Java was developed by a team of computer professionals under the guidance of James
Gosling at Sun Microsystems in 1991. They wanted to give a suitable name. Initially
they called it “oak” after seeing an oak tree from their window, but after some weeks
they were discussing another name and were sipping Java coffee, so one of them
suggested the name “Java”.
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.
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nice....... Thanx for provide such information :)
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ReplyDeleteThnks for sharing