Java Abstract Class and Interface

Question 1

Which of the following is FALSE about abstract classes in Java

• A. If we derive an abstract class and do not implement all the abstract methods, then the derived class should also be marked as abstract using 'abstract' keyword
• B. Abstract classes can have constructors
• C. A class can be made abstract without any abstract method
• D. A class can inherit from multiple abstract classes.

Answer: (D)

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Question 2

Which of the following is true about interfaces in java.

1) An interface can contain following type of members.
....public, static, final fields (i.e., constants) 
....default and static methods with bodies

2) An instance of interface can be created.

3) A class can implement multiple interfaces.

4) Many classes can implement the same interface.

• A. 1, 3 and 4
• B. 1, 2 and 4
• C. 2, 3 and 4
• D. 1, 2, 3 and 4

Answer: (A)

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Answer: (A)

Explanation: The instance of an interface can't be created because it acts as an abstract class.

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Question 3

Predict the output of the following program.

abstract class demo
{
    public int a;
    demo()
    {
        a = 10;
    }

    abstract public void set();
    
    abstract final public void get();

}

class Test extends demo
{

    public void set(int a)
    {
        this.a = a;
    }

    final public void get()
    {
        System.out.println("a = " + a);
    }

    public static void main(String[] args)
    {
        Test obj = new Test();
        obj.set(20);
        obj.get();
    }
}
 

• A. a = 10
• B. a = 20
• C. Compilation error

Answer: (C)

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Final method can’t be overridden. Thus, an abstract function can’t be final.

Question 4

Type IV JDBC driver is a driver

• A. which is written in C++
• B. which requires an intermediate layer
• C. which communicates through Java sockets
• D. which translates JDBC function calls into API not native to DBMS

Answer: (C)

ISRO CS 2017 Java Abstract Class and Interface 50 Java Language MCQs with Answers
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JDBC type 4 driver,works directly by connecting to the database server through socket connections and converts JDBC calls to vendor-specific database protocols.These drivers don't require any intermediate layer. So, option (C) is correct.

Question 5

Which of the following statement(s) with regard to an abstract class in JAVA is/are TRUE ? I. An abstract class is one that is not used to create objects. II. An abstract class is designed only to act as a base class to be inherited by other classes.

• A. Only I
• B. Only II
• C. Neither I nor II
• D. Both I and II

Answer: (D)

UGC NET CS 2017 Jan - III Java Abstract Class and Interface
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Abstract data class is not used to create objects in Java and it is designed only to act as a base class to be inherited by other classes. Both Statement are correct. For more information Refer:Abstract Classes in Java Option (D) is correct.

Question 6

Which of the following is used to make an Abstract class?

• A. Making atleast one member function as pure virtual function
• B. Making atleast one member function as virtual function
• C. Declaring as Abstract class using virtual keyword
• D. Declaring as Abstract class using static keyword

Answer: (A)

UGC NET CS 2015 Dec – III Java Abstract Class and Interface 50 Java Language MCQs with Answers
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Making atleast one member function as pure virtual function is the method to make abstract class. For more information on Abstract Class Refer:Pure Virtual Functions and Abstract Classes in C++ Option (A) is correct.

Question 7

We can make a class abstract by

• A. Declaring it abstract using the virtual keyword
• B. Making at least one member function as virtual function
• C. Making at least one member function as pure virtual function
• D. Making all member function const

Answer: (C)

ISRO CS 2008 Java Abstract Class and Interface 50 Java Language MCQs with Answers
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An abstract class is a class that is designed to be specifically used as a base class. An abstract class contains at least one pure virtual function. A pure virtual function can be declared by using a pure specifier ( = 0 ) in the declaration of a virtual member function in the class declaration. Option (C) is correct.