
INFOSYS POWER PREPARATION
Good
questions asked during Java interview
Is “abc” a primitive value? - The String literal
“abc” is not a primitive value. It is a String
object. "
What restrictions are placed on the values
of each case of a switch statement? - During
compilation, the values of each case of a
switch statement must evaluate to a value
that can be promoted to an int value.
What modifiers may be used with an interface
declaration? - An interface may be declared
as public or abstract.
Is a class a subclass of itself? - A class
is a subclass of itself.
What is the difference between a while statement
and a do statement? - 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.
What modifiers can be used with a local inner
class? - A local inner class may be final
or abstract.
What is the purpose of the File class? - The
File class is used to create objects that
provide access to the files and directories
of a local file system.
Can an exception be rethrown? - Yes, an exception
can be rethrown.
When does the compiler supply a default constructor
for a class? - The compiler supplies a default
constructor for a class if no other constructors
are provided.
If a method is declared as protected, where
may the method be accessed? - A protected
method may only be accessed by classes or
interfaces of the same package or by subclasses
of the class in which it is declared.
Which non-Unicode letter characters may be
used as the first character of an identifier?
- The non-Unicode letter characters $ and
_ may appear as the first character of an
identifier
What restrictions are placed on method overloading?
- Two methods may not have the same name and
argument list but different return types.
What is casting? - There are two types of
casting, casting between primitive numeric
types and casting between object references.
Casting between numeric types is used to convert
larger values, such as double values, to smaller
values, such as byte values. Casting between
object references is used to refer to an object
by a compatible class, interface, or array
type reference.
What is the return type of a program’s main()
method? - A program’s main() method has a
void return type.
What class of exceptions are generated by
the Java run-time system? - The Java runtime
system generates RuntimeException and Error
exceptions.
What class allows you to read objects directly
from a stream? - The ObjectInputStream class
supports the reading of objects from input
streams.
What is the difference between a field variable
and a local variable? - A field variable is
a variable that is declared as a member of
a class. A local variable is a variable that
is declared local to a method.
How are this() and super() used with constructors?
- this() is used to invoke a constructor of
the same class. super() is used to invoke
a superclass constructor.
What is the relationship between a method’s
throws clause and the exceptions that can
be thrown during the method’s execution? -
A method’s throws clause must declare any
checked exceptions that are not caught within
the body of the method.
Why are the methods of the Math class static?
- So they can be invoked as if they are a
mathematical code library.
What are the legal operands of the instanceof
operator? - The left operand is an object
reference or null value and the right operand
is a class, interface, or array type.
What an I/O filter? - An I/O filter is an
object that reads from one stream and writes
to another, usually altering the data in some
way as it is passed from one stream to another.
If an object is garbage collected, can it
become reachable again? - Once an object is
garbage collected, it ceases to exist. It
can no longer become reachable again.
What are E and PI? - E is the base of the
natural logarithm and PI is mathematical value
pi.
Are true and false keywords? - The values
true and false are not keywords.
What is the difference between the File and
RandomAccessFile classes? - The File class
encapsulates the files and directories of
the local file system. The RandomAccessFile
class provides the methods needed to directly
access data contained in any part of a file.
What happens when you add a double value to
a String? - The result is a String object.
What is your platform’s default character
encoding? - If you are running Java on English
Windows
platforms, it is probably Cp1252. If you are
running Java on English Solaris platforms,
it is most likely 8859_1.
Which package is always imported by default?
- The java.lang package is always imported
by default.
What interface must an object implement before
it can be written to a stream as an object?
- An object must implement the Serializable
or Externalizable interface before it can
be written to a stream as an object.
How can my application get to know when a
HttpSession is removed? - Define a Class HttpSessionNotifier
which implements HttpSessionBindingListener
and implement the functionality what you need
in valueUnbound() method. Create an instance
of that class and put that instance in HttpSession.
Whats the difference between notify() and
notifyAll()? - notify() is used to unblock
one waiting thread; notifyAll() is used to
unblock all of them. Using notify() is preferable
(for efficiency) when only one blocked thread
can benefit from the change (for example,
when freeing a buffer back into a pool). notifyAll()
is necessary (for correctness) if multiple
threads should resume (for example, when releasing
a “writer” lock on a file might permit all
“readers” to resume).
Why can’t I say just abs() or sin() instead
of Math.abs() and Math.sin()? - The import
statement does not bring methods into your
local name space. It lets you abbreviate class
names, but not get rid of them altogether.
That’s just the way it works, you’ll get used
to it. It’s really a lot safer this way.
However, there is actually a little trick
you can use in some cases that gets you what
you want. If your top-level class doesn’t
need to inherit from anything else, make it
inherit from java.lang.Math. That *does* bring
all the methods into your local name space.
But you can’t use this trick in an applet,
because you have to inherit from java.awt.Applet.
And actually, you can’t use it on java.lang.Math
at all, because Math is a “final” class which
means it can’t be extended.
Why are there no global variables in Java?
- Global variables are considered bad form
for a variety of reasons: Adding state variables
breaks referential transparency (you no longer
can understand a statement or expression on
its own: you need to understand it in the
context of the settings of the global variables),
State variables lessen the cohesion of a program:
you need to know more to understand how something
works. A major point of Object-Oriented programming
is to break up global state into more easily
understood collections of local state, When
you add one variable, you limit the use of
your program to one instance. What you thought
was global, someone else might think of as
local: they may want to run two copies of
your program at once. For these reasons, Java
decided to ban global variables.
What does it mean that a class or member is
final? - A final class can no longer be subclassed.
Mostly this is done for security reasons with
basic classes like String and Integer. It
also allows the compiler to make some optimizations,
and makes thread safety a little easier to
achieve. Methods may be declared final as
well. This means they may not be overridden
in a subclass. Fields can be declared final,
too. However, this has a completely different
meaning. A final field cannot be changed after
it’s initialized, and it must include an initializer
statement where it’s declared. For example,
public final double c = 2.998; It’s also possible
to make a static field final to get the effect
of C++’s const statement or some uses of C’s
#define, e.g. public static final double c
= 2.998;
What does it mean that a method or class is
abstract? - An abstract class cannot be instantiated.
Only its subclasses can be instantiated. You
indicate that a class is abstract with the
abstract keyword like this:
public abstract class Container extends Component
{
Abstract classes may contain abstract methods.
A method declared abstract is not actually
implemented in the current class. It exists
only to be overridden in subclasses. It has
no body. For example,
public abstract float price();
Abstract methods may only be included in
abstract classes. However, an abstract class
is not required to have any abstract methods,
though most of them do. Each subclass of an
abstract class must override the abstract
methods of its superclasses or itself be declared
abstract.
What is a transient variable? - transient
variable is a variable that may not be serialized.
How are Observer and Observable used? - Objects
that subclass the Observable class maintain
a list of observers. When an Observable object
is updated it invokes the update() method
of each of its observers to notify the observers
that it has changed state. The Observer interface
is implemented by objects that observe Observable
objects.
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.
How does Java handle integer overflows and
underflows? - It uses those low order bytes
of the result that can fit into the size of
the type allowed by the operation.
What is the difference between the >>
and >>> operators? - The >>
operator carries the sign bit when shifting
right. The >>> zero-fills bits that
have been shifted out.
Is sizeof a keyword? - The sizeof operator
is not a keyword.
Does garbage collection guarantee that a program
will not run out of memory? - 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
Can an object’s finalize() method be invoked
while it is reachable? - 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 value does readLine() return when it
has reached the end of a file? - The readLine()
method returns null when it has reached the
end of a file.
Can a for statement loop indefinitely? - Yes,
a for statement can loop indefinitely. For
example, consider the following: for(;;) ;
To what value is a variable of the String
type automatically initialized? - The default
value of an String type is null.
What is a task’s priority and how is it used
in scheduling? - A task’s priority is an integer
value that identifies the relative order in
which it should be executed with respect to
other tasks. The scheduler attempts to schedule
higher priority tasks before lower priority
tasks.
What is the range of the short type? - The
range of the short type is -(2^15) to 2^15
- 1.
What is the purpose of garbage collection?
- The purpose of garbage collection is to
identify and discard objects that are no longer
needed by a program so that their resources
may be reclaimed and reused.
What do you understand by private, protected
and public? - These are accessibility modifiers.
Private is the most restrictive, while public
is the least restrictive. There is no real
difference between protected and the default
type (also known as package protected) within
the context of the same package, however the
protected keyword allows visibility to a derived
class in a different package.
What is Downcasting ? - Downcasting is the
casting from a general to a more specific
type, i.e. casting down the hierarchy
Can a method be overloaded based on different
return type but same argument type ? - No,
because the methods can be called without
using their return type in which case there
is ambiquity for the compiler
What happens to a static var that is defined
within a method of a class ? - Can’t do it.
You’ll get a compilation error
How many static init can you have ? - As many
as you want, but the static initializers and
class variable initializers are executed in
textual order and may not refer to class variables
declared in the class whose declarations appear
textually after the use, even though these
class variables are in scope.
What is the difference amongst JVM Spec, JVM
Implementation, JVM Runtime ? - The JVM spec
is the blueprint for the JVM generated and
owned by Sun. The JVM implementation is the
actual implementation of the spec by a vendor
and the JVM runtime is the actual running
instance of a JVM implementation
Describe what happens when an object is created
in Java? - Several things happen in a particular
order to ensure the object is constructed
properly: Memory is allocated from heap to
hold all instance variables and implementation-specific
data of the object and its superclasses. Implemenation-specific
data includes pointers to class and method
data. The instance variables of the objects
are initialized to their default values. The
constructor for the most derived class is
invoked. The first thing a constructor does
is call the consctructor for its superclasses.
This process continues until the constrcutor
for java.lang.Object is called, as java.lang.Object
is the base class for all objects in java.
Before the body of the constructor is executed,
all instance variable initializers and initialization
blocks are executed. Then the body of the
constructor is executed. Thus, the constructor
for the base class completes first and constructor
for the most derived class completes last.
What does the “final” keyword mean in front
of a variable? A method? A class? - FINAL
for a variable: value is constant. FINAL for
a method: cannot be overridden. FINAL for
a class: cannot be derived
What is the difference between instanceof
and isInstance? - instanceof is used to check
to see if an object can be cast into a specified
type without throwing a cast class exception.
isInstance() Determines if the specified Object
is assignment-compatible with the object represented
by this Class. This method is the dynamic
equivalent of the Java language instanceof
operator. The method returns true if the specified
Object argument is non-null and can be cast
to the reference type represented by this
Class object without raising a ClassCastException.
It returns false otherwise.
Why does it take so much time to access an
Applet having Swing Components the first time?
- Because behind every swing component are
many Java objects and resources. This takes
time to create them in memory. JDK 1.3 from
Sun has some improvements which may lead to
faster execution of Swing applications.