Basic Concepts of Object-Oriented Programming

1. Basic Concepts of OO • An object has • state: defined by the set of fields or attributes. • behavior: defined by the set of methods (operations that can be applied to the object). • identity: determined at creation time. • Class • A template for creating objects. • Objects of the same class exhibit the same behavior. They may have different states.

2. Objects and Classes In real world terms: • An object represents an individual entity or thing. • Objects can be lumped together if they exhibit some common characteristics or behavior. • Examples of classes in real world: • Students • Graduate students • Undergraduate students • MS students • Ph.D. students

3. Objects and Classes In programming terms: • A class is a software component that usually represents a real world class. • The design of classes should utilize: • modularity • encapsulation • A class defines certain common characteristics: • Data variables • behaviors • Each object that is created is an instance of a class. • Each object has its own state, i.e., the values of the fields.

4. Relationship among Classes:Inheritance • A mechanism to organize classes by commonalities. • subclasses, specialization • superclass, generalization • Is-a relation • Example: • A graduate student is a student. • A Master student is a graduate student. • A Ph.D. student is a graduate student. • An undergraduate student is a student.

5. Class Diagram: Inheritance

6. Relationship among Classes:Composition • Has-a relation • Example: a student has a • address (type: Address) • faculty advisor (type: Faculty) • etc.

7. Class Diagram: Composition

8. Class Declaration • A class contains data (variable, field) declarations and method declarations • Variables declared at the class level can be used by all methods in that class • Variables declared within a method can only be used in that method • A method declaration specifies the code that will be executed when the method is invoked (or called)

9. Class Declaration Syntax [ ClassModifiers ] class ClassName  [ extends SuperClass ]  [ implements Interface1, Interface2  ...] {  ClassMemberDeclarations}

10. Class Visibility • publicAccessible everywhere. One public class allowed per file. The file must be named ClassName.java • default Accessible within the current package. • Other class modifiers: • abstractA class that contains abstract methods • finalNo subclasses

11. Method and Field Declaration [ MethodModifiers ] ResultType  MethodName ( [ ParameterList ]  ) {  Statements} [ FieldModifiers ] Type  FieldName1 [ = Initializer1 ] ,   FieldName2 [ = Initializer2 ] ...;

12. Encapsulation • external view (client view): for the users of the class • internal view(implementer view): for the developers of the class, • visibility: • public, • protected, • package (default), • private

13. public protected package private The class itself Yes Yes Yes Yes Classes in the  same package Yes Yes Yes No Subclasses in a  different package  Yes Yes No No Non-subclasses in  a different package Yes No No No Visibility

14. Example: Coin.java public class Coin { public final int HEADS = 0; public final int TAILS = 1; private int face; public Coin (){ flip(); } public void flip (){ face = (int) (Math.random() * 2); }

15. Example: Coin.java public int getFace () { return face; } public String toString() { String faceName; if (face == HEADS) faceName = "Heads"; else faceName = "Tails"; return faceName; } }

16. Example: CountFlips.java public class CountFlips { public static void main (String[] args) { final int NUM_FLIPS = 1000; int heads = 0, tails = 0; Coin myCoin = new Coin(); for (int count=1; count <= NUM_FLIPS; count++) { myCoin.flip(); if (myCoin.getFace() == myCoin.HEADS) heads++; else tails++; } System.out.println(“Number flips: " + NUM_FLIPS); System.out.println("number of heads: " + heads); System.out.println("number of tails: " + tails); } }

17. Example: FlipRace:java public class FlipRace { public static void main (String[] args) { final int GOAL = 3; int count1 = 0, count2 = 0; // Create two separate coin objects Coin coin1 = new Coin(); Coin coin2 = new Coin(); while (count1 < GOAL && count2 < GOAL) { coin1.flip(); coin2.flip(); System.out.print ("Coin 1: " + coin1); System.out.println (" Coin 2: " + coin2); count1 = (coin1.getFace() == coin1.HEADS) ? count1+1 : 0; count2 = (coin2.getFace() == coin2.HEADS) ? count2+1 : 0; }

18. Example: FlipRace:java // Determine the winner if (count1 < GOAL) { System.out.println("Coin 2 Wins!"); } else if (count2 < GOAL) { System.out.println("Coin 1 Wins!"); } else { System.out.println("It's a TIE!"); } } }

19. Method Overloading • Two or more methods/constructors with the same name but different numbers or different types of parameters:void methodA(int i)void methodA(int i, int j) void methodB(int i)void methodB(float f) • Avoid overloading

20. Special Methods • public boolean equals(Object o) • o1.equals(o2)versuso1 == o2 • The equals() method tests the equality of two objects. • The == operator tests the identity of two objects. • When comparing two strings, use s1.equals(s2) instead of s1 == s2. • public String toString() • returns a string representation of the state of the object • public void finalize() • invoked automatically by the Java runtimen just before an object is garbage-collected • public void dispose() • invoked deliberately by the programmer when an object is no longer needed

21. Example: Account.java import java.text.NumberFormat; public class Account { private NumberFormat fmt = NumberFormat.getCurrencyInstance(); private final double RATE = 0.045; // interest rate of 4.5% private long acctNumber; private double balance; private String name; public Account(String owner, long account, double initial) { name = owner; acctNumber = account; balance = initial; }

22. Example: Account.java public double deposit (double amount) { if (amount < 0) { // deposit value is negative System.out.println(); System.out.println("Error: ..."); System.out.println(acctNumber + " " + fmt.format(amount)); } else { balance = balance + amount; } return balance; }

23. Example: Account.java public double withdraw(double amount, double fee) { amount += fee; if (amount < 0) { // withdraw value is negative System.out.println ("Error: ..."); } else if (amount > balance) { // withdraw value exceeds balance System.out.println ("Error: ..."); } else { balance = balance - amount; } return balance; }

24. Example: Account.java public double addInterest () { balance += (balance * RATE); return balance; } public double getBalance () { return balance; } public long getAccountNumber () { return acctNumber; } public String toString () { return (acctNumber + "\t" + name + "\t" + fmt.format(balance)); }

25. Example: BankAccount.java public class BankAccounts { public static void 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); acct1.deposit (25.85); double smithBalance = acct2.deposit (500.00); System.out.println( "Smith balance after deposit: " + smithBalance); System.out.println( "Smith balance after withdrawal: " + acct2.withdraw (430.75, 1.50));

26. Example: BankAccount.java acct3.withdraw (800.00, 0.0); // exceeds balance acct1.addInterest(); acct2.addInterest(); acct3.addInterest(); System.out.println(); System.out.println(acct1); System.out.println(acct2); System.out.println(acct3); } }

27. StringTokenizer Class • separates a string into words (tokens) • white space as delimiters • Constructors: StringTokenizer(String str) StringTokenizer(String str, String delimiter) • Methods: boolean hasMoreTokens() String nextToken()

28. Example: PigLatinTranslator.java import java.util.StringTokenizer; public class PigLatinTranslator { public String translate(String sentence) { String result = ""; sentence = sentence.toLowerCase(); StringTokenizer tokenizer = new StringTokenizer (sentence); while (tokenizer.hasMoreTokens()) { result += translateWord(tokenizer.nextToken()); result += " "; } return result; }

29. Example: PigLatinTranslator.java private String translateWord (String word) { String result = ""; if (beginsWithVowel(word)) { result = word + "yay"; } else if (beginsWithPrefix(word)) { result = word.substring(2) + word.substring(0,2) + "ay"; } else { result = word.substring(1) + word.charAt(0) + "ay"; } return result; }

30. Example: PigLatinTranslator.java private boolean beginsWithVowel (String word) { String vowels = "aeiouAEIOU"; char letter = word.charAt(0); return (vowels.indexOf(letter) != -1); }

31. Example: PigLatinTranslator.java private boolean beginsWithPrefix (String str) { return (str.startsWith ("bl") || str.startsWith ("pl") || str.startsWith ("br") || str.startsWith ("pr") || str.startsWith ("ch") || str.startsWith ("sh") || str.startsWith ("cl") || str.startsWith ("sl") || str.startsWith ("cr") || str.startsWith ("sp") || str.startsWith ("dr") || str.startsWith ("sr") || str.startsWith ("fl") || str.startsWith ("st") || str.startsWith ("fr") || str.startsWith ("th") || str.startsWith ("gl") || str.startsWith ("tr") || str.startsWith ("gr") || str.startsWith ("wh") || str.startsWith ("kl") || str.startsWith ("wr") || str.startsWith ("ph") ); }

32. Example: PigLatin.java public class PigLatin { public static void main (String[] args) { String sentence, result, another; PigLatinTranslator translator = new PigLatinTranslator(); do { System.out.println(); System.out.println("Enter a sentence (no punctuation):"); sentence = Keyboard.readString(); System.out.println(); result = translator.translate (sentence); System.out.println("That sentence in Pig Latin is:"); System.out.println(result); System.out.println(); System.out.print("Translate another sentence (y/n)? "); another = Keyboard.readString(); } while (another.equalsIgnoreCase("y")); } }