Quiz 3 : Abstract Classes » Week 2 » Inheritance And Data Structures In Java

Q: We can instantiate an abstract class.

We can’t instantiate an abstract class.

Q: An abstract class can only contain abstract methods.

We can also declare concrete methods in an abstract class.

Q: A non-abstract class can contain an abstract method.

No, an abstract method can only be declared in an abstract class.

Q: A class which is declared as abstract must have at least one abstract method.

No, we can make a class abstract with all concrete methods to avoid the class being instantiated.

Q: What will be the output of the following program? <pre> abstract class A { public void display(); } class B extends A { @Override public void display() { System.out.print("Two"); } public void reset() { System.out.print("Zero"); } } public class C { public static void main(String args[]) { A screen = new B(); screen.display(); screen.reset(); } } </pre>

The program will throw a compile error, since we create an instance of the B class and store it in a variable "screen" of type A, which does not have a reset() method.

Q: What will be the output of the following program? <pre> abstract class Calculation { abstract void calculate(int number); } class Summation extends Calculation { int result; @Override public void calculate(int number) { this.result = number + number; } } class Main { public static void main(String args[]) { Summation summation = new Summation(); summation.result = 0; summation.calculate(2); System.out.print(summation.result); } } </pre>

The output is 4. The "summation.result = 0;" sets the summation.result to 0. The "summation.calculate(2);" method call resets the summation.result to 4.

Q: What will be the output of the following program? <pre> abstract class Calculation { abstract void calculate(int number); } class MySummation extends Calculation { int result; @Override public void calculate(int number) { if (number < 10) { this.result = 590; } else { this.result = number + number; } } } class Main { public static void main(String args[]) { MySummation mySummation1 = new MySummation(); mySummation1.calculate(9); MySummation mySummation2 = new MySummation(); mySummation2.calculate(10); System.out.print(mySummation1.result); System.out.print(", "); System.out.print(mySummation2.result); } } </pre>

The output is "590, 20". The "mySummation1.calculate(9);" method call sets the mySummation1.result to 590, and the "mySummation2.calculate(10);" method call sets the mySummation2.result to 20.