WEEK 4 - INTRODUCTION TO ABSTRACT CLASSES, POLYMORPHISM AND INTERFACES

Question 1

What is an abstract method?

Answer 1

A method with no implementation.

Question 2

What are the rules for abstract methods?

Answer 2

1. Declared with the keyword abstract
2. Never private
3. Ends with a semicolon. No method body and no curly brackets.

Example.
public abstract void paint();

Question 3

How to guarantee abstract method implementation?

Answer 3

Abstract methods MUST be implemented in the subclasses (if not, in a subclass of the subclass).

Question 4

What is the error thrown of an abstract method unsuccessfully implemented?

Answer 4

Syntax error.

Question 5

What does polymorphism mean?

Answer 5

Polymorphism means “many forms” or many shapes.

Question 6

Polymorphism can be applied to which three things?

Answer 6

• Types (classes)
• Methods
• Generics (< >)

Question 7

What does a polymorphic method indicate?

Answer 7

A polymorphic method indicates multiple “forms” or implementations.

This is alternatively known as dynamic binding.

Question 8

When is the implementation of polymorphic methods?

Answer 8

During runtime.

Question 9

Abstract methods are always what?

Answer 9

Polymorphic.

Question 10

What is an interface?

Answer 10

An interface represents a “contract” or agreement.
In Java, the implementing class will define all methods of the interface.

Question 11

What’s an interface similar to?

Answer 11

An interface is alike to an abstract class with all methods abstract.

Question 12

Interface is a special type of class, what keyword is used instead of class?

Answer 12

interface.

Question 13

What are the four rules for interfaces in Java?

Answer 13

1. Contains only public method declarations, no method implementations.
2. Can have field variables but they must be final
3. Interface name is often as adjective
   - Names often end in “…able”, e.g. Comparable
4. When using an interface, you write implements instead of extends

Question 14

What are the properties of interfaces?

Answer 14

1. Similar to abstract classes, we can’t create instances (objects) from interfaces.
2. An implementing class must implement all the interface methods (as if they were all abstract).
3. A class can implement more than one interface.
4. Many classes can implement the same interface without sharing the same class hierarchy.
5. Methods stubs in the interface are never ‘private’.

Question 15

What are the benefits and uses of interfaces?

Answer 15

• Can help achieve the object-oriented concept of encapsulation.
• Can be used to define an API (Application Programmer Interface).
• Commonly used to implement event handlers and callbacks.
• May be used to stimulate multiple inheritance in Java.

Question 16

What is the difference between the keywords extends and inheritance?

Answer 16

A class can only extend one other class (have at most one superclass).

A class can implement many interfaces.