is used to organize components in a container. The different layouts are available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.

# How are the elements of different layouts organized?- FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion. BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container. CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards. GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid. GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different size and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

# Which containers use a Border layout as their default layout?- Window, Frame and Dialog classes use a BorderLayout as their layout.

# Which containers use a Flow layout as their default layout?- Panel and Applet classes use the FlowLayout as their default layout.

# What are wrapper classes?- Wrapper classes are classes that allow primitive types to be accessed as objects.

संपले प्रेप Questions

Q:	What is the difference between an Interface and an Abstract class?
A:	An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods. .
	TOP

Q:	What is the purpose of garbage collection in Java, and when is it used?
A:	The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
	TOP

Q:	Describe synchronization in respect to multithreading.
A:	With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.
	TOP

Q:	Explain different way of using thread?
A:	The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.
	TOP

Q:	What are pass by reference and passby value?
A:	Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.
	TOP

Q:	What is HashMap and Map?
A:	Map is Interface and Hashmap is class that implements that.
	TOP

Q:	Difference between HashMap and HashTable?
A:	The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.
	TOP

Q:	Difference between Vector and ArrayList?
A:	Vector is synchronized whereas arraylist is not.
	TOP

Q:	Difference between Swing and Awt?
A:	AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.
	TOP

Q:	What is the difference between a constructor and a method?
A:	A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.
	TOP

Q:	What is an Iterator?
A:	Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.
	TOP

Q:	State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
A:	public : Public class is visible in other packages, field is visible everywhere (class must be public too) private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature. default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.
	TOP

Q:	What is an abstract class?
A:	Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
	TOP

Q:	What is static in java?
A:	Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.
	TOP

Q:

What is final?

A:

A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant). Q: What if the main method is declared as private? A: The program compiles properly but at runtime it will give "Main method not public." message. [ Received from Sandesh Sadhale] TOP Q: What if the static modifier is removed from the signature of the main method? A: Program compiles. But at runtime throws an error "NoSuchMethodError". [ Received from Sandesh Sadhale] TOP Q: What if I write static public void instead of public static void? A: Program compiles and runs properly. [ Received from Sandesh Sadhale] TOP Q: What if I do not provide the String array as the argument to the method? A: Program compiles but throws a runtime error "NoSuchMethodError". [ Received from Sandesh Sadhale] TOP Q: What is the first argument of the String array in main method? A: The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name. [ Received from Sandesh Sadhale] TOP Q: If I do not provide any arguments on the command line, then the String array of Main method will be empty or null? A: It is empty. But not null. [ Received from Sandesh Sadhale] TOP Q: How can one prove that the array is not null but empty using one line of code? A: Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length. [ Received from Sandesh Sadhale] TOP Q: What environment variables do I need to set on my machine in order to be able to run Java programs? A: CLASSPATH and PATH are the two variables. [ Received from Sandesh Sadhale] TOP Q: Can an application have multiple classes having main method? A: Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method. [ Received from Sandesh Sadhale] TOP Q: Can I have multiple main methods in the same class? A: No the program fails to compile. The compiler says that the main method is already defined in the class. [ Received from Sandesh Sadhale] TOP Q: Do I need to import java.lang package any time? Why ? A: No. It is by default loaded internally by the JVM. [ Received from Sandesh Sadhale] TOP Q: Can I import same package/class twice? Will the JVM load the package twice at runtime? A: One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class. [ Received from Sandesh Sadhale] TOP Q: What are Checked and UnChecked Exception? A: A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be. TOP Q: What is Overriding? A: When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private. TOP Q: What are different types of inner classes? A: Nested top-level classes, Member classes, Local classes, Anonymous classes Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety. Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class. Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable. Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor. Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile? A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol symbol : class ABCD location: package io import java.io.ABCD; [ Received from Sandesh Sadhale] TOP Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage. [ Received from Sandesh Sadhale] TOP Q: What is the difference between declaring a variable and defining a variable? A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization. e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions. [ Received from Sandesh Sadhale] TOP Q: What is the default value of an object reference declared as an instance variable? A: null unless we define it explicitly. [ Received from Sandesh Sadhale] TOP Q: Can a top level class be private or protected? A: No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected. [ Received from Sandesh Sadhale] TOP Q: What type of parameter passing does Java support? A: In Java the arguments are always passed by value . [ Update from Eki and Jyothish Venu] TOP Q: Primitive data types are passed by reference or pass by value? A: Primitive data types are passed by value. [ Received from Sandesh Sadhale] TOP Q: Objects are passed by value or by reference? A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object . [ Update from Eki and Jyothish Venu] TOP Q: When a thread is created and started, what is its initial state? A: A thread is in the ready state after it has been created and started. [ Received from Venkateswara Manam] TOP Q: What is the purpose of finalization? A: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected. [ Received from Venkateswara Manam] TOP Q: What is the Locale class? A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region. [ Received from Venkateswara Manam] TOP Q: What is the difference between a while statement and a do statement? A: A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once. [ Received from Venkateswara Manam] TOP Q: What is the difference between static and non-static variables? A: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance. [ Received from Venkateswara Manam] TOP Q: How are this() and super() used with constructors? A: This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor. [ Received from Venkateswara Manam] TOP Q: What are synchronized methods and synchronized statements? A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement. [ Received from Venkateswara Manam] TOP Q: What is daemon thread and which method is used to create the daemon thread? A: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread. Q: How does a try statement determine which catch clause should be used to handle an exception? A: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored. Q: What method must be implemented by all threads? A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface. [ Received from P Rajesh] TOP Q: What are synchronized methods and synchronized statements? A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement. [ Received from P Rajesh] Q: What modifiers are allowed for methods in an Interface? A: Only public and abstract modifiers are allowed for methods in interfaces. [ Received from P Rajesh] TOP Q: What are some alternatives to inheritance? A: Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass). [ Received from P Rajesh] TOP Q: What does it mean that a method or field is "static"? A: Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class. [ Received from P Rajesh] Q: Is Empty .java file a valid source file? A: Yes, an empty .java file is a perfectly valid source file. [Received from Sandesh Sadhale] TOP Q: Can a .java file contain more than one java classes? A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class. [ Received from Sandesh Sadhale] TOP Q: Is String a primitive data type in Java? A: No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package. [ Received from Sandesh Sadhale] TOP Q: Is main a keyword in Java? A: No, main is not a keyword in Java. [ Received from Sandesh Sadhale] TOP Q: Is next a keyword in Java? A: No, next is not a keyword. [ Received from Sandesh Sadhale] TOP Q: Is delete a keyword in Java? A: No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does. [ Received from Sandesh Sadhale] TOP Q: Is exit a keyword in Java? A: No. To exit a program explicitly you use exit method in System object. [ Received from Sandesh Sadhale] TOP Q: What happens if you dont initialize an instance variable of any of the primitive types in Java? A: Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false. [ Received from Sandesh Sadhale] TOP Q: What will be the initial value of an object reference which is defined as an instance variable? A: The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references. [ Received from Sandesh Sadhale] TOP Q: What are the different scopes for Java variables? A: The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time. 1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible. 2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope. 3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance. [ Received from Sandesh Sadhale] TOP Q: What is the default value of the local variables? A: The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized.. [ Received from Sandesh Sadhale] TOP Q: How many objects are created in the following piece of code? MyClass c1, c2, c3; c1 = new MyClass (); c3 = new MyClass (); A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized. [ Received from Sandesh Sadhale] TOP Q: Can a public class MyClass be defined in a source file named YourClass.java? A: No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension. [ Received from Sandesh Sadhale] TOP Q: Can main method be declared final? A: Yes, the main method can be declared final, in addition to being public static. [ Received fromSandesh Sadhale] TOP Q: What will be the output of the following statement? System.out.println ("1" + 3); A: It will print 13. [ Received from Sandesh Sadhale] TOP Q: What will be the default values of all the elements of an array defined as an instance variable? A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null. [ Received from Sandesh Sadhale]