Variations on Inheritance

Question 1

What’s an abstract class?

Answer 1

A class that cannot be instantiated

Question 2

Can abstract classes have constructors?

Answer 2

yes. if a concrete class derives from the abstract, when constructing the concrete class we need to construct the abstract class first.

Question 3

What’s an abstract method?

Answer 3

a method without implementation, postponed to inheriting class.

Question 4

What’s the difference between pure virtual function in c++ and an abstract method in java or c#?

Answer 4

PVF: may have implementation

abstract method: doesn’t have implementation

Question 5

What’s the difference between interface and abstract class in Java?

Answer 5

Interface:
only abstract methods and const definitions.
pure protocol specification.

Abstract class:
methods may have default implementation.
every class with an abstract method is abstract.
not every abstract class has an abstract method.

Question 6

What’s a final method?

Answer 6

cannot be overridden

Question 7

What’s a final class?

Answer 7

cannot be further derived

Question 8

What’s multiple dispatch?

Answer 8

the executed method is selected by the dynamic type of one (or more) argument(s)

Question 9

What’s MultiMethod?

Answer 9

a method that takes part in multiple dispatch

Question 10

What are the Drawbacks of MultiMethods?

Answer 10

• Expensive dispatch process
  Must examine dynamic types of all arguments
• Possible run time ambiguity
  Given A::f(A, A), B::f(B, A) and B::f(A, B)
  What happens if you pass two B’s to A::f?
• Can get the same functionality in standard Java

Question 11

What is a binary method?

Answer 11

a method that takes as an argument a parameter of the same type as the class the method is declared in.

Question 12

How mixins are implemented in c++?

Answer 12

Using templates.
let's say we want to add an undo functionality to different classes.
let's use a mixin:
typedef UndoableMixin UndoableInt;
typedef UndoableMixin Undoable char;

the new functionality will be in UndoableMixin.

Question 13

What are the drawbacks of c++’s mixins?

Answer 13

mixin is not a class.

Question 14

What is the drawback of mixins in java?

Answer 14

Adding a method to a mixin may silently override an inherited method

Question 15

What’s a trait?

Answer 15

A composable unit of behavior.
Serves as a type
No fields
Provides some methods (with behavior)
Requires other methods (abstract)
When composing traits, if a method has more than one implementation it becomes abstract