1 : 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
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated
2 :
What
is the difference between an Interface
and an Abstract class?
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.
3 : 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.
4 : Describe
synchronization in respect to multithreading.
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.
5 : 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.
6 : What
is HashMap and Map?
Map
is Interface and Hashmap is class that implements
that.
7 :
Difference between Vector and ArrayList?
Vector
is synchronized whereas arraylist is not.
AWT
are heavy-weight componenets. Swings are
light-weight components. Hence swing works
faster than AWT.
9 :
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.
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.
10:
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.
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.
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).
| 12 : | 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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