CS 121Week 10 - Monday

Last time

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

Questions?

Project 4

More Method Examples

Conversion

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

Right triangle

Write a method with the following header:public static void rightTriangle( double a, double b, double c)

This method uses the Pythagorean equation (a2 + b2 = c2) 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

Roulette

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)

Types in Java

Java types

There are two classifications of types in Java

Primitive types: int double boolean char

Object types: String arrays An infinite number of others…

Characteristics of primitive types

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 (||, &&, ^, !)

Characteristics of object types

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.

References

Reference types are different 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

How does this affect you?

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

How many hams?

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?

ham1Ham ham2 = ham1;

ham2

There is only one ham!

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;

y

37

x

37

Reference vs. primitive variables

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

Ham solo

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

ham1ham2.bite();

ham2

Remember that primitives make copies

We have ints 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--;

y

37

x

3738 36

Creating New Objects

A reference is just an arrow

If you declare a lot of references, you have not created any objects, just lots of arrowsEggplant aubergine;

DumpTruck truck1;Idea thought;

aubergine truck1 thought

Where do those arrows point?

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

Constructors

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

Invoking the constructor

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

Strings are funny

The objects you are most familiar with by now are Strings

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!");

Upcoming

Next time…

More on classes and objects

Reminders

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