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.

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

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.

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.

Class Diagram: Inheritance

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

Class Diagram: Composition

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)

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

Class Visibility public Accessible everywhere. One public class allowed per file. The file must be named ClassName.java default Accessible within the current package. Other class modifiers: abstract A class that contains abstract methods final No subclasses

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

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

Visibility publicprotectedpackageprivate The class itselfYes Classes in the same package Yes No Subclasses in a different package Yes No Non-subclasses in a different package YesNo

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); }

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

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); }

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; }

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!"); }

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

Special Methods public boolean equals(Object o) o1.equals(o2) versus o1 == 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

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; }

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; }

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; }

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)); }

Example: BankAccount.java public class BankAccounts { public static void main (String[] args) { Account acct1 = new Account("Ted Murphy", 72354, ); Account acct2 = new Account("Jane Smith", 69713, 40.00); Account acct3 = new Account("Edward Demsey", 93757, ); 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));

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); }

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()

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; }

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; }

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

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") ); }

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")); }