Multiple Choice Questions on JAVA (object oriented programming) bank 7 -- abstract classes

MCQs BANK
Page: 1
Object Oriented
Programming
Topic: Abstract Classes
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This MCQs Bank contains question
and solution on adjacent(even-odd)
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MCQs BANK No.: 7

MCQs Bank on Object Oriented Programming
Topic : Abstract Classes
MCQ No: 1
When the keyword _______ appears
in a class definition, it means that
zero or more of it’s methods are
abstract.
a) abstract
b) default
c) static
d) public
Page: 2

MCQ No: 1 (Solution)
Ans: a) abstract
Explanation:
When the keyword abstract
appears in a class definition, it
means that zero or more of it’s
methods are abstract.
Abstract class is a restricted class
that cannot be used to create
objects (to access it, it must be
inherited from another class).
Abstract method can only be used
in an abstract class, and it does
not have a body. The body is
provided by the subclass (inherited
from).
Page: 3

MCQ No: 2
An _________ method has no
body. Fill in the blank.
a) static
b) abstract
c) private
d) public
Page: 4

Ans: b) abstract
Explanation: An abstract method
has no body. Some of the
subclass has to override it and
provide the implementation.
Abstract methods are declaration
only and it will not have
implementation. It will not have a
method body. A Java class
containing an abstract class must
be declared as abstract class.
Page: 5

MCQ No: 3
Objects cannot be created out of
abstract class. True or False
a) True
b) False
Page: 6

Ans: a) True
Explanation:
Objects cannot be created out of
abstract class. Abstract classes
basically provide a guideline for the
properties and methods of an
object.
Page: 7

MCQ No: 4
An abstract class, which declared
with the “abstract” keyword, cannot
be instantiated. True or False?
a) True
b) False
Page: 8

Ans: a) True
Explanation:
An abstract class, which declared
with the “abstract” keyword, cannot
be instantiated. This is because in
abstract class the methods are
declared only without defining
them.
Abstract classes cannot be
instantiated, but they can be sub
classed.
Page: 9

MCQ No: 5
In order to use abstract classes,
they have to be ______________ .
Fill in the blank.
a) subclassed
b) sub-divided
c) overridden
d) overloaded
Page: 10

Ans: a) subclassed
Explanation: In order to use
abstract classes, they have to be
subclassed. There are situations in
which you want to define a
superclass that declares the
structure of a given abstraction
without providing a complete
implementation of every method.
That is, sometimes you want to
create a superclass that only defines
generalized form that will be shared
by all of its subclasses, leaving it to
each subclass to fill in the details.
Page: 11

MCQ No: 6
___________ are those methods
that must be overridden by
subclasses because they have no
implementation specified in the
superclass. Fill in the blank.
a) default methods
b) abstract methods
c) static methods
d) hidden methods
Page: 12

Ans: b) abstract methods
Explanation:
Consider a class Triangle. It has no
meaning if area( ) is not defined. In this
case, you want some way to ensure that
a subclass does, indeed, override all
necessary methods. Java’s solution to
this problem is the abstract method. You
can require that certain methods be
overridden by subclasses by specifying
the abstract type modifier.
These methods are sometimes referred
to as subclasser responsibility because
they have no implementation specified in
the superclass. Thus, a subclass must
override them—it cannot simply use the
version defined in the superclass.
Page: 13

MCQ No: 7
The correct way of declaring an
abstract method is
a) abstract return_type
method_name(parameter-list);
b) return_type
method_name(parameter-list)
abstract ;
c) return_type abstract
d) all are correct
Page: 14

Ans: a) abstract return_type
Explanation:
The correct way of declaring an
abstract method is :
abstract return_type
As you can see, no method body is
present. Any class that contains one
or more abstract methods must also
be declared abstract.
Page: 15

MCQ No: 8
There can be no objects of an
abstract class. True or False
a) True
b) False
Page: 16

Ans: a) True
Explanation:
There can be no objects of an
abstract class. That is, an abstract
class cannot be directly instantiated
with the new operator. Such objects
would be useless, because an
abstract class is not fully defined.
Page: 17

MCQ No: 9
Can you declare abstract
constructors, or abstract static
methods ?
a) Yes
b) No
Page: 18

Ans: b) No
Explanation: You cannot declare
abstract constructors, or abstract static
methods. Constructors are invoked
automatically when objects of a class
is created using new operator. As an
abstract class cannot be directly
instantiated with the new operator so
you cannot declare abstract
constructors. static methods are class
methods, that is shared by multiple
objects(instances of the class) of a
class, as an abstract class cannot be
directly instantiated so it is not possible
to declare abstract static methods.
Page: 19

MCQ No: 10
Any subclass of an abstract class
must either implement all of the
abstract methods in the superclass,
or be itself declared abstract. True or
False?
a) True
b) False
Page: 20

Ans: a) True
Explanation:
Any subclass of an abstract class
must either implement all of the
abstract methods in the superclass,
or be itself declared abstract.
Page: 21

MCQ No: 11
Is it possible to create a reference
to an abstract class so that it can
be used to point to a subclass
object ?
a) Yes, Possible
b) No, Not Possible
Page: 22

Ans: a) Yes, Possible
Explanation:
Although abstract classes cannot be
used to instantiate objects, they can
be used to create object references,
because Java’s approach to run-time
polymorphism is implemented
through the use of superclass
references. Thus, it must be possible
to create a reference to an abstract
class so that it can be used to point
to a subclass object. It is through
superclass reference variables that
overridden methods are resolved at
run time.
Page: 23

MCQ No: 12
Methods declared as final cannot
be overridden. True or False
a) True
b) False
Page: 24

Ans: a) True
Explanation:
While method overriding is one of
Java’s most powerful features, there
will be times when you will want to
prevent it from occurring. To disallow
a method from being overridden,
specify final as a modifier at the start
of its declaration.
Methods declared as final cannot be
overridden. If you attempt to do so, a
compile-time error will result.
Page: 25

MCQ No: 13
How would you will prevent a class
from being inherited?
a) By making the class abstract
b) By adding a comment statement
"Don't Inherit"
c) Precede the class declaration
with final
d) Requesting others to not inherit
the class
Page: 26

Ans: c) Precede the class
declaration with final
Explanation: Sometimes you will want
to prevent a class from being inherited. To
do this, precede the class declaration with
final.Declaring a class as final implicitly
declares all of its methods as final, too.
Here is an example of a final class:
final class A {
// ...
}
// The following class is illegal.
class B extends A { // ERROR! Can't
subclass A
// ...
}
Page: 27

MCQ No: 14
It is illegal to declare a class as
both abstract and final. Yes or No?
a) Yes, it is illegal
b) No, it is completely legal.
Page: 28

Ans: a) Yes, it is illegal
Explanation:
It is illegal to declare a class as both
abstract and final since an abstract
class is incomplete by itself and
relies upon its subclasses to provide
complete implementations.
Page: 29

Multiple Choice Questions on JAVA (object oriented programming) bank 7 -- abstract classes

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