What is the difference between an Interface and an Abstract ?
An abstract class can
have instance methods that implement a default behavior. An Interface can only declare
constants and instance methods, but cannot implement default behavior and all
methods are implicitly abstract. An interface has all public members and no
implementation. An abstract class is a class which may have the usual flavors
of class members (private, protected, etc.), but has some abstract methods.
What is the purpose of garbage collection in Java,
and when is it used?
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.
Describe synchronization in respect
to multithreading.
The Synchronization is the process of control the multiple thread sharing the samne resource. 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.
Explain
different way of using thread?
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.
What
are pass by reference and passby value?
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.
What
is HashMap and Map?
Map is
Interface and Hashmap is class that implements that.
Difference
between HashMap and HashTable?
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.
Difference
between Vector
and ArrayList?
Vector is synchronized whereas arraylist is not.
Difference between Swing and Awt?
AWT are heavy-weight componenets. Swings are light-weight components. Hence
swing works faster than AWT.
What is the difference between a constructor and a method?
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.
What is an Iterator?
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.
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.
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.
What is an abstract class?
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.
What is static in java?
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.
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