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CSC 1051 – Data Structures and Algorithms I

CSC 1051 – Data Structures and Algorithms I. Designing Classes – part 2. Dr. Mary-Angela Papalaskari Department of Computing Sciences Villanova University Course website: www.csc.villanova.edu/~map/1051/. Getting to know classes so far. Predefined classes from the Java API.

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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 – part 2 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. Getting to know classes so far • Predefined classes from the Java API. • We are also defining a few classes of our own: • Account • Die • Person • Book CSC 1051 M.A. Papalaskari, Villanova University

  3. Getting to know classes so far • Predefined classes from the Java API. • We have defined a few classes of our own: • Account • Die • Person • Book • Driver classes: • Transactions • RollingDice • PeopleBeingPeople • Bookshelf (Project 6) CSC 1051 M.A. Papalaskari, Villanova University

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

  5. Next: • Review what we learned so far • Focus on method definition • Encapsulation • UML diagrams CSC 1051 M.A. Papalaskari, Villanova University

  6. Review • driver classes //********************************************************************// 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); System.out.println (acct1); System.out.println (acct2); acct1.deposit (25.85); System.out.println (); System.out.println (acct1); System.out.println (acct2); }} CSC 1051 M.A. Papalaskari, Villanova University

  7. Review long acctNumber;double balance;String name; Constructor deposit() withdraw() • class declaration Data declarations Method declarations toString() CSC 1051 M.A. Papalaskari, Villanova University

  8. Review • using methods… CSC 1051 M.A. Papalaskari, Villanova University

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

  10. Review 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

  11. Review 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

  12. Review 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

  13. Review 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

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

  15. Review 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

  16. Review 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

  17. Review acct1 72354 acctNumber 102.56 balance "Ted Murphy" name Object creation: Constructors Account acct1 = new Account ("Ted Murphy", 72354, 102.56); public Account (String owner, long account, double initial){ acctNumber = account; balance = initial; name = owner;} CSC 1051 M.A. Papalaskari, Villanova University

  18. Review 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

  19. Review Invoking methods in same class paintSnowman(page, 200, 150); public void paintSnowman(Graphics page, int x, int y) { // snowman code goes here . . . } CSC 1051 M.A. Papalaskari, Villanova University

  20. Review static methods: 1) In another class OtherClass.doSomething(); public static void doSomething() { System.out.println(" At your service. " ); } OtherClass CSC 1051 M.A. Papalaskari, Villanova University

  21. Review static methods: 2) In same class doSomething(); public static void doSomething() { System.out.println(" At your service. " ); } CSC 1051 M.A. Papalaskari, Villanova University

  22. Review char calc (int num1, int num2, String message) { int sum = num1 + num2; char result = message.charAt (sum); return result; } • Method definition • parameters • return type • return statement ch = obj.calc (25, count, "Hello"); CSC 1051 M.A. Papalaskari, Villanova University

  23. compute myMethod myMethod(); Method Control Flow • If the called method is in the same class, only the method name is needed CSC 1051 M.A. Papalaskari, Villanova University

  24. main doIt helpMe obj.doIt(); helpMe(); Method Control Flow • The called method is often part of another class or object CSC 1051 M.A. Papalaskari, Villanova University

  25. More Method Examples: • Write a method that has oneintparameter num, and prints “Happy Birthday” num times • Write a method with two doubleparamentersa and b that computes and returns the sum of squares: a2 + b2of its two int parameters • Write a method that has oneintparameter num, and returns the String “Happy Birthday” numtimes CSC 1051 M.A. Papalaskari, Villanova University

  26. Another Example: RollingDice.java (Driver) //******************************************************************** // 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; intsum; 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

  27. Example: RollingDice.java(Driver) 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

  28. Example: RollingDice.java(Driver) 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

  29. 4 faceValue Example: Die.java //******************************************************************** // Die.java Author: Lewis/Loftus // // Represents one die (singular of dice) with faces showing values // between 1 and 6. //******************************************************************** public class Die { private final intMAX = 6; // maximum face value private intfaceValue; // current value showing on the die //----------------------------------------------------------------- // Constructor: Sets the initial face value. //----------------------------------------------------------------- public Die() { faceValue = 1; } CSC 1051 M.A. Papalaskari, Villanova University

  30. 4 faceValue Example: Die.java continue //----------------------------------------------------------------- // Rolls the die and returns the result. //----------------------------------------------------------------- public introll() { faceValue = (int)(Math.random() * MAX) + 1; return faceValue; } //----------------------------------------------------------------- // Face value mutator. //----------------------------------------------------------------- public void setFaceValue (intvalue) { faceValue = value; } //----------------------------------------------------------------- // Face value accessor. //----------------------------------------------------------------- public intgetFaceValue() { return faceValue; } continue CSC 1051 M.A. Papalaskari, Villanova University

  31. 4 faceValue Example: Die.java 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

  32. 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:

  33. die1 4 faceValue 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:

  34. Methods Client Data Encapsulation • An encapsulated object can be thought of as a black box -- its inner workings are hidden from the client • The client invokes the interface methods and they manage the instance data CSC 1051 M.A. Papalaskari, Villanova University

  35. Methods Client Data Violating Encapsulation • It is possible for a class to access the instance data of another class directly CSC 1051 M.A. Papalaskari, Villanova University

  36. deposit() withdraw() addInterest() Transactions.java name acctNumber balance Violating Encapsulation - Example • It is possible for a class to access the instance data of another class directly - but don’t do this! • See Account.java (modified) • See ImInUrClassMessingUrInstanceData.java Account.java acct1.name = “Joe”; CSC 1051 M.A. Papalaskari, Villanova University

  37. Visibility Modifiers • In Java, we enforce encapsulation through the appropriate use of visibility modifiers: • public – can be referenced from other classes • private – can be referenced only within that class: • protected – involves inheritance (discussed later) • Data declared without a visibility modifier have default visibility and can be referenced by any class in the same package • An overview of all Java modifiers is presented in Appendix E CSC 1051 M.A. Papalaskari, Villanova University

  38. public constants are ok - Example Account acct1 = new Account (”Sartre", 72354, 102.56); System.out.println (acct1); System.out.println ("Interest rate = " + acct1.RATE); CSC 1051 M.A. Papalaskari, Villanova University

  39. public constants are ok - Example Account acct1 = new Account (”Sartre", 72354, 102.56); System.out.println (acct1); System.out.println ("Interest rate = " + acct1.RATE); • Normally, constants are declared as static. • If RATE had been declared as follows: final static double RATE = ; • Then the last statement in this program would have been: System.out.println ("Interest rate = " + Account.RATE); CSC 1051 M.A. Papalaskari, Villanova University

  40. public private Variables Methods Visibility Modifiers – the RULES NO (but OK for public constants) Yes Yes, for support methods only Yes CSC 1051 M.A. Papalaskari, Villanova University

  41. deposit() withdraw() addInterest() Transactions.java name acctNumber balance Encapsulation – Accessing the data • Indirect access through methods • accessors and mutators (“getters” and “setters”) • Usually named getX() and setX() Account.java Methods Example acct1.getBalance() Data CSC 1051 M.A. Papalaskari, Villanova University

  42. Encapsulation – Mutators (setters) can restrict access to the data, as appropriate • Example: Say a class has an instance variable: private int quantity; • The mutator may also work to ensure that quantity, does not become negative: public voidsetQuantity(intnum){if (num<0) {System.out.println("*Error in setQuantity()");System.out.println("negative quantity.");System.out.println("quantity not changed."); }else quantity = num;} CSC 1051 M.A. Papalaskari, Villanova University

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