Week 10 - Monday

 What did we talk about last time?  Method overloading  Lab 9

 Recall that C = (5/9)(F – 32)  C is the temperature in degrees Celsius  F is the temperature in degrees Fahrenheit  Write a method that takes a double value in Fahrenheit and returns the answer in Celsius

 Write a method with the following header: public static void rightTriangle( double a, double b, double c)  This method uses the Pythagorean equation (a 2 + b 2 = c 2 ) to find the lengths of the sides of a right triangle  This method will compute the value for a, b, or c if the other two values are known  A user calls this method with the unknown value set to be zero  If no values are zero, check to make sure that the answer is correct  If exactly one value is zero, compute it, and print the answer  If two or more values are zero, print that it is impossible to determine the lengths

 Write a complete program that simulates betting on a roulette wheel  You start with $1000 in your bank  Each turn, you bet a dollar amount and either red ( r ) or black ( b )  Include a method with the following prototype:  public static boolean spin(char guess)  This method randomly picks a number between 1 and 38 (where 37 is 0 and 38 is 00) and determines whether or not your guess (r or b) was correct  Remember, there are 38 numbers on an American roulette wheel:  1, 3, 5, 7, 9, 12, 14, 16, 18, 19, 21, 23, 25, 27, 30, 32, 34, 36 are red  2, 4, 6, 8, 10, 11, 13, 15, 17, 20, 22, 24, 26, 28, 29, 31, 33, 35 are black  0 and 00 are green (neither red nor black)

 There are two classifications of types in Java  Primitive types:  int  double  boolean  char  Object types:  String  arrays  An infinite number of others…

Primitive types:  Have a fixed amount of storage  Think of them as a box designed to hold a particular kind of data  Have basic operations used to manipulate them  int, double ( +, -, *, /, % )  boolean ( ||, &&, ^, ! )

Object types  Hold arbitrarily complex kinds of data of any kind  Do not have a prespecified amount of storage  Think of them as arrows pointing to some concrete thing that holds primitive data  Use methods for interaction instead of operators  For example, String objects use length(), charAt(), etc.

 Variables that hold object types are called references  Unfortunately, I have lied to you:  References do not work the same as primitive variables  Up to this point, we have tried to ignore this difference  A primitive variable holds a value  A reference variable merely points to the location of the object

 Picture a ham…  Imagine that this ham is actually a Java object  You may want a reference of type Ham to point at this ham  Let’s call it ham1 ham1

 Now, what if we have another Ham reference called ham2  What happens if we set ham2 to have the same value as ham1 using the following code? ham1 Ham ham2 = ham1; ham2

 When you assign an object reference to another reference, you only change the thing it points to  This is different from primitive types  When you do an assignment with primitive types, you actually get a copy int x = 37; int y = x; int x = 37; int y = x; y 37 x

 Since reference variables are only pointers to real objects, an object can have more than one name  These names are called aliases  If the object is changed, it doesn’t matter which reference was used to change it

 Thus, if we tell ham2 to take a bite away, it will affect the ham pointed at by ham1  Remember, they are the same ham ham1 ham2.bite(); ham2

 We have int s x and y, both with value 37  If we change x, it only affects x  If we change y, it only affects y int x = 37; int y = x; x++; y--; int x = 37; int y = x; x++; y--; y 37 x 38 36

 If you declare a lot of references, you have not created any objects, just lots of arrows Eggplant aubergine; DumpTruck truck1; Idea thought; Eggplant aubergine; DumpTruck truck1; Idea thought; aubergine truck1thought

 When you first declare a reference variable, those arrows point to null  null is a Java keyword that just means nothingness  If you try to do something with null, thinking it is a real object, you can break your program

 To make those arrows point at a new object, you must call a constructor  A constructor is a kind of method that creates an object  Some constructors allow you to specify certain attributes of the object you are creating  The default constructor does not let you specify anything

 To call a constructor, you use the new keyword with the name of the class followed by parentheses:  Perhaps there is a Ham constructor that lets you take a double that is the number of pounds that the ham weighs: Ham ham1 = new Ham(); //default constructor Ham ham2 = new Ham( 4.2 ); //weight constructor

 The objects you are most familiar with by now are String s  They break a few rules:  It is possible to create them without explicitly using a constructor  They can be combined using the + operator  You can still create a String object using a constructor: String s = new String("Help me!");

 More on classes and objects

 Exam 2 is next Monday  Review is on Friday  Start working on Project 4