Java Coding 6

David Davenport

Computer Eng. Dept.,Bilkent UniversityAnkara - Turkey.

email: david@bilkent.edu.tr

Collections

IMPORTANT… Students…

This presentation is designed to be used in class as part of a guided discovery sequence. It is not self-explanatory! Please use it only for revision purposes after having taken the class. Simply flicking through the slides will teach you nothing. You must be actively thinking, doing and questioning to learn!

Instructors…You are free to use this presentation in your classes and to make any modifications to it that you wish. All I ask is an email saying where and when it is/was used. I would also appreciate any suggestions you may have for improving it.

thank you, David.

MusicCD Class ExampleMusicCD( title, artist, tracks)

String getTitle()String getArtist()Track getTrack(int)int getDuration()Date getReleaseDate()

String titleString ArtistDate releaseDate??? tracks

Date( String)String toString()int yearint monthint day

Track( title, length)String getTitle()int getLength()String titleint length

collection of

Easy Collections Use Java’s ArrayList class

ArrayList()boolean add(Object)void add(int, Object)int size()Object get(int)Object remove(int)Object set(int, Object)Object clone()String toString()boolean equals( Object)

ArrayList An object of the ArrayList class

provides a container which can hold any

number of other objects… NEAT!

Objects arranged in a sequence: LIST

[ milk, eggs, bread, honey ]

Easy Problem Read in a set of positive integer values and then

print out a table showing the average, each of the values and their difference from the average.

Average is 5

Value Diff--------------10 53 -26 11 -4--------------

Example output…

Umm… must remember all the values we read in

in order to print the table.

Could use ArrayList… BUTintegers are not Objects!

(use Integer wrapper class)

Not-so-easy Collections Arrays

Common data structure All elements of same type Are Objects in Java Basis of ArrayList class!

3 6 5 1101 2 3 40

grades

Each element has unique successor & predecessor

(except first & last.)

Each element identified by an index (label/subscript)

Name for entire structure

Array Syntax (1)

Arrays are Objects, so declare variable then instantiate

3 6 5 1101 2 3 40

grades

type[] variableName ;

variableName = new type[ noOfElements ];

int[] grades;grades = new int[5];

Note use of square brackets!

Array size cannot be changed after

creation!

Array Syntax (2)

Referring to an individual elementvariableName[index]

examples

grades[0] grades[ i]grades[1] grades[ i+1]names[99] names[ FIRST]

grades[0] = 10;grades[1] = grades[0] + 2;System.out.println( grades[0]);names[99] = scan.nextLine();

Where index is a literal, named constant, variable, or expression.

Processing all elements e.g. Printing contents of array grades

for ( int i = 0; i < grades.length; i++)System.out.println( grades[i] );

for each int k in grades arrayprint k

for ( int i = 0; i < ___________; i++)System.out.println( grades[i] );

System.out.println( grades[0] );System.out.println( grades[1] ); :

// alternate for syntaxfor ( int k : grades)

System.out.println( k);

Easy Problem using arrays! Printing table of differences from average

1. read set of values

2. compute average of set of values

3. print table of differences using average & set of values

Steps 2 & 3 are straightforward For step 1 need to know how many values

Fixed, e.g. 5 Ask user Use sentinel - but length of array is fixed!

Data Requirements:average – doublesetOfValues – int[]

Easy Problem with Methods! Identify method signatures from algorithm

1. read set of values

2. compute average of set of values

3. print table of differences using average & set of values

int[] readSetOfValues()

double computeAverage( int[] setOfValues)

void printTable( double average, int[] setOfValues)

Note: Object-type parameters can act as outputs too!

Arrays of objects Array contains only references to objects

Track[] tracks;tracks = new Track[5];

tracks[0] = new Track( “David”, 100);tracks[1] = new Track( “Gunes”, 200);

Still need to create actual objects

1 2 3 40

tracks

David100

Gunes200

tracks[0].getTitle()

tracks[4].getTitle()