1. Which of these keywords are used to define an abstract class?
a) abst
b) abstract
c) Abstract
d) abstract class
Explanation: abstract
2. Which of these is not abstract?
a) Thread
b) AbstractList
c) List
d) None of the Mentioned
Explanation: Thread is not an abstract class.
3. If a class inheriting an abstract class does not define all of its function then it will be known as?
a) Abstract
b) A simple class
c) Static class
d) None of the mentioned
Explanation: Any subclass of an abstract class must either implement all of the abstract method in the superclass or be itself declared abstract.
4. Which of these is not a correct statement?
a) Every class containing abstract method must be declared abstract
b) Abstract class defines only the structure of the class not its implementation
c) Abstract class can be initiated by new operator
d) Abstract class can be inherited
Explanation: Abstract class cannot be directly initiated with new operator, Since abstract class does not contain any definition of implementation it is not possible to create an abstract object.
5. Which of these packages contains abstract keyword?
a) java.lang
b) java.util
c) java.io
d) java.system
Explanation: None.
6. What will be the output of the following Java code?
class A
{
public int i;
private int j;
}
class B extends A
{
void display()
{
super.j = super.i + 1;
System.out.println(super.i + " " + super.j);
}
}
class inheritance
{
public static void main(String args[])
{
B obj = new B();
obj.i=1;
obj.j=2;
obj.display();
}
}
a) 2 2
b) 3 3
c) Runtime Error
d) Compilation Error
Explanation: Class contains a private member variable j, this cannot be inherited by subclass B and does not have access to it.
output:
Exception in thread "main" java.lang.Error: Unresolved compilation problem: The field A.j is not visible
7. What will be the output of the following Java code?
class A
{
public int i;
public int j;
A()
{
i = 1;
j = 2;
}
}
class B extends A
{
int a;
B()
{
super();
}
}
class super_use
{
public static void main(String args[])
{
B obj = new B();
System.out.println(obj.i + " " + obj.j)
}
}
}
a) 1 2
b) 2 1
c) Runtime Error
d) Compilation Error
Explanation: Keyword super is used to call constructor of class A by constructor of class B. Constructor of a initializes i & j to 1 & 2 respectively.
output:
1 2
8. What will be the output of the following Java code?
class A
{
int i;
void display()
{
System.out.println(i);
}
}
class B extends A
{
int j;
void display()
{
System.out.println(j);
}
}
class method_overriding
{
public static void main(String args[])
{
B obj = new B();
obj.i=1;
obj.j=2;
obj.display();
}
}
}
a) 0
b) 1
c) 2
d) Compilation Error
Explanation: class A & class B both contain display() method, class B inherits class A, when display() method is called by object of class B, display() method of class B is executed rather than that of Class A.
output:
2
9. What will be the output of the following Java code?
class A
{
public int i;
protected int j;
}
class B extends A
{
int j;
void display()
{
super.j = 3;
System.out.println(i + " " + j);
}
}
class Output
{
public static void main(String args[])
{
B obj = new B();
obj.i=1;
obj.j=2;
obj.display();
}
}
a) 1 2
b) 2 1
c) 1 3
d) 3 1
Explanation: Both class A & B have member with same name that is j, member of class B will be called by default if no specifier is used. I contains 1 & j contains 2, printing 1 2.
output:
1 2
10. Which of this keyword must be used to inherit a class?
a) super
b) this
c) extent
d) extends
Explanation: extends