494 CS Adv. SW Design and Development
A Tasting.... Course 1: Design patterns: Intro, example Course 2: Inheritance, Interfaces, OO Design
Reading Assignment
Read for understanding (code if it helps)
Basic Java program structure Classes and files; importing packages main() method Types in Java Primitive vs. class; class hierarchy; class Object Boxing References Defining classes In Just Java 2: Chapters 1-5
2001 T. Horton
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Tastings: Course 1
Idioms, Patterns, Frameworks
Idioms, Patterns, Frameworks
Idiom: a small language-specific pattern or technique
A more primitive building block
Design pattern: a description of a problem that reoccurs and an outline of an approach to solving that problem
Generally domain, language independent Also, analysis patterns
Framework:
A partially completed design that can be extended to solve a problem in a domain Horizontal vs. vertical Example: Microsoft's MFC for Windows apps using C++
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Examples of C++ Idioms
Use of an Init() function in constructors
If there are many constructors, make each one call a private function Init() Init() guarantees all possible attributes are initialized Initialization code in one place despite multiple constructors
Design Patterns: Essential Elements
Pattern name
A vocabulary of patterns is beneficial
Problem
When to apply the pattern, what context. How to represent, organize components Conditions to be met before using
Don't do real work in a constructor
Define an Open() member function Constructors just do initialization Open() called immediately after construction Constructors can't return errors They can throw exceptions
Solution
Design elements: relationships, responsibilities, collaborations A template for a solution that you implement
Consequences
Results and trade-offs that result from using the pattern Needed to evaluate design alternatives
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Patterns Are (and Aren't)
Name and description of a proven solution to a problem Documentation of a design decision They're not:
Reusable code, class libraries, etc. (At a higher level) Do not require complex implementations Always the best solution to a given situation Simply "a good thing to do"
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Example 1: Singleton Pattern
Global variables are bad!
Why?
We are tempted to use global variables?
Why?
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Example 1: Singleton Pattern
Context: Only one instance of a class is created. Everything in the system that needs this class interacts with that one object. Controlling access: Make this instance accessible to all clients Solution:
The class has a static variable called theInstance (etc) The constructor is made private (or protected) Clients call a public operation getInstance() that returns the one instance This may construct the instance the very first time or be given an initializer
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Singleton: Java implementation
public class MySingleton { private static theInstance = new MySingleton(); private MySingleton() { // constructor ... } public static MySingleton getInstance() { return theInstance; } }
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Static Factory Methods
Singleton pattern uses a static factory method
Factory: something that creates an instance
Advantages over a public constructor
They have names. Example: BigInteger(int, int, random) vs. BigInteger.probablePrime() Might need more than one constructor with same/similar signatures Can return objects of a subtype (if needed)
Wrapper class example: Double d1 = Double .valueOf("3.14"); Double d2 = new Double ("3.14"); More info: Bloch's Effective Java
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Tastings: Course 2
Interfaces and Collections in Java
Java Interfaces
Note that the word "interface"
Is a specific term for a language construct Is not the general word for "communication boundary" Is also a term used in UML (but not in C++)
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Why Use Inheritance?
Why inherit? Create a class that...
1. 2. 3. 4. Makes sense in problem domain Locates common implementation in superclass Defines a shared API (methods) so we can... Use polymorphism Define a reference or parameter in terms of the superclass
Two Types of Inheritance
How can inheritance support reuse? Implementation Inheritance
A subclass reuses some implementation from an ancestor In Java, keyword extends
If just last two, then use Java interface
No shared implementation You commit that part of what defines a class is that it meets a particular API We can write methods etc. that operate on objects of any class that meets or supports that interface
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Interface Inheritance
A "subclass" shares the interface with an "ancestor" In Java, keyword implements I.e. this class will support this set of methods
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Interfaces and Abstract Classes
Abstract classes:
Cannot create any instances
Interfaces in Other Languages
A modeling method in UML Interfaces in C++
All methods are pure virtual No data members Use multiple inheritance
Prefer Java interfaces over abstract classes!
Existing classes can add an interface Better support for mix-in classes E.g. Comparable interface -- supports compare() Do not need a hierarchical framework Composition preferred over inheritance E.g. wrapper classes
But, abstract classes have some implementation
Skeletal implementation classes, e.g. AbstractCollection
Disadvantage: released, once a public interface shouldn't be updated
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Collections in Java
ADT: more than one implementation meets same interface, models same data In Java, separate interface from implementation We'll illustrate with "fake" Java example:
Queue interface Two implementations
Defining an Interface
Java code: interface Queue { void add (Object obj); Object remove(); int size(); } Nothing about implementation here!
methods and no fields
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Using Objects by Interface
Say we had two implementations: Queue q1 = new CircularArrayQueue(100); or Queue q1 = new LinkedListQueue(); q1.add( new Widget() ); Queue q3 = new ... Queue q2 = mergeQueue(q2, q3);
Implementing an Interface
Example: class CircularArrayQueue implements Queue { CircularArrayQueue(int capacity) {...} public void add(Object o) {...} public Object remove() {...} public int size() {...} private Object[] elements; private int head; private int tail; }
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Implementing an Interface (2)
Implementation for LinkedListQueue similar Question: How to handle errors?
Array version is bounded. add() when full? Throw an exception, perhaps Not an issue for linked list version, though
Real Collection Interfaces in Java
All collections meet Collection interface: boolean add(Object obj); Iterator iterator(); int size(); boolean isEmpty(); boolean contains(Object obj); boolean containsAll (Collection other); ... See Java API documentation for all methods
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Iterator Interface
Three fundamental methods: Object next(); boolean hasNext(); void remove(); We use an iterator object returned by Collection.iterator() to visit or process items in the collection
Don't really know or care how its implemented
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Example Iterator Code
Traverse a collection of Widgets and get each object Iterator iter = c.iterator(); while ( iter.hasNext() ) { Object obj = iter.next(); // or: Widget w = (Widget) iter.next(); // do something with obj or w } Note the cast!
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New in Java 1.5 generics and foreach
Java 1.5 has generics, and... A foreach statement simplifies the previous idiom Collection<Double> c = new HashSet<Double>; for (Double d : c ) System.out.println("c has " + d );
Methods Defined by Other IF Methods
Some collection methods can be defined "abstractly" public boolean addAll (Collection from) { Iterator iterFrom = from.iterator(); boolean modified = false; while ( iterFrom.hasNext() ) if ( add(iterFrom.next()) ) modified = true; return modified; }
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Collections and Abstract Classes
To define a new Collection, one must implement all methods -- a pain! Better: define a skeletal implementation class
Leaves primitives undefined: add(), iterator() Defines other methods in terms of those
Java's Concrete Collection Classes
Vector is like array but grows dynamically
Insertion or deletion in the middle expensive
LinkedList class
Doubly-linked Ordered collection add() inserts at end of list How do we add in middle?
Concrete collection class inherits from skeletal class
Defines "primitives" Overrides any methods it chooses too
Java library: AbstractCollection
Implements...