COMP 110: Introduction to Programming

1. COMP 110:Introduction to Programming Tyler Johnson Mar 23, 2009 MWF 11:00AM-12:15PM Sitterson 014

2. Questions?

3. Today in COMP 110 • Review • Overloading • Programming Demo

4. Review • From last time

5. The Math Class Provides many standard mathematical methods All methods are static, no need for an object of the Math class Call methods of the Math class using class name Math.abs Math.max Math.min Math.pow Math.round Others Predefined constants Math.PI Math.E

6. Wrapper Classes • Each primitive type has an associated “Wrapper” class • Byte • Short • Integer • Long • Float • Double • Character • Boolean

7. Writing Methods • Solving a problem using decomposition • Divide into subproblems (pseudocode) • Solve each subproblem separately as a method • Use the methods you’ve created to solve the problem

8. Review • More broadly

9. Calling Methods within Methods • It’s possible to call methods within other methods • If calling a method of the same class, no need to specify receiving object public class Example { public void method1() { method2(); //no object needed, current object assumed } public void method2() { //do something } }

10. Calling Methods within Methods public class Example { public void method1() { System.out.println("method1!"); method2(); //no object needed, current object assumed } public void method2() { System.out.println("method2!"); } public static void main(String[] args) { Example example = new Example(); //create object of class Example example.method1(); } }

11. Input to Methods • The input to a method is in the form of arguments public class Account { private double balance; private double limit; public void addPurchase(double amount) { if(balance + amount <= limit) balance = balance + amount; //only add if the transaction //is valid } } The value is filled in by whomever calls the method

12. Input to Methods public class Account { private double balance; private double limit; public void addPurchase(double amount) { if(balance + amount <= limit) balance = balance + amount; //only add if the transaction is valid } } public class AccountTester { public static void main(String[] args) { Account accnt = new Account(); account.addPurchase(15.); //call addPurchase and w/ 15 for the amount account.addPurchase(20.); //call addPurchase and w/ 20 for the amount } } Separate Java Files! A Driver program (Used for testing)

13. Input to Methods • NEVER do this public class Account { private double balance; private double limit; public void addPurchase(double amount) { Scanner keyboard = new Scanner(System.in); amount = keyboard.nextDouble(); if(balance + amount <= limit) balance = balance + amount; //only add if the transaction is valid } } Overwriting the value that was passed in!

14. Methods that Return a Value public class Account { private double balance; private double limit; //a helper method to determine if a transaction is valid privateboolean transactionValid(double amount) { if(balance + amount <= limit) return true; else return false; } public void addPurchase(double amount) { boolean valid = transactionValid(amount); //is the transaction valid? if(valid) balance = balance + amount; //only add if the transaction is valid } }

15. Method Calls in If-Statements public class Account { private double balance; private double limit; //a helper method to determine if a transaction is valid privateboolean transactionValid(double amount) { if(balance + amount <= limit) return true; else return false; } public void addPurchase(double amount) { if(transactionValid(amount)) balance = balance + amount; //only add if the transaction is valid } } Call to a method inside an if-statement

16. Booleans • There’s no need to write if(systemsGo == true) System.out.println("Launch"); if(transactionValid(amount) == true) System.out.println("Accepted"); • The more concise and equivalent way is if(systemsGo) System.out.println("Launch"); if(transactionValid(amount)) System.out.println("Accepted");

17. Overloading • Section 6.4 in text

18. public class InputOne { public void readInput() { … } } public class InputTwo { public void readInput() { … } } Overloading • Methods in different classes can have the same name • public static void main(String[] args) { • InputOne iOne = new InputOne(); • InputTwo iTwo = new InputTwo(); • iOne.readInput(); //readInput method of class InputOne • iTwo.readInput(); //readInput method of class InputTwo • }

19. Overloading • Methods in the same class can also have the same name • This is called overloading • Distinguished by the number & types of the parameters

20. Overloading Example public class Average { //average two values public double getAverage(double a, double b) { return (a + b) / 2.; } //average three values public double getAverage(double a, double b, double c) { return (a + b + c) / 3.; } }

21. Overloading • You have already been using overloaded methods System.out.print(7); //print an integer System.out.print('7'); //print a character System.out.print("seven"); //print a string System.out.print(7.0); //print a double

22. Other Overloading Examples • The Math class • double Math.max(double a, double b) • int Math.max(int a, int b) • long Math.max(long a, long b) • Allows the following int m = Math.max(1,3); double d = Math.max(5.6, 5.7);

23. Overloading • Any kind of method can be overloaded • Void methods • Methods returning a value • Static methods • Non-static methods • Constructors

24. Constructor Overloading Pet myPet = new Pet(); Pet myPet = new Pet("Fang", 12, 10.); public class Pet { private String name; private int age; private double weight; public Pet() { name = “No name yet.”; age = 0; weight = 0; } public Pet(String initName, int initAge, double initWeight) { name = initName; age = initAge; weight = initWeight; } }

25. Method Signatures • A method’s signature consists of • Method name • Number of parameters • Types of parameters • Example public double getAverage(double a, double b) { … } • Signature • Name: getAverage • NumParams: 2 • Param Types: • Param1: double • Param2: double Return type is NOT considered part of the signature!

26. Method Signatures • Java does not allow you to define two methods with the same signature in the same class • Examples //these two are the same float getAverage(float a, float b) double getAverage(float a, float b) //these two are different float getAverage(float a, float b) double getAverage(double a, double b)

27. Automatic Type Conversion • Recall that automatic type conversion can sometimes occur with method calls double square(double x) { return x*x; //square the argument and return it } • We can call this method as follows square(7.0); //returns 49.0 square(7); //also returns 49.0, auto type conversion

28. Interaction with Overloading • The situation gets more complicated with overloading public class Example { double square(double x) { return x*x; } int square(int x) { return x*x; } public static void main(String[] args) { Example e = new Example(); e.square(7.0); e.square(7); } } Which method is being called?

29. Overloading/Type Conversion • Java will always use a method that is an exact match before it attempts type conversion

30. Exact Overloading Match public void example(int i, double d, char c) {…} • Are these calls to example an exact match? • example(23, 55, 'c'); • example(88, 76.0, ';'); • example(4.0, 25, '!'); No. Automatic type conversion used Yes. No need for Automatic type conversion No. Automatic type conversion not possible

31. Ambiguous Method Calls • Java will only perform type conversion if the method call is unambiguous • There is only ONE method for which automatic type conversion can be used to find a match

32. Ambiguous Method Calls public class Example { double sum(int a, double b) { return a + b; } double sum(double a, int b) { return a + b; } public static void main(String[] args) { Example e = new Example(); e.sum(7, 7); //error, this method call is ambiguous e.sum(7, 7.0); //this is ok e.sum(7.0, 7); //this is ok } }

33. In Summary • How Java determines which method you intend to call Match Based on Method Name, Num & Types of Parameters Exact Match? Use the Method Unambiguous Match using Type Conversion? Use the Method Error

34. Use of Overloading • Misuse of overloading can lead to strange bugs • Use only with good reason public Pet(double initWeight) { //constructor for weight weight = initWeight; } public Pet(int initAge) { //constructor for age age = initAge; } public static void main(String[] args) { Pet myPet = new Pet(10); //meant to set weight, set age instead }

35. Programming Demo • Room Occupancy • Create a class called Room that can be used to record the number of people in the rooms of a building

36. Room Occupancy • Attributes • numberInRoom – the number of people in a room • totalNumber – the total number of people in all rooms as a static variable • Methods • default constructor – sets number of people in room to 0 • addOneToRoom – add a person to the room • removeOneFromRoom – remove a person from the room (don’t go below 0 persons) • getNumber – returns the number of people in the room • getTotal – a static method that returns the total number of people in all rooms • validRemoval(int num) – returns whether num people can be removed from the room

37. Programming Demo • Programming

38. Wednesday • Array Basics (Section 7.1)