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CSC 1051 – Data Structures and Algorithms I

CSC 1051 – Data Structures and Algorithms I. Designing Classes. Dr. Mary-Angela Papalaskari Department of Computing Sciences Villanova University Course website: www.csc.villanova.edu/~map/1051/. Where do objects come from?. Where do objects come from?. Good question!

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CSC 1051 – Data Structures and Algorithms I

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  1. CSC 1051 – Data Structures and Algorithms I Designing Classes Dr. Mary-Angela Papalaskari Department of Computing Sciences Villanova University Course website: www.csc.villanova.edu/~map/1051/ CSC 1051 M.A. Papalaskari, Villanova University

  2. Where do objects come from? CSC 1051 M.A. Papalaskari, Villanova University

  3. Where do objects come from? Good question! Defining our own classes will make it possible to design our own objects We need to learn: • How do we define our own classes? • How do we define what happens when an object is instantiated? • How do we define methods that can be invoked through our objects? CSC 1051 M.A. Papalaskari, Villanova University

  4. long acctNumber;double balance;String name; Constructor deposit() withdraw() 1. How do we define our own classes?Example: Defining the Account Class • A class to represent a generic bank account Data declarations Method declarations toString() CSC 1051 M.A. Papalaskari, Villanova University

  5. 2. How do we define what happens when an object is instantiated? Creating Objects – old example • We have already seen something like this: scan = new Scanner (System.in); This calls the Scanner constructor, which is a special method that sets up the object CSC 1051 M.A. Papalaskari, Villanova University

  6. 2. How do we define what happens when an object is instantiated? acct1 72354 acctNumber 102.56 balance "Ted Murphy" name Creating Objects – User-defined Account class Account acct1 = new Account ("Ted Murphy", 72354, 102.56); This calls the Account constructor, which is a special method that sets up the object CSC 1051 M.A. Papalaskari, Villanova University

  7. 3. How do we define what happens when an object’s method is invoked? Invoking Methods • We have seen that once an object has been instantiated, we can use the dot operatorto invoke its methods ans = scan.nextLine(); numChars = title.length(); CSC 1051 M.A. Papalaskari, Villanova University

  8. 3. How do we define what happens when an object’s method is invoked? acct1 72354 acctNumber 102.56 balance "Ted Murphy" name Method invocation Me Account class acct1.deposit (25.85); Method definition //---------------------------------------------------// Deposits the specified amount into the account. //---------------------------------------------------publicvoid deposit (double amount){ balance = balance + amount;} CSC 1051 M.A. Papalaskari, Villanova University

  9. Chapter 4: Writing Classes • We've been using predefined classes from the Java API. Now we will learn to write our own classes. • class definitions • instance data • encapsulation and Java modifiers • method declaration and parameter passing • constructors • graphical objects • events and listeners • buttons and text fields CSC 1051 M.A. Papalaskari, Villanova University

  10. acct1 72354 acctNumber 102.56 balance "Ted Murphy" name acct2 69713 acctNumber 40.00 balance "Jane Smith" name Bank Account: Example of user-defined class CSC 1051 M.A. Papalaskari, Villanova University

  11. //********************************************************************// Account.java Author: Lewis/Loftus// Simplified version by MA Papalaskari//// Represents a bank account with basic services such as deposit.//********************************************************************publicclass Account{finaldouble RATE = 0.035; // interest rate of 3.5%long acctNumber;double balance; String name;//-----------------------------------------------------------------// Sets up the account by defining its owner, account number,// and initial balance.//-----------------------------------------------------------------public Account (String owner, long account, double initial) { name = owner; acctNumber = account; balance = initial; } continue long acctNumber;double balance;String name; Constructor deposit() withdraw() CSC 1051 M.A. Papalaskari, Villanova University

  12. long acctNumber;double balance;String name; Constructor deposit() withdraw() continue //-----------------------------------------------------------------// Deposits the specified amount into the account. //-----------------------------------------------------------------publicvoid deposit (double amount) { balance = balance + amount; }//-----------------------------------------------------------------// Withdraws the specified amount from the account and applies// the fee.//-----------------------------------------------------------------publicvoid withdraw (double amount, double fee) { balance = balance - amount - fee; } continue CSC 1051 M.A. Papalaskari, Villanova University

  13. continue//-----------------------------------------------------------------// Adds interest to the account and returns the new balance.//-----------------------------------------------------------------publicvoid addInterest () { balance += (balance * RATE); }//-----------------------------------------------------------------// Returns the current balance of the account.//-----------------------------------------------------------------publicdouble getBalance () {return balance; }//-----------------------------------------------------------------// Returns a one-line description of the account as a string.//-----------------------------------------------------------------public String toString () {return (acctNumber + "\t" + name + "\t" + balance); }} long acctNumber;double balance;String name; Constructor deposit() withdraw() CSC 1051 M.A. Papalaskari, Villanova University

  14. //********************************************************************// Transactions.java Author: MA Papalaskari // (based on Lewis/Loftus example)// Demonstrates the creation and use of multiple Account objects.//********************************************************************publicclass Transactions1{//-----------------------------------------------------------------// Creates some bank accounts and requests various services.//-----------------------------------------------------------------publicstaticvoid main (String[] args) { Account acct1 = new Account ("Ted Murphy", 72354, 102.56); Account acct2 = new Account ("Jane Smith", 69713, 40.00); Account acct3 = new Account ("Edward Demsey", 93757, 759.32); System.out.println (acct1); System.out.println (acct2); System.out.println (acct3); acct1.deposit (25.85); acct1.withdraw (60,2); System.out.println (); System.out.println (acct1); System.out.println (acct2); System.out.println (acct3); }} CSC 1051 M.A. Papalaskari, Villanova University

  15. Transactions class: Creating Account objects Account acct1 = new Account ("Ted Murphy", 72354, 102.56); CSC 1051 M.A. Papalaskari, Villanova University

  16. Transactions class: Creating Account objects Transactions class Account acct1 = newAccount ("Ted Murphy", 72354, 102.56); CSC 1051 M.A. Papalaskari, Villanova University

  17. longacctNumber;double balance;String name; Constructor deposit() withdraw() Transactions class: Creating Account objects Transactions class Account acct1 = newAccount ("Ted Murphy", 72354, 102.56); Account class publicAccount (String owner, long account, double initial) { name = owner; acctNumber = account; balance = initial; } Constructor method CSC 1051 M.A. Papalaskari, Villanova University

  18. acct1 72354 acctNumber 102.56 balance longacctNumber;double balance;String name; name "Ted Murphy" Constructor deposit() withdraw() Transactions class: Creating Account objects Transactions class Account acct1 = newAccount ("Ted Murphy", 72354, 102.56); Account class publicAccount (String owner, long account, double initial) { name = owner; acctNumber = account; balance = initial; } Constructor method CSC 1051 M.A. Papalaskari, Villanova University

  19. acct1 72354 acctNumber 102.56 balance "Ted Murphy" name acct2 69713 acctNumber 40.00 balance "Jane Smith" name Transactions class: Creating more Account objects Account acct1 = new Account ("Ted Murphy", 72354, 102.56); Account acct2 = new Account ("Jane Smith", 69713, 40.00); CSC 1051 M.A. Papalaskari, Villanova University

  20. acct1 72354 acctNumber 102.56 balance "Ted Murphy" name Account class: Using methods acct1.deposit (25.85); CSC 1051 M.A. Papalaskari, Villanova University

  21. acct1 72354 acctNumber 102.56 balance "Ted Murphy" name Account class: Using methods acct1.deposit (25.85); //---------------------------------------------------// Deposits the specified amount into the account. //---------------------------------------------------publicvoid deposit (double amount){ balance = balance + amount;} CSC 1051 M.A. Papalaskari, Villanova University

  22. acct1 72354 acctNumber 102.56 balance "Ted Murphy" name Account class: Using methods acct1.deposit (25.85); //---------------------------------------------------// Deposits the specified amount into the account. //---------------------------------------------------publicvoid deposit (double amount){ balance = balance + amount;} CSC 1051 M.A. Papalaskari, Villanova University

  23. acct1 72354 acctNumber 102.56 balance "Ted Murphy" name Account class: Using methods acct1.deposit (25.85); //---------------------------------------------------// Deposits the specified amount into the account. //---------------------------------------------------publicvoid deposit (double amount){ balance = balance + amount;} CSC 1051 M.A. Papalaskari, Villanova University

  24. acct1 72354 acctNumber 102.56 balance "Ted Murphy" name Account class: Using methods acct1.deposit (25.85); //---------------------------------------------------// Deposits the specified amount into the account. //---------------------------------------------------publicvoid deposit (double amount){ balance = balance + amount;} CSC 1051 M.A. Papalaskari, Villanova University

  25. acct1 72354 acctNumber 128.41 balance "Ted Murphy" name Account class: Another Example acct1.withdraw (60,2); CSC 1051 M.A. Papalaskari, Villanova University

  26. acct1 72354 acctNumber 128.41 balance "Ted Murphy" name Account class: Another Example acct1.withdraw (60,2); //-------------------------------------------------------// Withdraws the specified amount from the account// and applies the fee.//-----------------------------------------------------publicvoid withdraw (double amount, double fee) { balance = balance - amount - fee; } CSC 1051 M.A. Papalaskari, Villanova University

  27. acct1 72354 acctNumber 66.41 balance "Ted Murphy" name Account class: Another Example acct1.withdraw (60,2); //-------------------------------------------------// Withdraws the specified amount from the account// and applies the fee.//-----------------------------------------------publicvoid withdraw (double amount, double fee){ balance = balance - amount - fee;} CSC 1051 M.A. Papalaskari, Villanova University

  28. long acctNumber;double balance;String name; Constructor deposit() withdraw() Class definitions • A class can contain data declarations and method declarations Data declarations Method declarations toString() CSC 1051 M.A. Papalaskari, Villanova University

  29. Bank Account Example • There are some improvements that can be made to the Account class • The design of some methods could also be more robust, such as verifying that the amount parameter to the withdraw method is positive • Some of these improvements are in the book examples • Account.java, Transactions.java (simplified versions) • Account.java ,Transactions.java (book versions) CSC 1051 M.A. Papalaskari, Villanova University

  30. //********************************************************************// Transactions.java Author: MA Papalaskari // (based on Lewis/Loftus example)// Demonstrates the creation and use of multiple Account objects.//********************************************************************publicclass Transactions1{//-----------------------------------------------------------------// Creates some bank accounts and requests various services.//-----------------------------------------------------------------publicstaticvoid main (String[] args) { Account acct1 = new Account ("Ted Murphy", 72354, 102.56); Account acct2 = new Account ("Jane Smith", 69713, 40.00); Account acct3 = new Account ("Edward Demsey", 93757, 759.32); System.out.println (acct1); System.out.println (acct2); System.out.println (acct3); acct1.deposit (25.85); acct1.withdraw (60,2); System.out.println (); System.out.println (acct1); System.out.println (acct2); System.out.println (acct3); }} CSC 1051 M.A. Papalaskari, Villanova University

  31. //********************************************************************// Transactions.java Author: MA Papalaskari // (based on Lewis/Loftus example)// Demonstrates the creation and use of multiple Account objects.//********************************************************************publicclass Transactions1{//-----------------------------------------------------------------// Creates some bank accounts and requests various services.//-----------------------------------------------------------------publicstaticvoid main (String[] args) { Account acct1 = new Account ("Ted Murphy", 72354, 102.56); Account acct2 = new Account ("Jane Smith", 69713, 40.00); Account acct3 = new Account ("Edward Demsey", 93757, 759.32); System.out.println (acct1); System.out.println (acct2); System.out.println (acct3); acct1.deposit (25.85); acct1.withdraw (60,2); System.out.println (); System.out.println (acct1); System.out.println (acct2); System.out.println (acct3); }} Sample Run 72354 Ted Murphy $102.5669713 Jane Smith $40.0093757 Edward Demsey $759.3272354 Ted Murphy $66.4169713 Jane Smith $40.0093757 Edward Demsey $759.32 CSC 1051 M.A. Papalaskari, Villanova University

  32. acct1 72354 acctNumber 102.56 balance "Ted Murphy" name toString() method System.out.prinltn(acct1); public String toString () { NumberFormat fmt = NumberFormat.getCurrencyInstance();return (acctNumber +"\t”+ name +"\t”+ fmt.format(balance));} CSC 1051 M.A. Papalaskari, Villanova University

  33. static methods OtherClass.doSomething(acct1); public static void doSomething() { System.out.println(" At your service. " ); } OtherClass CSC 1051 M.A. Papalaskari, Villanova University

  34. Examples of Classes CSC 1051 M.A. Papalaskari, Villanova University

  35. Another example: The Die Class • See RollingDice.java • See Die.java CSC 1051 M.A. Papalaskari, Villanova University

  36. RollingDice Die faceValue : int main (args : String[]) : void roll() : int setFaceValue (int value) : void getFaceValue() : int toString() : String UML Class Diagrams • A UML class diagram for the RollingDice program:

  37. //********************************************************************//******************************************************************** // RollingDice.java Author: Lewis/Loftus // // Demonstrates the creation and use of a user-defined class. //******************************************************************** public class RollingDice { //----------------------------------------------------------------- // Creates two Die objects and rolls them several times. //----------------------------------------------------------------- public static void main (String[] args) { Die die1, die2; int sum; die1 = new Die(); die2 = new Die(); die1.roll(); die2.roll(); System.out.println ("Die One: " + die1 + ", Die Two: " + die2); continue CSC 1051 M.A. Papalaskari, Villanova University

  38. continue die1.roll(); die2.setFaceValue(4); System.out.println ("Die One: " + die1 + ", Die Two: " + die2); sum = die1.getFaceValue() + die2.getFaceValue(); System.out.println ("Sum: " + sum); sum = die1.roll() + die2.roll(); System.out.println ("Die One: " + die1 + ", Die Two: " + die2); System.out.println ("New sum: " + sum); } } CSC 1051 M.A. Papalaskari, Villanova University

  39. Sample Run Die One: 5, Die Two: 2 Die One: 1, Die Two: 4 Sum: 5 Die One: 4, Die Two: 2 New sum: 6 continue die1.roll(); die2.setFaceValue(4); System.out.println ("Die One: " + die1 + ", Die Two: " + die2); sum = die1.getFaceValue() + die2.getFaceValue(); System.out.println ("Sum: " + sum); sum = die1.roll() + die2.roll(); System.out.println ("Die One: " + die1 + ", Die Two: " + die2); System.out.println ("New sum: " + sum); } } CSC 1051 M.A. Papalaskari, Villanova University

  40. //********************************************************************//******************************************************************** // Die.java Author: Lewis/Loftus // // Represents one die (singular of dice) with faces showing values // between 1 and 6. //******************************************************************** public class Die { private final int MAX = 6; // maximum face value private int faceValue; // current value showing on the die //----------------------------------------------------------------- // Constructor: Sets the initial face value. //----------------------------------------------------------------- public Die() { faceValue = 1; } CSC 1051 M.A. Papalaskari, Villanova University

  41. continue //----------------------------------------------------------------- // Rolls the die and returns the result. //----------------------------------------------------------------- public int roll() { faceValue = (int)(Math.random() * MAX) + 1; return faceValue; } //----------------------------------------------------------------- // Face value mutator. //----------------------------------------------------------------- public void setFaceValue (int value) { faceValue = value; } //----------------------------------------------------------------- // Face value accessor. //----------------------------------------------------------------- public int getFaceValue() { return faceValue; } continue CSC 1051 M.A. Papalaskari, Villanova University

  42. continue //----------------------------------------------------------------- // Returns a string representation of this die. //----------------------------------------------------------------- public String toString() { String result = Integer.toString(faceValue); return result; } } CSC 1051 M.A. Papalaskari, Villanova University

  43. Homework • Chapter 4, Sections 4.1, 4.2 • Designing classes exercises Some slides in this presentation are adapted from the slides accompanying Java Software Solutions by Lewis & Loftus CSC 1051 M.A. Papalaskari, Villanova University

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