Java Abstraction and Exception

Java Abstraction

G.N.P.Ariysinghe

Multiple Select

Which of the following can be used to achieve Abstraction? (Select all which is appropriate)

• Abstract classes

• Constructors

• Interfaces

• Enums

Abstract Classes and Methods

• Data abstraction is the process of hiding certain details and showing only essential information to the user.

• 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).

• An abstract class can have both abstract and regular methods

Multiple Choice

abstract class Animal {

public abstract void animalSound();

public void sleep() {

System.out.println("Zzz");}}

What will be the output of the following code?

Animal myObj = new Animal();

• Zzz

• An error

• myObj

• animalSound

An error will be generated.

To access the abstract class, it must be inherited from another class.

Example

abstract class Animal {

public abstract void animalSound(); // Abstract method (does not have a body)

public void sleep() { // Regular method

System.out.println("Zzz");

}

}

class Pig extends Animal {

public void animalSound() {

System.out.println("The pig says: wee wee"); // The body of animalSound() is provided here

}

}

class Main {

public static void main(String[] args) {

Pig myPig = new Pig(); // Create a Pig object

myPig.animalSound();

myPig.sleep();

}

}

Multiple Choice

Which of these keywords are used to define an abstract class?

• abst

• abstract class

• abstract

• Abstract

Multiple Choice

If a class inheriting an abstract class does not define all of its function then it will be known as?

• A simple class

• A Static class

• Abstract class

• None of the mentioned

Any subclass of an abstract class must either implement all of the abstract method in the superclass or be itself declared abstract.

Multiple Choice

Which of these is not a correct statement?

• Every class containing abstract method must be declared abstract

• Abstract class defines only the structure of the class not its implementation

• Abstract class can be initiated by new operator

• Abstract class can be inherited

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.

Multiple Choice

Which of these packages contains abstract keyword?

• java.lang

• java.util

• java.io

• java.system

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();

}

}

Class contains a private member variable j, this cannot be inherited by subclass B and does not have access to it.

$ javac inheritance.java

Exception in thread "main" java.lang.Error:

Unresolved compilation problem:

The field A.j is not visible