COMP 14Introduction to Programming

Miguel A. Otaduy

June 3, 2004

Review Homework 5

• Algorithm

• How to solve the problem without objects– not totally true, Strings are objects too

• How to solve the problem with objects

Algorithm• Create teams• Create semifinal games• Output initial message• Get score of semifinal 1• Determine finalist 1• Get score of semifinal 2• Determine finalist 2• Create final game• Get score of final• Determine winner

Without objects

String UNC=“UNC”;String Duke=“Duke”;String semi1=UNC + “ Vs. ” + Duke;String finalist1;…if(score1 > score2)

finalist1=UNC;else

finalist1=Duke;

Add method

String UNC=“UNC”;String Duke=“Duke”;String semi1=UNC + “ Vs. ” + Duke;String finalist1;…finalist1=gameWinner(semi1,

score1, score2, UNC, Duke);

With objects

Team UNC=new Team(“UNC”);Team Duke=new Team(“Duke”);Game semi1=new Game(UNC, Duke);Team finalist1;…

finalist1=semi1.gameWinner(score1, score2);

What’s going on?

String name=“UNC”

String name=“Duke”

Team objects

Team team1Team team2

Game object

Team UNCTeam Duke

Game semi1

Team finalist1

Reference variables

What is different?• We declare data inside objects, not directly in the

main() method– The Strings become data members of the Team objects

• We no longer need Strings for the games– If we don’t use objects, we duplicate the data.– The String for the game can be generated using the ones

stored in the Teams.• The method gameWinner is a method associated

to a particular game object– No need to pass the game as a parameter.– No need to pass the teams as parameters

• Why does the code look almost the same?– Because Strings and Teams and Games are ALL

classes/objects– Our classes are simply wrappers of Strings

Structure of a Team object• Data

– String name

• Methods– Constructors– toString()

Structure of a Game object• Data

– Pointer to team1 (Team team1)– Pointer to team2 (Team team2)

• Methods– Constructors– toString()– gameWinner()

receives as parameter the score, and returns the pointer to the winner team.

Classes

• Objects are collections of data and operations (methods)

• We need to define the pattern/footprint of objects classes!!!

Writing classespublic class ClassName{

//Data membersprivate int a;…

//Constructor(s)public ClassName{

//Code of the method}

//other methodspublic return_type method1(type param1, type param2, …){

//Code of the method}

}

Writing the main()public class MainClassName{

//Data membersStatic BufferedReader keyboard=…;

//main methodpublic static void main(…){

//Code of the main method}

//other methodspublic static return_type method1(type param1, …){

//Code of the method}

}

This is wrong!!public class ClassName{

//Data membersprivate int a;…

//Constructor(s)…

OtherClass obj=new OtherClass();

…

//other methodspublic return_type method1(type param1, …){

//Code of the method}

}

No instructions directly in the body of a class!!!They have to be inside a method

Exception:initialization of a static variable (e.g. keyboard)

ReviewArrays• Declaration

int[] counts;

• Instantiationcounts = new int[50];

• Initialization / Accessfor (int i=0; i<counts.length; i++) {counts[i] = 0;

}

• Initializer List– declaration, instantiation, and initializationdouble[] grades = {98.7, 72.4, 87.5};int[] numbers = {num, num+1, num+2, num+3};

can use variables andexpressions as initial values

0 1 2 3

Arrays and Assignment

counter

01

23

4

int[] counter = new int[5];

int[] temp;

temp

temp = counter;

doesn't make a copy of the array!

temp == counter is true sincethe reference variables containthe same address

Copying Arrayscounter

01

23

4

int[] counter = {1, 2, 3, 4, 5};

1

23

4

5

int[] temp = new int[counter.length];

01

23

4

temp

for (int i=0; i<counter.length; i++) {temp[i] = counter[i];

}

1

23

4

5

i

012345

References and Assignment

counter

01

23

4

1

23

4

5

01

23

4

temp

temp = counter;

1

23

4

5

Remember that arrays use reference variables just like objects.

References and nullcounter

01

23

4

1

23

4

5

01

23

4

temp

temp = null;

1

23

4

5

Remember that arrays use reference variables just like objects.

null is a reserved wordthat means "empty/nothing”

Arrays as Parameters

• Entire array can be passed as a parameter– method can change elements of the

array permanently– since we're passing a reference

• Elements of an array can be passed as parameters, too– normal rules apply…

public class Tester{ public static void swap (int[] scores, int x, int y) {

int temp = scores[x];scores[x] = scores[y];scores[y] = temp;

}

public static void main (String[] args) {

int[] grades = new int[4];

for (int i=0; i<grades.length; i++) {grades[i] = i*10;

}

swap (grades, 2, 3); }

}

grades[2]: 20grades[3]: 30grades[2]: 30grades[3]: 20

scores[2]: 20scores[3]: 30temp : 20

scores[2]: 30scores[3]: 30temp : 20

scores[2]: 30scores[3]: 20temp : 20

Arrays of Objects

• Can use arrays to manipulate objects• Create array of objects

• Must instantiate each object in array

classname[] array = new classname[size];

for(int j=0; j <array.length; j++) {array[j] = new classname();

}

Example• 16 students• Each student:

String nameint ageString majorint year

• Create arrays of all the data

• Problem: print the names according toincreasing age

• Analyze: without objects or with objects? Why?

To do

• Homework 6– Start with requirements 1 and 4.

• Objects/classes. Exercise:– Create classes House and Roommate.– House will have an array of Roommates.– Create methods to set number of

Roommates, etc (whatever you come up with)

– Test the classes by writing calls to the methods in the main() method

– Come to discuss problems