COMP 14 Introduction to Programming Adrian Ilie July 18 2005 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL 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 2 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Algorithm 3 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 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Without objects String UNC UNC String Duke Duke String semi1 UNC Vs Duke String finalist1 if score1 score2 finalist1 UNC else finalist1 Duke 4 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Add method String UNC UNC String Duke Duke String semi1 UNC Vs Duke String finalist1 finalist1 gameWinner score1 score2 UNC Duke 5 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL With objects Team Team Game Team UNC new Team UNC Duke new Team Duke semi1 new Game UNC Duke finalist1 finalist1 semi1 gameWinner score1 score2 6 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL What s going on String name UNC Team UNC Team Duke Game semi1 String name Duke Team objects Team finalist1 Reference variables 7 Adrian Ilie Team team1 Team team2 Game object The UNIVERSITY of NORTH CAROLINA at CHAPEL 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 8 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Structure of a Team object Data String name Methods Constructors toString 9 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL 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 10 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Classes Objects are collections of data and operations methods We need to define the pattern footprint of objects classes 11 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Writing classes public class ClassName Data members private int a Constructor s public ClassName Code of the method other methods public return type method1 type param1 type param2 Code of the method 12 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Writing the main public class MainClassName Data members Static BufferedReader keyboard main method public static void main Code of the main method other methods public static return type method1 type param1 Code of the method 13 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL This is wrong public class ClassName Data members private int a No instructions directly in the body of a class They have to be inside a method Constructor s Exception initialization of a static variable e g keyboar OtherClass obj new OtherClass other methods public return type method1 type param1 Code of the method 14 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Review 0 1 2 3 Arrays Declaration can use variables and expressions as initial values int counts Instantiation counts new int 50 Initialization Access for int i 0 i counts length i counts i 0 Initializer List declaration instantiation and initialization double grades 98 7 72 4 87 5 int numbers num num 1 num 2 num 3 15 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Arrays and Assignment int counter new int 5 temp counter int temp 0 1 temp counter doesn t make a copy of the array temp counter is true since the reference variables contain the same address 16 Adrian Ilie 2 3 4 The UNIVERSITY of NORTH CAROLINA at CHAPEL Copying Arrays counter int counter 1 2 3 4 5 int temp new int counter length temp 0 1 2 3 4 17 1 2 3 2 3 2 3 4 4 5 for int i 0 i counter length i temp i counter i Adrian Ilie 1 0 1 4 5 i 1 4 2 5 0 3 The UNIVERSITY of NORTH CAROLINA at CHAPEL References and Assignment counter temp counter temp 0 1 2 3 4 18 1 2 3 4 5 0 1 2 3 4 1 2 3 4 5 Remember that arrays use reference variables just like objects Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL References and null temp null null is a reserved word that means temp null empty nothing counter temp 0 1 2 3 4 19 1 2 3 4 5 0 1 2 3 4 1 2 3 4 5 Remember that arrays use reference variables just like objects Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL 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 20 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL public class Tester public static void swap int scores int x int y 30 scores 2 20 int temp scores x scores 3 30 20 scores x scores y temp 20 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 grades 2 30 20 grades 3 20 30 swap grades 2 3 21 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Arrays of Objects Can use arrays to manipulate objects Create array of objects classname array new classname size for int Must instantiate each object in j 0 j array length j array array j new classname 22 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL Example 16 students Each student String name int age String major int year Create arrays of all the data Problem print the names according to increasing age Analyze without objects or with objects Why 23 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL 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 24 Adrian Ilie The UNIVERSITY of NORTH CAROLINA at CHAPEL