Q: What are the different ways to handle exceptions?
A: There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch
block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and
let the caller of the method hadle those exceptions.
Q: Are the imports checked for validity at compile time? e.g. will the
code containing an import such as java.lang.ABCD com compile
sucessfully?
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;
Q: What are checked exceptions?
A: Checked exception are those which the Java compiler forces you to
catch. e.g. IOException are checked Exceptions
Q: How can container knows that JSP has been changed ? ex: I created
one.jsp and the result has been displayed. Next i modified one.jsp and
got the new result. but how can container know that one.jsp has got
changed?
A: By checking the "time stamp" of the jsp file
.
Q: What is the difference between Exception & RuntimeException in Java?
A: RuntimeException is a child class of Exception class. You can see
the details here. This is one of the many child classes of Exception
class. RuntimeException is the superclass of those exceptions that can
be thrown during the normal operation of the Java Virtual Machine. A
method is not required to declare in its throws clause any subclasses
of RuntimeException that might be thrown during the execution of the
method but not caught.
The hierchy is
java.lang.Object
---java.lang.Throwable
-------java.lang.Exception
-------------java.lang.RuntimeException
.
Q: Is it possible to use try-catch in the finally block of java
A: Yes it is possible to use try catch inside the finally block of
java. As a matter of fact it is a good practice to do so as the
methods called in finally block may throw an exception. Importance:
Highest
.
Q: What is the difference between ApplicationServer and webserver?
A: Web Server is limited to Web Technology and more over it can't
deploy the entriprise applications. So inorder to deploy entriprise
applications(EAR Files), we need Application Server. And More Over Web
server supports all kinds of protocols not only http.It can support
FTP and any, provided the concern jar files must be placed in the lib
folder of the Web Server.
Q: Write a recursive programme to reverse a string i.e given an input
"catch" the output should be "hctac"
A: public String reverse(String str)
{
if ((null == str) || (str.length() <= 1))
{
return str; /*End */
}
return reverse(str.substring(1)) + str.charAt(0); /* Recursion */
}
.
Q: 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.
.
Q: Can you call one constructor from another if a class has multiple
constructors
A: Yes. Use this() syntax
.
Q: 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.
.
Q: How can a subclass call a method or a constructor defined in a superclass?
A: To call a method 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
.
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.
.
: What is the difference betweeen error and exception?
A: An error is an irrecoverable condition occurring at runtime. Such
as OutOfMemory error. These are JVM errors and you can not repair them
at runtime. While exceptions are conditions that occur because of bad
input etc. e.g. FileNotFoundException will be thrown if the specified
file does not exist. Or a NullPointerException will take place if you
try using a null reference. In most of the cases it is possible to
recover from an exception (probably by giving user a feedback for
entering proper values etc)
.
What are Checked and Unchecked Exceptions?
A: A checked exception is a subclass of Exception, excluding class
RuntimeException and its subclasses. Making an exception checked
forces client programmes to deal with the exception that may be
thrown. Checked exceptions must be caught at compile time. Example:
IOException.
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. Example: ArrayIndexOutOfBoundsException.
.
Q: Does the order of placing catch statements matter in the catch block?
A: Yes, it does. The FileNoFoundException is inherited from the
IOException. So FileNoFoundException is caught before IOException.
Exception's subclasses have to be caught first before the General
Exception
.
Q: What is the difference between throw and throws keywords?
A: The throw keyword denotes a statement that causes an exception to
be initiated. It takes the Exception object to be thrown as an
argument. The exception will be caught by an enclosing try-catch block
or propagated further up the calling hierarchy. The throws keyword is
a modifier of a method that denotes that an exception may be thrown by
the method. An exception can be rethrown.
.
Q: Explain the user defined Exceptions
A: User defined Exceptions are custom Exception classes defined by the
user for specific purpose. A user defined exception can be created by
simply sub-classing an Exception class or a subclass of an Exception
class. This allows custom exceptions to be generated (using throw
clause) and caught in the same way as normal exceptions.
Example:
class CustomException extends Exception { }
Explain Garbage collection mechanism in Java?
A: Garbage collection is one of the most important features of Java.
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.
Garbage collection is also called automatic memory management as JVM
automatically removes the unused variables/objects (value is null)
from the memory. 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. In 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. Garbage collection is an automatic
process and can't be forced. There is no guarantee that Garbage
collection will start immediately upon request of System.gc().
.
Q: Does garbage collection guarantee that a program will not run out of memory?
A: Garbage collection does not guarantee that a program will not run
out of memory. It is possible for programs to use up memory resources
faster than they are garbage collected. It is also possible for
programs to create objects that are not subject to garbage collection.
.
Q: What is the purpose of finalization?
A: The purpose of finalization is to give an unreachable object the
opportunity to perform any cleanup, before the object gets garbage
collected. For example, closing an opened database Connection.
.
Q: Can an object's finalize() method be invoked while it is reachable?
A: An object's finalize() method cannot be invoked by the garbage
collector while the object is still reachable. However, an object's
finalize() method may be invoked by other objects.
.
What is serialization?
A: Serialization is the process of writing the state of an object to a
byte stream. The serialization is a kind of mechanism that makes a
class or a bean persistent by having its properties or fields and
state information saved and restored to and from storage.
Whenever an object is to be sent over the network or saved in a file,
objects are serialized.
.
Q: What happens to the static fields of a class during serialization?
A: There are three exceptions in which serialization doesn't
necessarily read and write to the stream. These are
Serialization ignores static fields, because they are not part of any
particular state.
Base class fields are only handled if the base class itself is serializable.
Transient fields.
You must make sure that all the included objects are also
serializable. If any of the objects is not serializable then it throws
a NotSerializableException.
.
Q: What are Transient and Volatile Modifiers?
A: A transient variable is a variable that may not be serialized i.e.
the value of the variable can't be written to the stream in a
Serializable class. If you don't want some field to be serialized, you
can mark that field transient or static. In such a case when the class
is retrieved from the ObjectStream the value of the variable is null.
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.
.
Q: What is Externalizable?
A: Externalizable is an interface which contains two methods
readExternal and writeExternal. These methods give you a control over
the serialization mechanism. Thus if your class implements this
interface, you can customize the serialization process by implementing
these methods.
.
Q: What is the difference between Serializalble and Externalizable
interface? How can you control over the serialization process i.e. how
can you customize the seralization process?
A: When you use Serializable interface, your class is serialized
automatically by default. But you can override writeObject() and
readObject() two methods to control more complex object serailization
process. When you use Externalizable interface, you have a complete
control over your class's serialization process. This interface
contains two methods namely readExternal and writeExternal. You should
implement these methods and write the logic for customizing the
serialization process.
.
Q: How to make a class or a bean serializable? How do I serialize an
object to a file?
A: An object must implement the Serializable or Externalizable
interface before it can be written to a stream as an object. The class
whose instances are to be serialized should implement an interface
Serializable. Then you pass the instance to the ObjectOutputStream
which is connected to a fileoutputstream. This will save the object to
a file.
.
How to create multithreaded program? Explain different ways of using thread?
A: There are two ways to create multithreaded program. First one is by
extending the Thread class. The other way is by making the class
implement "Runnable" interface.
The latter is more advantageous, because while going for multiple
inheritance, only interface can help. . If the programme is 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. If the class is not extending any other class, then you
can extend Thread class as it will save few lines of coding.
.
Q: What is synchronization?
A: With respect to multithreading, Synchronization is a process of
controlling the access of shared resources by 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.
You can either synchronizing a function or synchronizing a piece of code.
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.
}
}
Synchronized blocks place locks for shorter periods than synchronized methods.
.
Q: What is mutual exclusion? How can you take care of mutual exclusion
using Java threads?
A: 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 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
.
Q: What invokes a thread's run() method?
A: 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.
.
Q: What is deadlock?
A: 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.
.
Q: What's the difference between the methods sleep() and wait()?
A: 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.
For More Updates .
