Java Interview Questions
Posted On Thursday, July 2, 2009 at
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11:28 AM
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Java Interview 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.
.
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.
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.
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.
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.
Q:
What is HashMap and Map?
A: Map is Interface and Hashmap is class that implements that.
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.
Q:
Difference between Vector and ArrayList?
A: Vector is synchronized whereas arraylist is not.
Q:
Difference between Swing and Awt?
A: AWT are heavy-weight componenets. Swings are light-weight
components. Hence swing works faster than AWT.
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.
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.
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.
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.
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.
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.
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".
Q:
What if I write static public void instead of public static void?
A: Program compiles and runs properly.
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".
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.
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.
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.
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.
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.
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.
Q:
Do I need to import java.lang package any time? Why ?
A: No. It is by default loaded internally by the JVM.
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.
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.
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.
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;
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.
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.
Q:
What is the default value of an object reference declared as an
instance variable?
A: null unless we define it explicitly.
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.
Q:
What type of parameter passing does Java support?
A: In Java the arguments are always passed by value .
Q:
Primitive data types are passed by reference or pass by value?
A: Primitive data types are passed by value.
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 .
Q:
What is serialization?
A: Serialization is a mechanism by which you can save the state
of an object by converting it to a byte stream.
Q:
How do I serialize an object to a file?
A: 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.
Q:
Which methods of Serializable interface should I implement?
A: The serializable interface is an empty interface, it does not
contain any methods. So we do not implement any methods.
Q:
How can I customize the seralization process? i.e. how can one have a
control over the serialization process?
A: Yes it is possible to have control over serialization process.
The class should implement Externalizable interface. This interface
contains two methods namely readExternal and writeExternal. You should
implement these methods and write the logic for customizing the
serialization process.
Q:
What is the common usage of serialization?
A: Whenever an object is to be sent over the network, objects
need to be serialized. Moreover if the state of an object is to be
saved, objects need to be serilazed.
Q:
What is Externalizable interface?
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:
When you serialize an object, what happens to the object references
included in the object?
A: The serialization mechanism generates an object graph for
serialization. Thus it determines whether the included object
references are serializable or not. This is a recursive process. Thus
when an object is serialized, all the included objects are also
serialized alongwith the original obect.
Q:
What one should take care of while serializing the object?
A: One should 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 happens to the static fields of a class during serialization?
A:
There are three exceptions in which serialization doesnot necessarily
read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of
ay particular state state.
2. Base class fields are only hendled if the base class itself is
serializable.
3. Transient fields.
Q:
Does Java provide any construct to find out the size of an object?
A: No there is not sizeof operator in Java. So there is not
direct way to determine the size of an object directly in Java.
Q:
Give a simplest way to find out the time a method takes for execution
without using any profiling tool?
A: Read the system time just before the method is invoked and
immediately after method returns. Take the time difference, which will
give you the time taken by a method for execution.
To put it in code...
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();
System.out.println ("Time taken for execution is " + (end - start));
Remember that if the time taken for execution is too small, it might
show that it is taking zero milliseconds for execution. Try it on a
method which is big enough, in the sense the one which is doing
considerable amout of processing.
Q:
What are wrapper classes?
A: Java provides specialized classes corresponding to each of the
primitive data types. These are called wrapper classes. They are e.g.
Integer, Character, Double etc.
Q:
Why do we need wrapper classes?
A: It is sometimes easier to deal with primitives as objects.
Moreover most of the collection classes store objects and not
primitive data types. And also the wrapper classes provide many
utility methods also. Because of these resons we need wrapper classes.
And since we create instances of these classes we can store them in
any of the collection classes and pass them around as a collection.
Also we can pass them around as method parameters where a method
expects an object.
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:
What are runtime exceptions?
A: Runtime exceptions are those exceptions that are thrown at
runtime because of either wrong input data or because of wrong
business logic etc. These are not checked by the compiler at compile
time.
Q:
What is the difference between error and an exception?
A: An error is an irrecoverable condition occurring at runtime.
Such as OutOfMemory error. These 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.).
Q:
How to create custom exceptions?
A: Your class should extend class Exception, or some more
specific type thereof.
Q:
If I want an object of my class to be thrown as an exception object,
what should I do?
A: The class should extend from Exception class. Or you can
extend your class from some more precise exception type also.
Q:
If my class already extends from some other class what should I do if
I want an instance of my class to be thrown as an exception object?
A: One can not do anytihng in this scenarion. Because Java does
not allow multiple inheritance and does not provide any exception
interface as well.
Q:
How does an exception permeate through the code?
A: An unhandled exception moves up the method stack in search of
a matching When an exception is thrown from a code which is wrapped in
a try block followed by one or more catch blocks, a search is made for
matching catch block. If a matching type is found then that block will
be invoked. If a matching type is not found then the exception moves
up the method stack and reaches the caller method. Same procedure is
repeated if the caller method is included in a try catch block. This
process continues until a catch block handling the appropriate type of
exception is found. If it does not find such a block then finally the
program terminates.
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:
What is the basic difference between the 2 approaches to exception
handling.
1> try catch block and
2> specifying the candidate exceptions in the throws clause?
When should you use which approach?
A: In the first approach as a programmer of the method, you
urself are dealing with the exception. This is fine if you are in a
best position to decide should be done in case of an exception.
Whereas if it is not the responsibility of the method to deal with
it's own exceptions, then do not use this approach. In this case use
the second approach. In the second approach we are forcing the caller
of the method to catch the exceptions, that the method is likely to
throw. This is often the approach library creators use. They list the
exception in the throws clause and we must catch them. You will find
the same approach throughout the java libraries we use.
Q:
Is it necessary that each try block must be followed by a catch block?
A: It is not necessary that each try block must be followed by a
catch block. It should be followed by either a catch block OR a
finally block. And whatever exceptions are likely to be thrown should
be declared in the throws clause of the method.
Q:
If I write return at the end of the try block, will the finally block
still execute?
A: Yes even if you write return as the last statement in the try
block and no exception occurs, the finally block will execute. The
finally block will execute and then the control return.
Q:
If I write System.exit (0); at the end of the try block, will the
finally block still execute?
A: No in this case the finally block will not execute because
when you say System.exit (0); the control immediately goes out of the
program, and thus finally never executes.
Q:
How are Observer and Observable used?
A: 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.
Q:
What is synchronization and why is it important?
A: With respect to multithreading, synchronization is the
capability to control
the access of multiple threads to shared resources. Without
synchronization, it is possible for one thread to modify a shared
object while another thread is in the process of using or updating
that object's value. This often leads to significant errors.
Q:
How does Java handle integer overflows and underflows?
A: It uses those low order bytes of the result that can fit into
the size of the type allowed by the operation.
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 difference between preemptive scheduling and time slicing?
A: 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 reenters the pool of
ready tasks. The scheduler then determines which task should execute
next, based on priority and other factors.
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.
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.
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.
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.
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.
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.
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.
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:
Can applets communicate with each other?
A: At this point in time applets may communicate with other
applets running in the same virtual machine. If the applets are of the
same class, they can communicate via shared static variables. If the
applets are of different classes, then each will need a reference to
the same class with static variables. In any case the basic idea is to
pass the information back and forth through a static variable.
An applet can also get references to all other applets on the same
page using the getApplets() method of java.applet.AppletContext. Once
you get the reference to an applet, you can communicate with it by
using its public members.
It is conceivable to have applets in different virtual machines that
talk to a server somewhere on the Internet and store any data that
needs to be serialized there. Then, when another applet needs this
data, it could connect to this same server. Implementing this is
non-trivial.
Q:
What are the steps in the JDBC connection?
A: While making a JDBC connection we go through the following
steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
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:
Can an unreachable object become reachable again?
A: An unreachable object may become reachable again. This can
happen when the object's finalize() method is invoked and the object
performs an operation which causes it to become accessible to
reachable objects.
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.
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.
Q:
What is Externalizable?
A: Externalizable is an Interface that extends Serializable
Interface. And sends data into Streams in Compressed Format. It has
two methods, writeExternal(ObjectOuput out) and
readExternal(ObjectInput in)
Q:
What modifiers are allowed for methods in an Interface?
A: Only public and abstract modifiers are allowed for methods in
interfaces.
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).
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.
Q:
What is the difference between preemptive scheduling and time slicing?
A: 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 reenters the pool of
ready tasks. The scheduler then determines which task should execute
next, based on priority and other factors.
Q:
What is the catch or declare rule for method declarations?
A: If a checked exception may be thrown within the body of a
method, the method must either catch the exception or declare it in
its throws clause.
Q:
Is Empty .java file a valid source file?
A: Yes, an empty .java file is a perfectly valid source file.
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.
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.
Q:
Is main a keyword in Java?
A: No, main is not a keyword in Java.
Q:
Is next a keyword in Java?
A: No, next is not a keyword.
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.
Q:
Is exit a keyword in Java?
A: No. To exit a program explicitly you use exit method in System
object.
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.
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.
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.
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..
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.
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.
Q:
Can main method be declared final?
A: Yes, the main method can be declared final, in addition to
being public static.
Q:
What will be the output of the following statement?
System.out.println ("1" + 3);
A: It will print 13.
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.
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.
.
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.
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.
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.
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.
Q:
What is HashMap and Map?
A: Map is Interface and Hashmap is class that implements that.
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.
Q:
Difference between Vector and ArrayList?
A: Vector is synchronized whereas arraylist is not.
Q:
Difference between Swing and Awt?
A: AWT are heavy-weight componenets. Swings are light-weight
components. Hence swing works faster than AWT.
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.
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.
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.
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.
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.
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.
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".
Q:
What if I write static public void instead of public static void?
A: Program compiles and runs properly.
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".
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.
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.
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.
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.
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.
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.
Q:
Do I need to import java.lang package any time? Why ?
A: No. It is by default loaded internally by the JVM.
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.
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.
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.
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;
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.
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.
Q:
What is the default value of an object reference declared as an
instance variable?
A: null unless we define it explicitly.
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.
Q:
What type of parameter passing does Java support?
A: In Java the arguments are always passed by value .
Q:
Primitive data types are passed by reference or pass by value?
A: Primitive data types are passed by value.
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 .
Q:
What is serialization?
A: Serialization is a mechanism by which you can save the state
of an object by converting it to a byte stream.
Q:
How do I serialize an object to a file?
A: 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.
Q:
Which methods of Serializable interface should I implement?
A: The serializable interface is an empty interface, it does not
contain any methods. So we do not implement any methods.
Q:
How can I customize the seralization process? i.e. how can one have a
control over the serialization process?
A: Yes it is possible to have control over serialization process.
The class should implement Externalizable interface. This interface
contains two methods namely readExternal and writeExternal. You should
implement these methods and write the logic for customizing the
serialization process.
Q:
What is the common usage of serialization?
A: Whenever an object is to be sent over the network, objects
need to be serialized. Moreover if the state of an object is to be
saved, objects need to be serilazed.
Q:
What is Externalizable interface?
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:
When you serialize an object, what happens to the object references
included in the object?
A: The serialization mechanism generates an object graph for
serialization. Thus it determines whether the included object
references are serializable or not. This is a recursive process. Thus
when an object is serialized, all the included objects are also
serialized alongwith the original obect.
Q:
What one should take care of while serializing the object?
A: One should 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 happens to the static fields of a class during serialization?
A:
There are three exceptions in which serialization doesnot necessarily
read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of
ay particular state state.
2. Base class fields are only hendled if the base class itself is
serializable.
3. Transient fields.
Q:
Does Java provide any construct to find out the size of an object?
A: No there is not sizeof operator in Java. So there is not
direct way to determine the size of an object directly in Java.
Q:
Give a simplest way to find out the time a method takes for execution
without using any profiling tool?
A: Read the system time just before the method is invoked and
immediately after method returns. Take the time difference, which will
give you the time taken by a method for execution.
To put it in code...
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();
System.out.println ("Time taken for execution is " + (end - start));
Remember that if the time taken for execution is too small, it might
show that it is taking zero milliseconds for execution. Try it on a
method which is big enough, in the sense the one which is doing
considerable amout of processing.
Q:
What are wrapper classes?
A: Java provides specialized classes corresponding to each of the
primitive data types. These are called wrapper classes. They are e.g.
Integer, Character, Double etc.
Q:
Why do we need wrapper classes?
A: It is sometimes easier to deal with primitives as objects.
Moreover most of the collection classes store objects and not
primitive data types. And also the wrapper classes provide many
utility methods also. Because of these resons we need wrapper classes.
And since we create instances of these classes we can store them in
any of the collection classes and pass them around as a collection.
Also we can pass them around as method parameters where a method
expects an object.
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:
What are runtime exceptions?
A: Runtime exceptions are those exceptions that are thrown at
runtime because of either wrong input data or because of wrong
business logic etc. These are not checked by the compiler at compile
time.
Q:
What is the difference between error and an exception?
A: An error is an irrecoverable condition occurring at runtime.
Such as OutOfMemory error. These 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.).
Q:
How to create custom exceptions?
A: Your class should extend class Exception, or some more
specific type thereof.
Q:
If I want an object of my class to be thrown as an exception object,
what should I do?
A: The class should extend from Exception class. Or you can
extend your class from some more precise exception type also.
Q:
If my class already extends from some other class what should I do if
I want an instance of my class to be thrown as an exception object?
A: One can not do anytihng in this scenarion. Because Java does
not allow multiple inheritance and does not provide any exception
interface as well.
Q:
How does an exception permeate through the code?
A: An unhandled exception moves up the method stack in search of
a matching When an exception is thrown from a code which is wrapped in
a try block followed by one or more catch blocks, a search is made for
matching catch block. If a matching type is found then that block will
be invoked. If a matching type is not found then the exception moves
up the method stack and reaches the caller method. Same procedure is
repeated if the caller method is included in a try catch block. This
process continues until a catch block handling the appropriate type of
exception is found. If it does not find such a block then finally the
program terminates.
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:
What is the basic difference between the 2 approaches to exception
handling.
1> try catch block and
2> specifying the candidate exceptions in the throws clause?
When should you use which approach?
A: In the first approach as a programmer of the method, you
urself are dealing with the exception. This is fine if you are in a
best position to decide should be done in case of an exception.
Whereas if it is not the responsibility of the method to deal with
it's own exceptions, then do not use this approach. In this case use
the second approach. In the second approach we are forcing the caller
of the method to catch the exceptions, that the method is likely to
throw. This is often the approach library creators use. They list the
exception in the throws clause and we must catch them. You will find
the same approach throughout the java libraries we use.
Q:
Is it necessary that each try block must be followed by a catch block?
A: It is not necessary that each try block must be followed by a
catch block. It should be followed by either a catch block OR a
finally block. And whatever exceptions are likely to be thrown should
be declared in the throws clause of the method.
Q:
If I write return at the end of the try block, will the finally block
still execute?
A: Yes even if you write return as the last statement in the try
block and no exception occurs, the finally block will execute. The
finally block will execute and then the control return.
Q:
If I write System.exit (0); at the end of the try block, will the
finally block still execute?
A: No in this case the finally block will not execute because
when you say System.exit (0); the control immediately goes out of the
program, and thus finally never executes.
Q:
How are Observer and Observable used?
A: 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.
Q:
What is synchronization and why is it important?
A: With respect to multithreading, synchronization is the
capability to control
the access of multiple threads to shared resources. Without
synchronization, it is possible for one thread to modify a shared
object while another thread is in the process of using or updating
that object's value. This often leads to significant errors.
Q:
How does Java handle integer overflows and underflows?
A: It uses those low order bytes of the result that can fit into
the size of the type allowed by the operation.
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 difference between preemptive scheduling and time slicing?
A: 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 reenters the pool of
ready tasks. The scheduler then determines which task should execute
next, based on priority and other factors.
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.
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.
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.
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.
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.
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.
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.
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:
Can applets communicate with each other?
A: At this point in time applets may communicate with other
applets running in the same virtual machine. If the applets are of the
same class, they can communicate via shared static variables. If the
applets are of different classes, then each will need a reference to
the same class with static variables. In any case the basic idea is to
pass the information back and forth through a static variable.
An applet can also get references to all other applets on the same
page using the getApplets() method of java.applet.AppletContext. Once
you get the reference to an applet, you can communicate with it by
using its public members.
It is conceivable to have applets in different virtual machines that
talk to a server somewhere on the Internet and store any data that
needs to be serialized there. Then, when another applet needs this
data, it could connect to this same server. Implementing this is
non-trivial.
Q:
What are the steps in the JDBC connection?
A: While making a JDBC connection we go through the following
steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
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:
Can an unreachable object become reachable again?
A: An unreachable object may become reachable again. This can
happen when the object's finalize() method is invoked and the object
performs an operation which causes it to become accessible to
reachable objects.
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.
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.
Q:
What is Externalizable?
A: Externalizable is an Interface that extends Serializable
Interface. And sends data into Streams in Compressed Format. It has
two methods, writeExternal(ObjectOuput out) and
readExternal(ObjectInput in)
Q:
What modifiers are allowed for methods in an Interface?
A: Only public and abstract modifiers are allowed for methods in
interfaces.
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).
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.
Q:
What is the difference between preemptive scheduling and time slicing?
A: 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 reenters the pool of
ready tasks. The scheduler then determines which task should execute
next, based on priority and other factors.
Q:
What is the catch or declare rule for method declarations?
A: If a checked exception may be thrown within the body of a
method, the method must either catch the exception or declare it in
its throws clause.
Q:
Is Empty .java file a valid source file?
A: Yes, an empty .java file is a perfectly valid source file.
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.
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.
Q:
Is main a keyword in Java?
A: No, main is not a keyword in Java.
Q:
Is next a keyword in Java?
A: No, next is not a keyword.
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.
Q:
Is exit a keyword in Java?
A: No. To exit a program explicitly you use exit method in System
object.
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.
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.
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.
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..
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.
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.
Q:
Can main method be declared final?
A: Yes, the main method can be declared final, in addition to
being public static.
Q:
What will be the output of the following statement?
System.out.println ("1" + 3);
A: It will print 13.
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.
