Ch 12 - Abstract Classes

A class provides default variables

Abstract class

A class that provides default code to other classes that use that class (Abstract class or interface)

Abstract class

T/F: Abstract classes provide compile-time polymorphism

False, Abstract classes provide runtime polymorphism, which enables a programer to use an abstract method without worrying about which concrete class implements the abstract method.

T/F: Consider a program that catalogs the types of trees in a forest. Each tree object contains the tree's species type, age, and location. This program will benefit from an abstract class to represent the trees.

False, Because there is no information specific to each species of tree, each tree object can simply possess the species type, age, and location information

Consider a program that maintains a grocery list. Each item, like eggs, has an associated price and weight. Each item belongs to a category like produce, meat, or cereal, where each category has additional features, such as meat having a "sell by" date. This program will benefit from an abstract class.

False, Normal inheritance is sufficient. A superclass like Item might implement price and weight. Then subclasses like MeatItem might add behavior like a "sell by" date. But no behavior was mentioned that must be implemented by all subclasses.

T/F: An abstract class can be instantiated as an object

False, an abstract class is missing some behaviors. A subclass that implements those behavior can then be instantiated

A class that provides an API that must be implemented and no other code

Interface

A class that provides only static final fields

Interfaces, they can define public static final fields and don't restrict a class' inheritance

T/F: An abstract class cannot be instantiated, or created, because it has missing methods

True

T/F: UML uses italics to denote abstract classes

True

T/F: a class can implement multiple interfaces

True

T/F: an abstract class can include both method signatures for abstract methods and complete code for non-abstract methods

True

T/F: if a sublcass does not implement an abstract method, then the subclass must also be defined as abstract

True

T/F: Consider a program that catalogs the types of trees in a forest. Each tree object contains the tree's species type, age, location, and estimated size based on age. Each species uses a different formula to estimate size based on age. This program will benefit from an abstract class.

True, a superclass tree might store the age and location, and specify an estimateSize() behavior. Subclasses for each species like OakTree then implements the species particular growth rate formula

abstract class

a class that guides the design of subclasses but cannot itself be instantiated as an object; specifies how the subclass must be implemented

concrete class

a class that is not abstract, and hence can be instantiated

abstract method

a method that each subclass must implement to be a concrete class

public class Circle extends Shape implements Serializable, DrawableInterface {

how to implement an interface

interface

specifies a set of methods that an implementing class must override and define

what word is needed to create an interface?

the keyword interface is needed in the class definition: public interface DrawableInterface {