1 COS 260 DAY 9 Tony Gauvin. 2 Agenda Questions? 3 rd Mini quiz Good results -All As Threw out...

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COS 260 DAY 9

Tony Gauvin

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Agenda• Questions? • 3rd Mini quiz

– Good results ->All A’s– Threw out question on name overloading

• 4th Mini quiz next class– Chapter 4

• Assignment 2 posted – Due Oct 5 prior to class (next Monday)

• Capstone Discussion– Capstone Project Description Fall 2015.pdf

• Finish Discussion on Grouping objects • Begin Discussion on More Sophisticated Behaviors

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Overloading• Name overloading

– this.name = name;• Method overloading

– this.name() = name();• Operator overloading

– 3 + 4 – 3.0 + 4.0 – “3” + “4”

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

Grouping objects

Introduction to collections

5.0

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Review• Collections allow an arbitrary

number of objects to be stored.• Class libraries usually contain tried-

and-tested collection classes.• Java’s class libraries are called

packages.• We have used the ArrayList class

from the java.util package.


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ArrayList 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.– Can store any object type


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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. • for (ElementType element : collection) { do stuff }


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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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The auction project• The auction project provides

further illustration of collections and iteration.

• Examples of using null.• Anonymous objects.• Chaining method calls.


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The auction project

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null• Used with object types.• Used to indicate, 'no object'.• We can test if an object variable

holds the null value:

if(highestBid == null) …

• Used to indicate ‘no bid yet’.

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Anonymous objects• Objects are often created and

handed on elsewhere immediately:Lot furtherLot = new Lot(…);lots.add(furtherLot);

• We don’t really need furtherLot:lots.add(new Lot(…));

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Chaining method calls• Methods often return objects.• We often immediately call a

method on the returned object.Bid bid = lot.getHighestBid();Person bidder = bid.getBidder();

• We can use the anonymous object concept and chain method calls:lot.getHighestBid().getBidder()

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Chaining method calls

String name = lot.getHighestBid().getBidder().getName();

• Each method in the chain is called on the object returned from the previous method call in the chain.

Returns a Bid object from the Lot

Returns a Person object from the Bid

Returns a String object from the Person

Grouping objects

Arrays(fixed size collection)

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Fixed-size collections• Sometimes the maximum collection

size can be pre-determined.• A special fixed-size collection type is

available: an array.• Unlike the flexible List collections,

arrays can store object references or primitive-type values. (generic collections can only store object references)

• Arrays use a special syntax. [ ]Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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The weblog-analyzer project

• Web server records details of each access.

• Supports analysis tasks:– Most popular pages.– Busiest periods.– How much data is being delivered.– Broken references.

• Analyze accesses by hour.• u_ex150930.log


18 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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Creating an array object


public class LogAnalyzer{ private int[] hourCounts; private LogfileReader reader; public LogAnalyzer() { hourCounts = new int[24]; reader = new LogfileReader(); } ...}

Array object creation

— specifies size

Array variable declaration— does not contain size

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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.• The target of an assignment:

hourCounts[hour] = ...;• In an expression:

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


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Standard array use


private int[] hourCounts;private String[] names;

... hourCounts = new int[24];

...

hourcounts[i] = 0;hourcounts[i]++;System.out.println(hourcounts[i]);

declaration

creation

use

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Array literals

• Array literals in this form can only be used in declarations.

• Related uses require new:


private int[] numbers = { 3, 15, 4, 5 };declaration, creation and initialization

numbers = new int[] { 3, 15, 4, 5};

• The size is inferred from the data.

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Array length

• NB: length is a field rather than a method!

• It cannot be changed – ‘fixed size’.


private int[] numbers = { 3, 15, 4, 5 };

int n = numbers.length;no brackets!

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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


for(initialization; condition; post-body action) { statements to be repeated}

General form of the for loop

Equivalent in while-loop form

initialization;while(condition) { statements to be repeated post-body action}

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A Java example


for(int hour = 0; hour < hourCounts.length; hour++) { System.out.println(hour + ": " + hourCounts[hour]);}

int hour = 0;while(hour < hourCounts.length) { System.out.println(hour + ": " + hourCounts[hour]); hour++;}

for loop version

while loop version

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Practice• Given an array of numbers, print out all

the numbers in the array, using a for loop.


int[] numbers = { 4, 1, 22, 9, 14, 3, 9};

for ...

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Practice• Fill an array with the Fibonacci sequence.


int[] fib = new int[100];

fib[0] = 0;fib[1] = 1;

for ...

0 1 1 2 3 5 8 13 21 34 ...

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for loop with bigger step


// Print multiples of 3 that are below 40.

for(int num = 3; num < 40; num = num + 3) {

System.out.println(num);

}

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Review• Arrays are appropriate where a

fixed-size collection is required.• Arrays use a special syntax.• For loops are used when an index

variable is required.• For loops offer an alternative to

while loops when the number of repetitions is known.

• Used with a regular step size.Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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Chapter Terms• Collections• Array• Iterator• For-each• For• While• index

• Import statement• Library• Package• Anonymous

object• Definite Iteration• Indefinite

Iteration


More sophisticated behavior

Using library classes to implement some more advanced

functionality

5.0

34

Main concepts to be covered

• Using library classes• Reading 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.• Library classes are often inter-

related.• Arranged into packages.• https://docs.oracle.com/javase/8/docs/api/


https://docs.oracle.com/javase/8/docs/api/

https://docs.oracle.com/javase/8/docs/api/

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Working with the library• A competent Java programmer must

be able to work with the libraries.• 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, interactive dialog system

• Idea based on ‘Eliza’ by Joseph Weizenbaum (MIT, 1960s)– http://www.masswerk.at/elizabot/

• Explore tech-support-complete …


http://www.masswerk.at/elizabot/

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Main loop structureboolean finished = false;

while(!finished) {

do something

if(exit condition) { finished = true; } else { do something more }}


A common iteration pattern.

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Main loop body

String input = reader.getInput();...String response = responder.generateResponse();System.out.println(response);


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The exit conditionString input = reader.getInput();

if(input.startsWith("bye")) { finished = true;}

• Where does ‘startsWith’ come from?

• What is it? What does it do?• How can we find out?


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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


42 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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Interface vs implementation

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 methodthe interface of the class


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Interface vs implementation

The documentation does not include

• private fields (most fields are private)• private methods• the bodies (source code) of methods

the implementation of the class


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Documentation for startsWith

• startsWith– public boolean startsWith(String prefix)

• Tests if this string starts with the specified prefix.

• Parameters:– prefix - the prefix.

• Returns:– true if the …; false otherwise


http://download.oracle.com/javase/7/docs/api/java/lang/String.html

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Methods from String• contains• endsWith• indexOf• substring• toUpperCase• trim• Beware: strings are immutable!

• Cannot be changed!


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Using library classes• Classes organized into packages.• Classes from the library must be

imported using an import statement (except classes from the java.lang package).

• They can then be used like classes from the current project.


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Fixing the Support System termination input

• Trim input <get rid of white spaces>– “bye” vs. “ bye ”

• Handle change in case– “Bye” vs “bye”

• Check for equality instead of “startsWith”


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Packages and import• Single classes may be imported:import java.util.ArrayList;

• Whole packages can be imported:import java.util.*;

• Importation does not involve source code insertion.


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Using Random (need for games)

• The library class Random can be used to generate random numbers (look up in documentation)


import java.util.Random;...Random rand = new Random();...int num = rand.nextInt();int value = 1 + rand.nextInt(100);int index = rand.nextInt(list.size());

random-check.zip

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Selecting random responses for TechSupport


public Responder(){ randomGenerator = new Random(); responses = new ArrayList<String>(); fillResponses();}

public void fillResponses(){ fill responses with a selection of response strings}

public String generateResponse(){ int index = randomGenerator.nextInt(responses.size()); return responses.get(index);}

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Parameterized classes• The documentation includes

provision for a type parameter:– ArrayList<E>

• These type names reappear in the parameters and return types:– E get(int index)– boolean add(E e)


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Parameterized classes• The types in the documentation are

placeholders for the types we use in practice:– An ArrayList<TicketMachine>

actually has methods:– TicketMachine get(int index)– boolean add(TicketMachine e)– Replace “E” with the type you will

use Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

54

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 (its interface).

• Some classes are parameterized with additional types.• Parameterized classes are also known as

generic classes or generic types.


More sophisticated behavior

Using library classes to implement some more advanced functionality

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Main concepts to be covered

• Further library classes• Set• Map

• Writing documentation• javadoc


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Using setsimport java.util.HashSet;

...HashSet<String> mySet = new HashSet<String>();

mySet.add("one");mySet.add("two");mySet.add("three");

for(String element : mySet) { do something with element}


Compare with code

for an ArrayList!

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Tokenising Stringspublic HashSet<String> getInput() { System.out.print("> "); String inputLine = reader.nextLine().trim().toLowerCase();

String[] wordArray = inputLine.split(" "); HashSet<String> words = new HashSet<String>();

for(String word : wordArray) { words.add(word); } return words;}


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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.• Example: a telephone book.


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Using maps• A map with strings as keys and values


"Charles Nguyen"

:HashMap

"(531) 9392 4587""Lisa Jones" "(402) 4536 4674"

"William H. Smith" "(998) 5488 0123"

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Using mapsHashMap <String, String> phoneBook =

new HashMap<String, String>();

phoneBook.put("Charles Nguyen", "(531) 9392 4587");phoneBook.put("Lisa Jones", "(402) 4536 4674");phoneBook.put("William H. Smith", "(998) 5488 0123");

String phoneNumber = phoneBook.get("Lisa Jones");System.out.println(phoneNumber);


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List, Map and Set• Alternative ways to group objects.• Varying implementations available:

– ArrayList, LinkedList– HashSet, TreeSet

• But HashMap is unrelated to HashSet, despite similar names.

• The second word reveals organizational relatedness.


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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 potential '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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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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javadocClass comment:

/** * The Responder class represents a response * generator object. It is used to generate an * automatic response. * * @author Michael Kölling and David J. Barnes * @version 1.0 (2011.07.31) */


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javadocMethod comment:/** * Read a line of text from standard input (the text * terminal), and return it as a set of words. * * @param prompt A prompt to print to screen. * @return A set of Strings, where each String is * one of the words typed by the user */public HashSet<String> getInput(String prompt) { ...}


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Public vs private• Public elements are accessible to

objects of other classes:• Fields, constructors and methods

• Fields should not be public.• Private elements are accessible

only to objects of 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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Code completion• The BlueJ editor supports lookup

of methods.• Use Ctrl-space after a method-

call dot to bring up a list of available methods.

• Use Return to select a highlighted method.


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Code completion in BlueJ


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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 on its own (class comment, method comments).


Class and constant variables


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Class variables• A class variable is shared between

all instances of the class.• In fact, it belongs to the class and

exists independent of any instances.• Designated by the static keyword.• Public static variables are accessed

via the class name; e.g.:– Thermometer.boilingPoint


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Class variables


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Constants• A variable, once set, can have its

value fixed.• Designated by the final keyword.

– final int max = list.size();• Final fields must be set in their

declaration or the constructor.• Combing static and final is

common.Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

77

Class constants• static: class variable• final: constantprivate static final int gravity = 3;

• Public visibility is less of an issue with final fields.

• Upper-case names often used for class constants:

public static final int BOILING_POINT = 100;


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1 COS 260 DAY 9 Tony Gauvin. 2 Agenda Questions? 3 rd Mini quiz Good results -All As Threw out...

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