Principles of Programming CM10227 Lecture D.8.: Java: Groups of - - PowerPoint PPT Presentation

Grouping Objects Object Behaviour

Principles of Programming CM10227

Lecture D.8.: Java: Groups of Objects - Object Behaviour

• Dr. Marina De Vos

University of Bath Ext: 5053

Academic Year 2012-2013

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Grouping Objects Object Behaviour

Resources

Objects First with Java. David J. Barnes and Michael K¨

• lling. Third edition

How to Think Like a Computer Scientist: Java. http://www.greenteapress.com/thinkapjava/ Big Java. Gay Horstman. Thinking in Java. Bruce Eckel’s www.mindview.net/Books/TIJ4 Sun Java Tutorials Series http://java.sun.com/ docs/books/tutorial/index.html

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Grouping Objects Object Behaviour

Outline

1

Grouping Objects

2

Object Behaviour

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Main concepts to be covered

Collections Loops Iterators Arrays

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The requirement to group objects

Many applications involve collections of objects:

Personal organizers. Library catalogs. Student-record system.

The number of items to be stored varies.

Items added. Items deleted.

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A personal notebook

Notes may be stored. Individual notes can be viewed. There is no limit to the number of notes. It will tell how many notes are stored. Explore the notebook1 project.

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Class libraries

Collections of useful classes. We do not have to write everything from scratch. Java calls its libraries: Packages. In Python they were called Modules Grouping objects is a recurring requirement. The java. util package contains classes for doing this.

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Class Libraries

Learning to program is to learn how to reuse code written by other. There is no point in reinventing the wheel. Java comes with a library of useful classes, which are referred to as the Java API They are organized in packages that follow a directory structure

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Class Libraries

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Class Libraries

You can access the classes from your code, using the import statement

import java. util .Vector; import java. util .∗;

Grouping objects is a recurring requirement. The java. util package contains classes for doing this. We will use ArrayList as a first example

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Class libraries

⌥

import java . u t i l . ArrayList ; /∗∗ ∗ . . . ∗/ public class Notebook { / / Storage f o r an a r b i t r a r y number of notes . private ArrayList<String> notes ; /∗∗ ∗ Perform any i n i t i a l i z a t i o n required f o r the ∗ notebook . ∗/ public Notebook ( ) { notes = new ArrayList<String >() ; } . . . }

⌦ ⌃ ⇧

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Collections

We specify:

the type of collection: ArrayList the type of objects it will contain: <String>

We say, ArrayList of String.

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Object structures with collections

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Adding a third note

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Features of the collection

It increases its capacity as necessary. It keeps a private count (size() accessor). It keeps the objects in order. Details of how all this is done are hidden.

Does that matter? Does not knowing how prevent us from using it?

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Using the collection

⌥

public class Notebook { private ArrayList <String> notes ; . . . public void storeNote ( String note ) { notes . add ( note ) ; } public int numberOfNotes ( ) { return notes . size ( ) ; } . . . }

⌦ ⌃ ⇧

Adding a new note Returning the number

• f notes (delegation)

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Index numbering

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Retrieving an object

⌥

public void showNote ( int noteNumber ) { i f ( noteNumber < 0) { / / This i s not a v a l i d note number . } else i f ( noteNumber < numberOfNotes ( ) ) { System . out . p r i n t l n ( notes . get ( noteNumber ) ) ; } else { / / This i s not a v a l i d note number . } }

⌦ ⌃ ⇧

Index validity check Retrieve and print the note

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Removal may affect numbering

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Generic classes

Collections are known as parameterized or generic types.

ArrayList implements list functionality: add, get, size, etc.

The type parameter says what we want a list of:

ArrayList<Person> ArrayList<TicketMachine>

etc.

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Review

Collections allow an arbitrary number of objects to be stored. Class libraries usually contain tried-and-tested collection classes. Javas class libraries are called packages. We have used the ArrayList class from the java. util package.

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Review

Items may be added and removed. Each item has an index. Index values may change if items are removed (or further items added). The main ArrayList methods are add, get, remove and size.

ArrayList is a parameterized or generic type.

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Iteration

We often want to perform some actions an arbitrary number of times.

E.g., print all the notes in the notebook. How many are there?

Most programming languages include loop statements to make this possible. Java has several sorts of loop statement. We will start with its for-each loop.

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Iteration fundamentals

We often want to repeat some actions over and over. Loops provide us with a way to control how many times we repeat those actions. With collections, we often want to repeat things once for every object in a particular collection.

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For-each loop pseudo code

General form of the for-each loop

⌥

for ( ElementType element : c o l l e c t i o n ) { loop body }

⌦ ⌃ ⇧

for keyword loop header Statement(s) to be repeated Pseudo-code expression of the actions of a for-each loop For each element in collection, do the things in the loop body.

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A Java example

⌥

/∗ ∗ ∗ L i s t a l l notes in the notebook . ∗/ public void l i s t N o t e s ( ) { for ( String note : notes ) { System . out . p r i n t l n ( note ) ; } }

⌦ ⌃ ⇧

for each note in notes, print out noted

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The while loop

A for-each loop repeats the loop body for each object in a collection. Sometimes we require more variation than this. We can use a boolean condition to decide whether or not to keep going. A while loop provides this control.

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While loop pseudo code

General form of a while loop

⌥

while ( loop condition ) { loop body }

⌦ ⌃ ⇧

while keyword boolean test Statement(s) to be repeated Pseudo-code expression of the actions of a while loop while we wish to continue, do the things in the loop body.

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A Java example

⌥

/∗ ∗ ∗ L i s t a l l notes in the notebook . ∗/ public void l i s t N o t e s ( ) { int index = 0; while ( index < notes . size ( ) ) { System . out . p r i n t l n ( notes . get ( index ) ) ; index ++; } }

⌦ ⌃ ⇧

while the value of index is less than the size of the collection, print the next note, and then increment index

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Increments

i++ : the value of i is used first before it is being incremented with one

⌥

a = 5; b = a++; a == 6; b == 5;

⌦ ⌃ ⇧

++i: increments i with one, the new value of i is then used.

⌥

a = 5; b = ++a ; a == 6; b == 6;

⌦ ⌃ ⇧

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for-each versus while

for-each:

easier to write. safer: it is guaranteed to stop.

while:

we do not have to process the whole collection. does not even have to be used with a collection. take care: could be an infinite loop.

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While without a collection

⌥

/ / P r i n t a l l even numbers from 0 to 30. int index = 0; while ( index <= 30) { System . out . p r i n t l n ( index ) ; index = index + 2; }

⌦ ⌃ ⇧

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Searching a collection

⌥

int index = 0; boolean found = false ; while ( index < notes . size ( ) && ! found ) { String note = notes . get ( index ) ; i f ( note . contains ( searchString ) ) { / / We don ’ t need to keep looking . found = true ; } else { index ++; } } / / Either we found i t ,

• r we searched the whole

/ / c o l l e c t i o n .

⌦ ⌃ ⇧

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Using an Iterator object

⌥

I t e r a t o r <ElementType> i t = myCollection . i t e r a t o r ( ) ; while ( i t . hasNext ( ) ) { c a l l i t . next ( ) to get the next

• bject

do something with that

• bject

}

⌦ ⌃ ⇧

java.util.Iterator returns an Iterator object

⌥

public void l i s t N o t e s ( ) { I t e r a t o r <String> i t = notes . i t e r a t o r ( ) ; while ( i t . hasNext ( ) ) { System . out . p r i n t l n ( i t . next ( ) ) ; } }

⌦ ⌃ ⇧

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Iterators

An iterator is an object that provides functionality to iterate

• ver all elements of a collection or container class
• java. util . Iterator

two main methods: hasNext() to check if the Iterator has more elements and next() to take the next object from the Iterator. You can access an iterator for all java collections. Not all collections can be accessed using indexes.

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Index versus Iterator

Ways to iterate over a collection:

for-each loop.

Use if we want to process every element.

while loop.

Use if we might want to stop part way through. Use for repetition that doesn’t involve a collection.

Iterator object.

Use if we might want to stop part way through. Often used with collections where indexed access is not very efficient, or impossible.

Iteration is an important programming pattern.

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The auction project

The auction project provides further illustration of collections and iteration. Two further points to follow up:

The null value.

• Casting. Used to store the result of get into a variable:

⌥

String message = ( String ) notes . get (0) ;

⌦ ⌃ ⇧

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The auction project

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The Lot Class

⌥

public void bidFor ( Person bidder , long value ) { i f ( ( highestBid == null ) || ( highestBid . getValue ( ) < value ) ) { / / This bid i s the best so f a r . setHighestBid (new Bid ( bidder , value ) ) ; } else { System . out . p r i n t l n ( ” Lot number : ” + getNumber ( ) + ” ( ” + getDescription ( ) + ” ) ” + ” already has a bid

• f :

” + highestBid . getValue ( ) ) ; } }

⌦ ⌃ ⇧

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The null Keyword

highestBid == null;

The Java keyword null is used to mean no object when a variable is not currently holding a reference to a particular

• bject.

setHighestBid(new Bid(bidder, value));

two things are done here:

we create a new object Bid we pass this new object immediately to the method

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The Auction Class

⌥

public void showLots ( ) { I t e r a t o r i t = l o t s . i t e r a t o r ( ) ; while ( i t . hasNext ( ) ) { Lot l o t = ( Lot ) i t . next ( ) ; System . out . p r i n t l n ( l o t . getNumber ( ) + ” : ” + l o t . getDescription ( ) ) ; / / Include any d e t a i l s

• f a highest

bid . Bid highestBid = l o t . getHighestBid ( ) ; i f ( highestBid != null ) { System . out . p r i n t l n ( ” Bid : ” + highestBid . getValue ( ) ) ; } else { System . out . p r i n t l n ( ” (No bid ) ” ) ; } } }

⌦ ⌃ ⇧

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Type Casting I

Lot lot = (Lot) it.next();

the return value of the Iterator method next() is an object of type Object. To store this in an object of type Lot we need to explicitly convert it that type. This is called Casting This can only been done if the objects we have added to the container were originally of type Lot.

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Type Casting II

Collections in Java allow for the storage of any type of

• bjects. In order to do so it transforms everything you add

into an object of type Object. When retrieving objects from a collection we normally cast them back into their original types.

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Wrapper Classes

Container classes in Java can only contain objects. Primitive types can therefore not be added. To solve this, Java provides wrapper classes: Integer, Float, Double

Integer a = new Integer(10); int b = a.intValue() ;

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The auction project

The auction project provides further illustration of collections and iteration. One further point to follow up: the null value.

Used to indicate, ’no object’. We can test if an object variable holds the null variable.

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Review

Loop statements allow a block of statements to be repeated. The for−each loop allows iteration over a whole collection. The while loop allows the repetition to be controlled by a boolean expression. All collection classes provide special Iterator objects that provide sequential access to a whole collection.

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Fixed-size collections

Sometimes the maximum collection size can be pre-determined. Programming languages usually offer a special fixed-size collection type: an array. Java arrays can store objects or primitive-type values. arrays use a special syntax.

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The weblog-analyzer project

Web server records details of each access. Supports webmasters tasks.

Most popular pages. Busiest periods. How much data is being delivered. Broken references.

Analyze accesses by hour.

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Creating an array object

⌥

public class LogAnalyzer { private int [ ] hourCounts ; private LogfileReader reader ; public LogAnalyzer ( ) { hourCounts = new int [ 2 4 ] ; reader = new LogfileReader ( ) ; } . . . }

⌦ ⌃ ⇧

Array variable declaration Array object creation

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The hourCounts array

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Using an array

Square-bracket notation is used to access an array element: hourCounts[...] Elements are used like ordinary variables.

On the left of an assignment:

hourCounts[hour] = ...;

In an expression:

adjusted = hourCounts[hour] − 3; hourCounts[hour]++;

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The for loop

There are two variations of the for loop, for-each and for. The for loop is often used to iterate a fixed number of times. Often used with a variable that changes a fixed amount on each iteration.

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For loop pseudo-code

General form of a for loop

⌥

for ( i n i t i a l i z a t i o n ; condition ; post−body action ) { statements to be repeated }

⌦ ⌃ ⇧

Equivalent in while-loop form

⌥

i n i t i a l i z a t i o n ; while ( condition ) { statements to be repeated post−body action }

⌦ ⌃ ⇧

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A Java example

General form of a for loop

⌥

for ( int hour = 0; hour < hourCounts . length ; hour ++) { System . out . p r i n t l n ( hour + ” : ” + hourCounts [ hour ] ) ; }

⌦ ⌃ ⇧

Equivalent in while-loop form

⌥

int hour = 0; while ( hour < hourCounts . length ) { System . out . p r i n t l n ( hour + ” : ” + hourCounts [ hour ] ) ; hour ++; }

⌦ ⌃ ⇧

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for loop with bigger step

⌥

/ / P r i n t multiples

• f 3 that

are below 40. for ( int num = 3; num < 40; num = num + 3) { System . out . p r i n t l n (num) ; }

⌦ ⌃ ⇧

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Review

arrays are appropriate where a fixed-size collection is required. arrays use special syntax. For loops offer an alternative to while loops when the number of repetitions is known. For loops are used when an index variable is required.

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Coding Conventions

Iterations

Always use { , even if there is only one statement to be repeated

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Glossary

Java API packages import ArrayList Iterator Generic classes type cast

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Main concepts to be covered

Using library classes Reading documentation Writing documentation

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The Java class library

Thousands of classes Tens of thousands of methods Many useful classes that make life much easier A competent Java programmer must be able to work with the libraries.

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Working with the library

You should:

know some important classes by name; know how to find out about other classes.

Remember:

We only need to know the interface, not the implementation.

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A Technical Support System

A textual dialog system Idea based on ‘Eliza’ by Joseph Weizenbaum (MIT, 1960s) This is a system that when entering questions will try to answer them in an “intelligent” way.

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A Technical Support System

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A Technical Support System Output

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Main loop structure

⌥

/ / from the s t a r t method in SupportSystem boolean fi ni s h ed = false ; while ( ! f i n i sh e d ) { / / se n t i n e l c o n t r o l l e d loop do something i f ( e x i t condition ) { fi ni s h ed = true ; } else { do something more } }

⌦ ⌃ ⇧

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Main loop body

⌥

String input = reader . getInput ( ) ; . . . String response = responder . generateResponse ( ) ; System . out . p r i n t l n ( response ) ;

⌦ ⌃ ⇧

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The exit condition

⌥

String input = reader . getInput ( ) ; i f ( input . startsWith ( ” bye ” ) ) { fi ni s h ed = true ; }

⌦ ⌃ ⇧

Where does startsWith come from? What is it? What does it do? How can we find out?

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String Info

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Reading class documentation

Documentation of the Java libraries in HTML format; Readable in a web browser Class API: Application Programmers Interface Interface description for all library classes http://java.sun.com/j2se/1.5.0/docs/api/

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Interface vs implementation I

The documentation includes: the name of the class; a general description of the class; a list of constructors and methods return values and parameters for constructors and methods a description of the purpose of each constructor and method

⇒ the interface of the class

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Interface vs implementation II

The documentation does not include private fields (most fields are private) private methods the bodies (source code) for each method

⇒ the implementation of the class

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Interface vs implementation

The interface of a method consists of

signature

access modifier return type method name a list of parameters

comment

discussion of all signature items purpose of method

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Using library classes

Classes from the library must be imported using an import statement (except classes from java.lang). They can then be used like classes from the current project.

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Packages and import

Classes are organised in packages. Single classes may be imported:

⌥

import java . u t i l . ArrayList ;

⌦ ⌃ ⇧

Whole packages can be imported:

⌥

import java . u t i l . ∗ ;

⌦ ⌃ ⇧

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Side Note 1: Strings

Strings are immutable objects - just like in Python immutable objects cannot change content or state once they have been created

⌥

String input = reader . getInput ( ) ; input = input . trim ; i f ( input . startsWith ( ‘ ‘ bye ’ ’ ) ) { fi n i sh e d = true ; } else { Code omitted }

⌦ ⌃ ⇧

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Side Note 1: StringBuffer

A string buffer implements a mutable sequence of characters. A StringBsuffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls.

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Side note 2: String equality

⌥

i f ( input == ” bye ” ) { . . . }

⌦ ⌃ ⇧

tests identity

⌥

i f ( input . equals ( ” bye ” ) ) { . . . }

⌦ ⌃ ⇧

tests equality

⇒ Strings should (almost) always be compared with .equals

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Identity vs equality 1

Other (non-String) objects: :Person :Person “Fred“ “Jill“ person1 == person2 ?

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Identity vs equality 2

Other (non-String) objects: :Person :Person “Fred“ “Fred“ person1 == person2 ?

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Identity vs equality 3

Other (non-String) objects: :Person :Person “Fred“ “Fred“ person1 == person2 ?

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Identity vs equality (Strings)

= tests identity

⌥

String input = reader . getInput ( ) ; i f ( input == ” bye ” ) { . . . }

⌦ ⌃ ⇧

== ??

:String :String “bye“ “bye“ input

⇒ (may be) false!

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Identity vs equality (Strings)

equals tests equality (content)

⌥

String input = reader . getInput ( ) ; i f ( input . equals ( ” bye ” ) ) { . . . }

⌦ ⌃ ⇧

equals ??

:String :String “bye“ “bye“ input

⇒ true!

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Using Random

The library class Random can be used to generate random numbers

⌥

import java . u t i l .Random; . . . Random randomGenerator = new Random ( ) ; . . . int index1 = randomGenerator . n e xtIn t ( ) ; int index2 = randomGenerator . n e xtIn t (100) ;

⌦ ⌃ ⇧

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Generating random responses

⌥

public Responder ( ) { randomGenerator = new Random ( ) ; responses = new ArrayList ( ) ; fillResponses ( ) ; } public String generateResponse ( ) { int index = randomGenerator . n e xtIn t ( responses . size ( ) ) ; return ( String ) responses . get ( index ) ; } public void fillResponses ( ) . . .

⌦ ⌃ ⇧

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Maps

Maps are collections that contain pairs of values. Pairs consist of a key and a value. Lookup works by supplying a key, and retrieving a value. An example: a telephone book. Just like the Python dictionary

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Using maps

A map with Strings as keys and values :HashMap ”Charles Nguyen” ”Lisa Jones” ”William H. Smith” ”(531) 9392 4587” ”(402) 4536 4674” ”(998) 5488 0123”

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Using maps

⌥

HashMap phoneBook = new HashMap ( ) ; phoneBook . put ( ” Charles Nguyen ” , ” (531) 9392 4587 ” ) ; phoneBook . put ( ” Lisa Jones ” , ” (402) 4536 4674 ” ) ; phoneBook . put ( ” William H. Smith ” , ” (998) 5488 0123 ” ) ; String number = ( String ) phoneBook . get ( ” Lisa Jones ” ) ; System . out . p r i n t l n ( number ) ;

⌦ ⌃ ⇧

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Maps in TechSupport

⌥

public class Responder { private HashMap responseMap ; Code Omitted public String generateResponse ( String word ) { String response = ( String ) responseMap . get ( word ) ; i f ( response != null ) { return response ; } else { return pickDefaultResponse ( ) ; } }

⌦ ⌃ ⇧

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Using sets

⌥

import java . u t i l . HashSet ; import java . u t i l . I t e r a t o r ; . . . HashSet mySet = new HashSet ( ) ; mySet . add ( ” one ” ) ; mySet . add ( ” two ” ) ; mySet . add ( ” three ” ) ; I t e r a t o r i t = mySet . i t e r a t o r ( ) ; while ( i t . hasNext ( ) ) { c a l l i t . next ( ) to get the next

• bject

do something with that

• bject

}

⌦ ⌃ ⇧

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Sets and List

A Set is a collection that stores each individual element at most once. It does not maintain any specific order. A List is a collection that stores all elements it is been giving regardless of duplication. It maintains a order at all times.

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Writing class documentation

Your own classes should be documented the same way library classes are. Other people should be able to use your class without reading the implementation. Make your class a ’library class’!

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Elements of documentation

Documentation for a class should include: the class name a comment describing the overall purpose and characteristics of the class a version number the authors names documentation for each constructor and each method

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javadoc

Class comment:

⌥

/∗ ∗ ∗ The Responder class represents a response ∗ generator

• bject .

I t i s used to generate an ∗ automatic response . ∗ ∗ @author Michael K l l i n g and David J . Barnes ∗ @version 1.0 ( 1 . Feb.2002) ∗/

⌦ ⌃ ⇧

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Elements of documentation

The documentation for each constructor and method should include: the name of the method the return type the parameter names and types a description of the purpose and function of the method a description of each parameter a description of the value returned

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javadoc

Method comment:

⌥

/∗ ∗ ∗ Read a l i n e

• f

t e x t from standard input ( the t e x t ∗ terminal ) , and return i t as a set

• f words .

∗ ∗ @param prompt A prompt to p r i n t to screen . ∗ @return A set

• f

Strings , where each String i s ∗

• ne of

the words typed by the user ∗/ public HashSet getInput ( String prompt ) { . . . }

⌦ ⌃ ⇧

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Public vs private

Public attributes (fields, constructors, methods) are accessible to other classes. Fields should not be public.

Data encapsulation

Private attributes are accessible only within the same class. Only methods that are intended for other classes should be public.

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Information hiding

Data belonging to one object is hidden from other objects. Know what an object can do, not how it does it. Information hiding increases the level of independence. Independence of modules is important for large systems and maintenance.

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The Balls Project

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Class and Object Level

So far we have created classes for the sake of objects which should have each a state of their own. Sometimes, objects should be able to share common properties

For People this could be the gravitational pull For Employees of the same company this could be the companys details

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Class and Object Level

Instead of having these details stored in each object it would be better to store this at one central point:

THE CLASS

Classes themselves can also have state

class variables or static variables Exactly one copy exists of a class variable at all times, independent of the number of created instances The key word static is Javas syntax to define class variables methods at object level can access static variables

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Class and Object Level

⌥

public class BouncingBall { private static f i n a l int g r a v i t y = 3; / / e f f e c t

• f

g r a v i t y private int ballDegradation = 2; private Ellipse2D . Double c i r c l e ; private Color color ; private int diameter ; private int xPosition ; private int yPosition ; private f i n a l int groundPosition ; / / y p o si ti o n

• f ground

private Canvas canvas ; private int ySpeed = 1;

⌦ ⌃ ⇧

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Class variables

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Constants

Variables can change their value by means of assignment. Sometimes, when programming you need a mechanism that can guarantee you that the value does not change at all Constants provide you a way in doing so

similar to maths or physics: pi, i, The key word final is Javas syntax to express constants Once given a value, initialized, they can change value Trying to do so will result in a syntax error.

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Constants

⌥

private static f i n a l int g r a v i t y = 3;

⌦ ⌃ ⇧

private: access modifier, as usual static: class variable final: constant

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Static Methods I

Previous we discussed class and object level. We introduced class variables static keyword A similar thing can be done for methods providing services for the class instead an object

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Static Methods II

⌥

public class StaticTest { static count = 0; public StaticTest ( ) { count ++; } public static int giveCount ( ) { return count ; }

⌦ ⌃ ⇧

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Review

Java has an extensive class library. A good programmer must be familiar with the library. The documentation tells us what we need to know to use a class (interface). The implementation is hidden (information hiding). We document our classes so that the interface can be read

• n its own (class comment, method comments).

Classes can have variables and methods. We use the keyword static to indicate this

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Glossary

Interface javadoc class level

• bject level

static variables class variables constants final static

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