2. What is the difference between private, protected, and public?
These keywords are for allowing privilages to components such as functions and
variables.
Public: accessible to all classes
Private: accessible only to the class to which they belong
Protected: accessible to the class to which they belong and any subclasses.
3. What is a cartesian product in PL/SQL?
When a Join condition is not specified by the programmer or is invalid(fails),
PL/SQL forms a Cartesian product.
In a Cartesian product, all combinations of rows will be displayed.
For example, All rows in the first table are joined to all rows in the second
table. It joins a bunch of rows and it's result is rarely useful unless you have
a need to combine all rows from all tables.
4. What is mutual exclusion? How can you take care of mutual exclusion using
Java threads?
Mutual exclusion is where no two processes can access critical regions of memory
at the same time.
Java provides many utilities to deal with mutual exclusion with the use of
threaded programming.
For mutual exclusion, you can simply use the synchronized keyword and explicitly
or implicitly provide an Object, any Object, to synchronize on.
The runtime system/Java compiler takes care of the gruesome details for you. 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.
5. What are some advantages and disadvantages of Java Sockets?
Some advantages of Java Sockets:
Sockets are flexible and sufficient. Efficient socket based programming can be
easily implemented for general communications.
Sockets cause low network traffic. Unlike HTML forms and CGI scripts that
generate and transfer whole web pages for each new request, Java applets can
send only necessary updated information.
Some disadvantages of Java Sockets:
Security restrictions are sometimes overbearing because a Java applet running in
a Web browser is only able to establish connections to the machine where it came
from, and to nowhere else on the network
Despite all of the useful and helpful Java features, Socket based communications
allows only to send packets of raw data between applications. Both the
client-side and server-side have to provide mechanisms to make the data useful
in any way.
Since the data formats and protocols remain application specific, the re-use of
socket based implementations is limited.
Q1. How could Java classes direct program messages to the system console, but
error messages, say to a file?
A. The class System has a variable out that represents the standard output, and
the variable err that represents the standard error device. By default, they
both point at the system console. This how the standard output could be
re-directed:
Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st);
System.setOut(st);
Q2. What's the difference between an interface and an abstract class?
A. An abstract class may contain code in method bodies, which is not allowed in
an interface. With abstract classes, you have to inherit your class from it and
Java does not allow multiple inheritance. On the other hand, you can implement
multiple interfaces in your class.
Some Similarities and Differences
1.Abstract class is a class which contain one or more abstract methods, which
has to be implemented by sub classes. Interface is a Java Object containing
method declaration and doesn't contain implementation. The classes which have
implementing the Interfaces must provide the method definition for all the
methods.2. Abstract class is a Class prefix with a abstract keyword followed by
Class definition. Interface is a Interface which starts with interface
keyword.3. Abstract class contains one or more abstract methods. where as
Interface contains all abstract methods and final declarations.4. Abstract class
contains the method definition of the some methods. but Interface contains only
method declaration, no definition provided.5. Abstract classes are useful in a
situation that Some general methods should be implemented and specialization
behavior should be implemented by child classes. Interfaces are useful in a
situation that all properties should be implemented we can use this scenario
Q3. Why would you use a synchronized block vs. synchronized method?
A. Synchronized blocks place locks for shorter periods than synchronized
methods.
Q4. Explain the usage of the keyword transient?
A. This keyword indicates that the value of this member variable does not have
to be serialized with the object. When the class will be de-serialized, this
variable will be initialized with a default value of its data type (i.e. zero
for integers).
Q5. How can you force garbage collection?
A. You can't force GC, but could request it by calling System.gc(). JVM does not
guarantee that GC will be started immediately.
Q6. How do you know if an explicit object casting is needed?
A. If you assign a superclass object to a variable of a subclass's data type,
you need to do explicit casting. For example:
Object a; Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting is
performed automatically.
Q7. What's the difference between the methods sleep() and wait()
A. The code sleep(1000); puts thread aside for exactly one second. The code
wait(1000), causes a wait of up to one second. A thread could stop waiting
earlier if it receives the notify() or notifyAll() call. The method wait() is
defined in the class Object and the method sleep() is defined in the class
Thread.
Q8. Can you write a Java class that could be used both as an applet as well as
an application?
A. Yes. Add a main() method to the applet.
Q9. What's the difference between constructors and other methods?
A. Constructors must have the same name as the class and can not return a value.
They are only called once while regular methods could be called many times.
Q10. Can you call one constructor from another if a class has multiple
constructors
A. Yes. Use this() syntax.
Q11. Explain the usage of Java packages.
A. This is a way to organize files when a project consists of multiple modules.
It also helps resolve naming conflicts when different packages have classes with
the same names. Packages access level also allows you to protect data from being
used by the non-authorized classes.
Q12. If a class is located in a package, what do you need to change in the OS
environment to be able to use it?
A. You need to add a directory or a jar file that contains the package
directories to the CLASSPATH environment variable. Let's say a class Employee
belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java.
In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class
contains the method main(), you could test it from a command prompt window as
follows:
c:\>java com.xyz.hr.Employee
Q13. What's the difference between J2SDK 1.5 and J2SDK 5.0?
A. There's no difference, Sun Microsystems just re-branded this version.
Q14. What would you use to compare two String variables - the operator == or the
method equals()?
A. I'd use the method equals() to compare the values of the Strings and the ==
to check if two variables point at the same instance of a String object.
Q15. Does it matter in what order catch statements for FileNotFoundException and
IOExceptipon are written?
A. Yes, it does. The FileNoFoundException is inherited from the IOException.
Exception's subclasses have to be caught first.
Q16. Can an inner class declared inside of a method access local variables of
this method?
A. It's possible if these variables are final.
Q17. What can go wrong if you replace && with & in the following code:
String a=null; if (a!=null && a.length()>10) {...}
A. A single ampersand here would lead to a NullPointerException.
Q18. What's the main difference between a Vector and an ArrayList
A. Java Vector class is internally synchronized and ArrayList is not.
Q19. When should the method invokeLater()be used?
A. This method is used to ensure that Swing components are updated through the
event-dispatching thread.
Q20. How can a subclass call a method or a constructor defined in a superclass?
A. Use the following syntax: super.myMethod(); To call a constructor of the
superclass, just write super(); in the first line of the subclass's constructor.
Q21. What's the difference between a queue and a stack?
A. Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule
Q22. You can create an abstract class that contains only abstract methods. On
the other hand, you can create an interface that declares the same methods. So
can you use abstract classes instead of interfaces?
A. Sometimes. But your class may be a descendent of another class and in this
case the interface is your only option.
Q23. What comes to mind when you hear about a young generation in Java?
A. Garbage collection.
Q24. What comes to mind when someone mentions a shallow copy in Java?
A. Object cloning.
Q25. If you're overriding the method equals() of an object, which other method
you might also consider?
A. hashCode()
Q26. You are planning to do an indexed search in a list of objects. Which of the
two Java collections should you use:
ArrayList or LinkedList?
A. ArrayList
Q27. How would you make a copy of an entire Java object with its state?
A. Have this class implement Cloneable interface and call its method clone().
Q28. How can you minimize the need of garbage collection and make the memory use
more effective?
A. Use object pooling and weak object references.
Q29. There are two classes: A and B. The class B need to inform a class A when
some important event has happened. What Java technique would you use to
implement it?
A. If these classes are threads I'd consider notify() or notifyAll(). For
regular classes you can use the Observer interface.
Q30. What access level do you need to specify in the class declaration to ensure
that only classes from the same directory can access it?
A. You do not need to specify any access level, and Java will use a default
package access level.
These are some questions contributed by Rajesh Kotari.
1. Define abstract and final?
2. What are the types of Java Applications?
3. Default value of Boolean is?
4. What is garbage collection?
5. what happens when u add a double value to String?
6. what is the method gets invoked in standalone application?
7. How does a try stmt determine which catch clause should be used to handle an
exception?
8. What is the collections API?
9. Is numbers(0 to 9) used as first characters while naming the identifiers?
10. synchronization importance?
11. what is consumer and producer problem? and Monitors---------
1.abstract and final are the keyword in java. any class or any method which is
not implement it should be declared as abstract. we can't create objects to
abstract class. any class which extends the abstract class it provides the
implementation of the abstract class. final is the keyword any class which is
declared as final it isn't extend by any subclass.
2.there are 3 types of java application
i) Standalone application or desktop application or standard application (
includes all core programming)
ii) web application or enterprise application including all j2ee programs like
servlet,jsp,ejb)
iii) mobile application (midlets for mobiles etc)
3.the default value for Boolean is FALSE
4.one of the feature in java is garbage collection. garbage collection is a
mechanism to destroy the unused objects. jvm will handle gc at some regular
intervals. we can externally call garbage collector by calling the gc() method
in the System class eg.System.gc();
5 it converts the double value into the string format.
eg String st=new String(12.33);
6. the main method is invoked.
9. identifier are started with letter or some special symbols like$,_, we cant
start with numbers
eg $var,_var1,var1 are valid identifiers
1var3 are not valid identifiers
10.synchnorized key word is to avoid dead lock situations. no other thread will
not execute syn method is one thread is executed.
11. it is inter thread communication
# Question: What is transient variable?
Answer: Transient variable can't be serialize. For example if a variable is
declared as transient in a Serializable class and the class is written to an
ObjectStream, the value of the variable can't be written to the stream instead
when the class is retrieved from the ObjectStream the value of the variable
becomes null.
# Question: Name the containers which uses Border Layout as their default
layout?
Answer: Containers which uses Border Layout as their default are: window, Frame
and Dialog classes.
# Question: What do you understand by Synchronization?
Answer: 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 one 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.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}
# Question: What is Collection API?
Answer: The Collection API is a set of classes and interfaces that support
operation on collections of objects. These classes and interfaces are more
flexible, more powerful, and more regular than the vectors, arrays, and
hashtables if effectively replaces.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.
Example of interfaces: Collection, Set, List and Map.
# Question: Is Iterator a Class or Interface? What is its use?
Answer: Iterator is an interface which is used to step through the elements of a
Collection.
# Question: What is similarities/difference between an Abstract class and
Interface?
Answer: Differences are as follows:
* Interfaces provide a form of multiple inheritance. A class can extend only one
other class.
* Interfaces are limited to public methods and constants with no implementation.
Abstract classes can have a partial implementation, protected parts, static
methods, etc.
* A Class may implement several interfaces. But in case of abstract class, a
class may extend only one abstract class.
* Interfaces are slow as it requires extra indirection to to find corresponding
method in in the actual class. Abstract classes are fast.
Similarities:
* Neither Abstract classes or Interface can be instantiated.
# Question: How to define an Abstract class?
Answer: A class containing abstract method is called Abstract class. An Abstract
class can't be instantiated.
Example of Abstract class:
abstract class testAbstractClass {
protected String myString;
public String getMyString() {
return myString;
}
public abstract string anyAbstractFunction();
}
# Question: How to define an Interface?
Answer: In Java Interface defines the methods but does not implement them.
Interface can include constants. A class that implements the interfaces is bound
to implement all the methods defined in Interface.
Emaple of Interface:
public interface sampleInterface {
public void functionOne();
public long CONSTANT_ONE = 1000;
}
# Question: Explain the user defined Exceptions?
Answer: User defined Exceptions are the separate Exception classes defined by
the user for specific purposed. An user defined can created by simply
sub-classing it to the Exception class. This allows custom exceptions to be
generated (using throw) and caught in the same way as normal exceptions.
Example:
class myCustomException extends Exception {
// The class simply has to exist to be an exception
}
# Question: Explain the new Features of JDBC 2.0 Core API?
Answer: The JDBC 2.0 API includes the complete JDBC API, which includes both
core and Optional Package API, and provides inductrial-strength database
computing capabilities.
New Features in JDBC 2.0 Core API:
* Scrollable result sets- using new methods in the ResultSet interface allows
programmatically move the to particular row or to a position relative to its
current position
* JDBC 2.0 Core API provides the Batch Updates functionality to the java
applications.
* Java applications can now use the ResultSet.updateXXX methods.
* New data types - interfaces mapping the SQL3 data types
* Custom mapping of user-defined types (UTDs)
* Miscellaneous features, including performance hints, the use of character
streams, full precision for java.math.BigDecimal values, additional security,
and support for time zones in date, time, and timestamp values.
# Question: Explain garbage collection?
Answer: Garbage collection is one of the most important feature of Java. Garbage
collection is also called automatic memory management as JVM automatically
removes the unused variables/objects (value is null) from the memory. User
program cann't directly free the object from memory, instead it is the job of
the garbage collector to automatically free the objects that are no longer
referenced by a program. Every class inherits finalize() method from
java.lang.Object, the finalize() method is called by garbage collector when it
determines no more references to the object exists. In Java, it is good idea to
explicitly assign null into a variable when no more in use. I Java on calling
System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there
is no guarantee when all the objects will garbage collected.
# Question: How you can force the garbage collection?
Answer: Garbage collection automatic process and can't be forced.
# Question: What is OOPS?
Answer: OOP is the common abbreviation for Object-Oriented Programming.
There are three main principals of oops which are called Polymorphism,
Inheritance and Encapsulation.
# Question: Explain the Encapsulation principle.
Answer: Encapsulation is a process of binding or wrapping the data and the codes
that operates on the data into a single entity. This keeps the data safe from
outside interface and misuse. One way to think about encapsulation is as a
protective wrapper that prevents code and data from being arbitrarily accessed
by other code defined outside the wrapper.
# Question: Explain the Inheritance principle.
Answer: Inheritance is the process by which one object acquires the properties
of another object.
# Question: Explain the Polymorphism principle.
Answer: The meaning of Polymorphism is something like one name many forms.
Polymorphism enables one entity to be used as as general category for different
types of actions. The specific action is determined by the exact nature of the
situation. The concept of polymorphism can be explained as "one interface,
multiple methods".
From a practical programming viewpoint, polymorphism exists in three distinct
forms in Java:
* Method overloading
* Method overriding through inheritance
* Method overriding through the Java interface
# Question: What are Access Specifiers available in Java?
Answer: Access specifiers are keywords that determines the type of access to the
member of a class. These are:
* Public
* Protected
* Private
* Defaults
# Question: Describe the wrapper classes in Java.
Answer: Wrapper class is wrapper around a primitive data type. An instance of a
wrapper class contains, or wraps, a primitive value of the corresponding type.
Following table lists the primitive types and the corresponding wrapper classes:
Primitive Wrapper
boolean - java.lang.Boolean
byte - java.lang.Byte
char - java.lang.Character
double - java.lang.Double
float - java.lang.Float
int - java.lang.Integer
long - java.lang.Long
short - java.lang.Short
void - java.lang.Void
# Question: Read the following program:
public class test {
public static void main(String [] args) {
int x = 3;
int y = 1;
if (x = y)
System.out.println("Not equal");
else
System.out.println("Equal");
}
}
What is the result?
A. The output is "Equal"
B. The output in "Not Equal"
C. An error at " if (x = y)" causes compilation to fall.
D. The program executes but no output is show on console.
Answer: C
Answer: Transient variable can't be serialize. For example if a variable is
declared as transient in a Serializable class and the class is written to an
ObjectStream, the value of the variable can't be written to the stream instead
when the class is retrieved from the ObjectStream the value of the variable
becomes null.
# Question: Name the containers which uses Border Layout as their default
layout?
Answer: Containers which uses Border Layout as their default are: window, Frame
and Dialog classes.
# Question: What do you understand by Synchronization?
Answer: 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 one 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.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}