Q: What is the difference between an Interface and
an Abstract class?
A: An abstract class can
have instance methods that implement a default behaviour. An Interface can only
declare constants and instance methods, but cannot implement default behaviour
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 flavours
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 synchronization, 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 pass by 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 doesn’t
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: 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 i.e., final class may not be sub classed. A final method can't be
overridden when its class is inherited. You can't change value of a final
variable (is a constant).
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