1. Questions?
Math Class
Wrapper Classes
Writing / Testing Methods

2. Today in COMP 110 Review Overloading Programming Demo
Math class / Wrapper classes / Decomposition
Calling methods
Overloading
Programming Demo

3. 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

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

5. 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

6. 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

7. 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();

8. 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

9. Input to Methods A Driver program (Used for testing)
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)

10. 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!

11. Methods that Return a Value
public class Account {
private double balance;
private double limit;
//a helper method to determine if a transaction is valid
private boolean 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

12. Method Calls in If-Statements
public class Account {
private double balance;
private double limit;
//a helper method to determine if a transaction is valid
private boolean 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

13. 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)
if(transactionValid(amount))

14. Overloading

15. Overloading Methods in different classes can have the same name
public class InputOne {
public void readInput() { … }
public class InputTwo {
public void readInput() { … }
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 }

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

17. 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.;

18. 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

19. 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);

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

21. 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;

22. Method Signatures A method’s signature consists of Example 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!

23. 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
double getAverage(double a, double b)

24. 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

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

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

27. 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

28. 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

29. Ambiguous Method Calls
public class Example {
double sum(int a, double b) {
return a + b; }
double sum(double a, int 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

30. 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

31. 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(65); //meant to set weight, set age instead

32. 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

33. Room Occupancy Attributes Methods
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

34. Programming Demo
Programming

35. Wednesday
Array Basics