Core Java Interview Questions Page1
Q 1 : How should I create an immutable class ?
Ans: An immutable class is a class to which values
assigned to the variables can not be altered because they are declared as final,
all the variables must be assigned in the constructor. As per the specification
declared in java document an immutable class should not contain any modifier
method. But a developer should be careful while using an immutable class that
the constructor and accessor methods should not contain references of mutable
fields.
Q 2 : Is println overloading or overriding ?
Ans : The println() method is the method of
the PrintWriter class which is an example of
overloading because PrintWriter class includes several methods
having the same name and same return type. E.g. the methods such as println(boolean),
println(int) and println(String) have the same basic
method name as "println", and also have same return type as void,
but only method signature varies according to the type of the arguments passed,
that is sufficient for the java interpreter in identifying the appropriate
method in order to call at runtime.
Q 3 : What is dynamic method dispatch ?
Ans : Dynamic method dispatch is the mechanism that is
used by Java runtime system in order to determine which method implementation to
call in an inheritance hierarchy. E.g. the method toString() is the
method of Object class that is inherited by all the subclasses,
while the String class overrides this method in order to return its
string content. If an object refers to a String or other type by using
application logic then the java compiler does not have knowledge in advance
where a class to the toString() method is resolved but it must be determined
dynamically at runtime.
Q 4 : How can I call a constructor from a
constructor ?
Ans : When a class have a number of
constructors then calling to these constructors by using this() takes place in
the same way as super class constructor super(). Lets take
an example, Suppose you have a "good citizen" that takes a String
and a boolean and a shorthand version that only receives a String then you may
pass a default value to the two argument constructor.
Q 5 : What if two interface methods clash in
implementation ?
Ans : Suppose there are two interfaces having the same
method signature regardless of any other intentions. A concrete class
implementing both interfaces can only provide one implementation of a given
method signature that avoids ambiguity for the java compiler deal with such type
of situations. Whenever the condition arises in which two interface
methods having same method signatures and intended behavior then it would be
preferable to rename one of the interface methods indicating a more distinct
purpose.
Q 6 : Why use interfaces to develop Java
applications ?
Ans : Interfaces should be used in large
applications because it allows the whole system to modify easily and also
extending and integrating new features. To start with, a given interface may
have only a single implementation, but in special circumstances where slightly
different behavior is required, you only require to write a class implementing
to one of the existing interfaces that may be dropped in place without requiring
major modifications. Interfaces also enables to a developer to use a class
from different hierarchy to work in an existing application. The class only
requires to specify that it implements the interface that provides necessary
methods in such a way as if it were created for the job.
Q 7 : Can we create an object for an interface
?
Ans : Yes an interface always requires to create
an object that implements the given interface. Interfaces are not allowed to
instantiate in their own right, therefore it is required to write a class that
implements the given interface by implementing all the methods declared in that
interface.
Q 8 : Can an interface extend an abstract class
?
Ans: Java does not allow an interface to extend
an abstract class. An interface can only extend a super-interface (i.e. an
interface which comes above in the hierarchy). Java programming language allows
an abstract class to implement an interface. It separates interfaces and classes
regarding inheritance that only come together when a class implements an
interface, the reverse in not true.
Q 9 : What is a marker interface ?
Ans : In java programming
language, Marker interfaces are those interfaces that don't have any
method, but signify their compatibility with certain operations. java.io.Serializable, java.lang.Clonable, java.util.EventListener
interfaces are some of the
examples of typical marker interfaces. E.g. marker interface
java.io.Serializable does not contain any method but the class that implement to
this interface have the functionality of serializing and de-serializing to their
objects.
Q 10 : How does Class.forName(dbDriver) work with
DriverManager.getConnection ?
Ans : The method forName()
is a static method of the class Class. This method instantiates a
class and reduces hard coded dependencies of any java applications. Most of the
user are well known about the database driver that is intend to use while
writing code first, but reconfiguring for a different database product does not
require re-writing the client application if you are using a string variable for
your class name.
Q 11 : What
happens to singletons when two JVMs are running ?
ANS
: A singleton class is used only at those places where the user requires only one instance
of a class to be operating through out the application.
* It is mostly used in the multi-threaded environments.
* They often work as global variables permitting the allocation and
initialization whenever required.
Benefits of using a singleton class:
Singleton class provides
* access only to the unique instances,
* permits a variable number of instances,
* provides more flexibility than class operations.
* it reduces name space, and
* allows refinement of operations and representations,
A singleton class ensures that only one
instance is created for the JVM running currently.
A Singleton class have a private default constructor, this restricts the
direct instantiation of that class by any other class.
In a single JVM there must be only one instance of a true singleton class.
If we have 2 JVMs running simultaneously, Then there must exist 2 independent
instances of a singleton class.
If these two singletons accesses to the common system resource, then there
are chances of occuring the conflicts between the two systems.
Q 12 : What is a factory method ?
Ans : In java
programming language the factory methods are static constructor methods which
are required to return the instances of the native class. One can say that a
factory is a sort of a constructor that is required to produce a number of
multiple kind of objects. This requires the user to implement the factory
method as a static method, rather than a simple
constructor. The factory methods can?t be directly overriden.
The return type of a this method is an interface or superclass type. This is
to give the liberty to govern the actual class that is returned through
polymorphism.
Few of the examples of the factory methods in the JDK are :
- Pattern.compile
- LogManager.getLogManager
- Calendar.getInstance
- Collections.unmodifiableCollection, Collections.synchronizeCollection
public class ComplexNo { public static ComplexNo valueOf(float Real, float Imaginary) { return new ComplexNo(Real, Imaginary); } private ComplexNo(float Real, float Imaginary) { qReal = Real; qImaginary = Imaginary; } private float qReal; private float qImaginary; } |
Q 13 : When should I use the abstract class rather
than an interface ?
ANS : A Java interface is an abstract data type like a class
having all its methods abstract i.e. without any implementation. That means
we cannot create objects of an interface. Typically, an interface in java
programming language consists of one or more abstract methods that must be
implemented by a class to confirm to the type.
| public
interface A{
abstract_method1(); abstract_method2(); |
Similarly an abstract class is restricted by the compiler to get
instantiated as it may contain one or more abstract methods which violates
the essential things needed to create an object
abstract class A{ |
Interfaces Vs Abstract classes
Few of the differences between Interface and abstract classes are:
| Features | Interface | Abstract Class |
| Methods | An interface contains all the methods with empty implementation. | An abstract class must have at least one method with empty implementation. |
| Variables | The variables in the interfaces are final and static by default. | An Abstract class may contain final as well as static variables but the user needs to mention explicitly. |
| Multiple Inheritance | In java programming language, multiple inheritance is achieved with the help of the interfaces. This is achieved by implementing more than one interface at a time. | Abstract
classes does not provide this functionality. They just allow to introduce the single inheritance in java programming language. |
| Additional Functions | If we try to add a method to an interface then we need to implement this method in all the subsequent classes which have implemented this interface. | In an abstract class we can add a method with default implementation and then we can use it by extending the abstract class. |
Q 14 : What is hashcode? When is hashCode() used ?
Ans : It is always faster to sort out the things with
the numerically keys rather than going for a long alphabetic key, same thing
is followed with the hashcode. A hashCode is a technique or a mechanism of computing a numeric key from a
long String. The hash code speeds up the search process among the multiple strings, so
there lies a greater chance of being different from other instance. There
is a chance that we have two different Strings having the same hashCode in
that case we use the equals method
to make an exact match.
Q 15 : How can I get a class reference without using new operator ?
Ans : We mostly use the new keyword to construct the objects. However in
few conditions we need to get a class reference in absence of the new
operator. We do it when we require to load a class in memory at runtime,
though we are not aware of the class to be loaded in advance. In such cases,
what we do is to just configuring name of the required class in the
properties file and then we allow to load it with the help of the the static Class.forName(String)
method.
Q 16 : What is the return type for the Class.forName(String) method ?
Ans : Always the method Class.forName(String)
returns a Class object reference. Some times we need to get a
class reference in absence of the new operator to construct an
object. So we do it when we require to load a class in memory at
runtime, though we are not aware of the class to be loaded in advance. In
such circumstances, we just configure the name of the required class in the
properties file and then we allow to load it with the help of the the static Class.forName(String)
method. The user is required to use the newInstance() method on
the class reference then only he can obtain an instance of the named class.
Example of using Class.forName(String) in the jdbc code
Class.forName("jdbc.DriverABC"); |