Abstract Data Types (ADTs) Basic Elements of an ADT. Translating an ADT into Code. Problem Magic! Solution. Class #26: Abstract Data Types. Data.

Transcription

1 Abstract Data Types (ADTs) Problem Magic! Solution Class #26: Abstract Data Types Software Design II (CS 220): M. Allen, 11 March 16! An ADT is like a sketch of a solution to a problem! The basic elements we need to solve the problem, without any unnecessary detail! Sometimes, this can require us to do a lot of work to fill it in Friday, 11 Mar. 2016" Software Design II (CS 220)" 2" Basic Elements of an ADT! We can start by breaking down the problem into its simple main components 1. Data: the information we need to keep track of 2. Operations: what we can do with that information! Only once we understand the basic elements of the type we want to use, can we begin properly implementing it Data Operations Friday, 11 Mar. 2016" Software Design II (CS 220)" 3" Translating an ADT into Code! Once we have our general picture, we make it more and more specific, until we finally turn it into actual code 1. ADT" Data Operations 2. Class Diagram" myclass! + int var1! + int method()! + void method2(int i)! 3. Class Specification"! public void method2(int j)! Description of what method2 actually does (preconditions, output, etc.)! public class myclass! Friday, 11 Mar. 2016" Software Design II (CS 220)" 4" 4. Java Class Code" // code! 1

2 Template-Based Design! When solving a design problem, it often helps to create a template! Isolate the basic elements we will need! Create slots for those elements in our design! Fill in the slots later, to create a particular item Template-Based Design in Java! Inheritance provides one basic mechanism for templates! We create a super-class with all the basic features we want! Every sub-class that inherits from the super-class will have those features! While this works for any class, some types are only meant to be inherited! Abstract classes! Interfaces TemplateClass SomeSubclass Friday, 11 Mar. 2016" Software Design II (CS 220)" 5" Friday, 11 Mar. 2016" Software Design II (CS 220)" 6" Abstract Classes! An abstract class creates a pattern for other classes! Must be inherited (cannot be instantiated on its own)! Can be partially specified (abstract methods must be over-ridden by children) Vehicle {abstract + void setmileage( int )! protected instance variables (to be inherited) Normal methods (to be specified in Vehicle class) abstract methods (must be specified in child classes) Friday, 11 Mar. 2016" Software Design II (CS 220)" 7" Coding Abstract Classes Vehicle {abstract public abstract class Vehicle protected int mileage;! protected String makemodel;! // generic constructor! public Vehicle() mileage = 0;! makemodel = "Generic Vehicle";! // set/get methods! public void setmileage( int i ) {! mileage = i;! public void setmakemodel( String s ) {! makemodel = s;! public int getmileage() {! return mileage;! public String getmakemodel() {! return makemodel;! // abstract method (must be over-ridden)! public abstract String getinfo();! Friday, 11 Mar. 2016" Software Design II (CS 220)" 8" 2

3 Abstract Class Constructors! An abstract class will have a constructor, as usual // generic constructor! public Vehicle() mileage = 0;! makemodel = "Generic Vehicle";!! Such objects cannot be instantiated, however!! The following code will not compile: Vehicle v = new Vehicle();! Generates error message:! Vehicle is abstract; cannot be instantiated! Friday, 11 Mar. 2016" Software Design II (CS 220)" 9"! To be used, an abstract class must be extended Vehicle {abstract Child class must over-ride the abstract methods Friday, 11 Mar. 2016" Software Design II (CS 220)" 10" public class Car extends Vehicle! // constructor! public Car( String s ) // returns info about Car as String! // (over-ridden abstract method)! public String getinfo() String info = "This Car is a " + makemodel;!! As when we extend a normal class, we don t have to override complete methods that we want to use directly from Vehicle!! We only need to ensure our new class can actually be constructed, and has filled in any abstract methods that weren t fully complete Child class has its own (actually useable) constructor method Child class over-rides all of the abstract methods with its own suitable code Friday, 11 Mar. 2016" Software Design II (CS 220)" 11"! Again, like a regular class, we can extend an abstract class in multiple different ways Vehicle {abstract Truck! - int capacity! + Truck( String, int )! Friday, 11 Mar. 2016" Software Design II (CS 220)" 12" 3

4 public class Car extends Vehicle public Car( String s ) public String getinfo() String info = "This Car is a " + makemodel;! public class Truck extends Vehicle private int capacity;! public Truck( String s, int cap ) capacity = cap;! Each child class extends the same abstract parent template class Each child class then must over-ride all abstract methods in its own way public String getinfo() String info = "This Truck is a " + makemodel;! info += ", and carrying capacity of " + capacity + " tons";! Friday, 11 Mar. 2016" Software Design II (CS 220)" 13" public void filllot()! Vehicle[] vehicles = new Vehicle[10];! Polymorphism & Abstraction for (int i = 0; i < vehicles.length; i++)! if (i % 2 == 0)! vehicles[i] = new Car( "Honda Element );! else! vehicles[i] = new Truck("Ford Ranger, 2);! printinfo( vehicles );! private void printinfo( Vehicle[] vehicles )! for (int i = 0; i < vehicles.length; i++)! System.out.println( i + ". " + vehicles[i].getinfo() );! This array has an abstract type (so it can store multiple types of actual class object references) We fill it with references to objects of a non-abstract (normal) type Calling getinfo() works for both child types, using version from the actual class type each time Friday, 11 Mar. 2016" Software Design II (CS 220)" 14" Abstract Classes in the Java Libraries! Java already contains many abstract classes! For example, the hierarchy we have seen before: java.lang.object! java.awt.component! java.lang.container! javax.swing.jcomponent! These are regular classes These are abstract classes Abstract Classes in the Java Libraries javax.swing.jcomponent! Square!! JComponent is an interesting example: it is declared to be abstract, but it contains no abstract methods! Instead, this is just so that we have a certain amount of basic functionality that we can extend and inherit! Since a JComponent is just an invisible box for holding graphics, there is no reason to use it on its own! Thus, its programmers simply made it abstract, so that users would (we assume) write in code to make it do something a little more interesting when they extended it Friday, 11 Mar. 2016" Software Design II (CS 220)" 15" Friday, 11 Mar. 2016" Software Design II (CS 220)" 16" 4

5 Enjoy Your Spring Break! Please Do!! Reading: Chapter 06 due Monday 21 March Friday, 11 Mar. 2016" Software Design II (CS 220)" 17" 5