comp 110: spring 20091 announcements lab 0 was due today (noon) lab 1 is on friday bring laptops new...

COMP 110: Spring 20091

Announcements

Lab 0 was due today (noon)Lab 1 is on Friday

Bring laptops

New students see me after class


Today in COMP 110

Objects and Classes Review

Algorithms and pseudocode

Variables & Operations

Programming DemoVending Machine Change


Object-Oriented Programming

What are objects?Entities that can represent real-world things or abstractionsE.g. cars, houses, books, System.outObjects are concrete instances of Classes• Car is a class, myCar is an Object of type Car

Objects perform or have actions performed on them• car.start()• plane.fly()• book.turnPage()


Object-Oriented Programming

What is object-oriented programming?

Programming methodology where programs are composed of a collection of objects that can act alone or interact with one another

ExampleA race car simulation program might have car, driver, racetrack, and onlooker objects• A driver might ‘drive’ a car object• An onlooker object might ‘cheer’ a driver object


Object Attributes

Classes have general attributes, objects have specific attributes

Car class attributes vs. specific object attributes

Model : “Scion”Year : 2006Owner : “John Hansen”Location : “Parking Lot”

An individual attribute is referred to as a variable or field; the set of an object’s attributes is called its state


Object Actions

Objects perform actions, or have actions performed on them. These are generally called methods

Sometimes called functions or procedures

ExampleA car object might allow the following actions• Accelerate• Brake• Start


Object Actions

Performing actions on a object can change its state

ExampleIf we sell a car, we change its ownerIf we accelerate the car, we will change its location


Classes

A Class describes a general template for creating a certain kind of objectExample

We have two car objects• One is a 2000 Toyota Camry owned by John Doe• A second is a 2001 Ford Focus owned by Samantha

Smart

While the two car objects are different, they are both types of cars and have the same type of attributesBoth cars are thus members of the Class Car


Classes and Objects

Class CarMakeModelYearOwnerLocation

Focus objectMake = FordModel = FocusYear = 2001Owner = Samantha SmartLocation = School

Camry objectMake = ToyotaModel = CamryYear = 2000Owner = John DoeLocation = Garage


Creating a car object

Class CarMakeModelYearOwner

Person john = new Person(“John Hansen”);

Car myScion= new Car();myScion.setMake(“Toyota”);myScion.setModel(“Camry”);myScion.setYear(2006);myScion.setOwner(john);

scion objectMake = ToyotaModel = Scion xBYear = 2006Owner = John Hansen


Questions on Classes or Objects?


Algorithms

What is an algorithm?A set of instructions for solving a problem


Algorithm Example

How to make a peanut butter and jelly sandwich?

Get two slices of breadGet some peanut butterGet some jellyGet a knifeSpread peanut butter on the first slice of breadSpread jelly on the second slice of breadPut the two slices of bread together


Pseudocode

Combination of code and English used to express an algorithm before writing the algorithm into code

useful in planning out programs before you actually write them


Algorithm Example 2

How to compute the total cost of all the groceries in your shopping cart?

Simple, just start with the number 0 and add the cost of each item one by one.

Let’s look at how we might describe this in pseudocode


Algorithm Example 2

Computing the total cost of groceries in pseudocode

total = 0for each item in the carttotal = total + (cost of that item)

At the end of the algorithm, “total” is the total cost of all items in the cart


Variables

Used to store data in a programThink of it is a container for a value

ExamplemyVariable = 6;

Set the value of “myVariable” to 6

Can change value throughout programmyVariable = 8;

The value of “myVariable” is now 8


Variables

Name of a variable is called its identifierChoose variable names that are meaningful!Example

a = a + b;No one will know what these variables mean

accntBalance = accntBalance + deposit;Is much better


How to use Variables

Declare a variableAssign a value to the variableChange the value of the variable


Variable Declarations

Tells the computer what type of data the variable will holdAllocates space in memory for the data

Examples:int count, score, myInt;char letter;double totalCost, ratio;


Syntax

Set of grammatical rules in a languageSimple syntax for English sentences

Subject Verb Object.E.g “I won the game.”

Syntax for variable declarations in Java:Type Variable_1, Variable_2, …;

• Examples:– int count, score, myInt;

char letter;double totalCost, ratio;


Variables and Memory

•A variable corresponds to a location in memory

variable n1

• Use this cell to store the value of n1

• Prevent this cell from being used by other variables later

main memory


Type

• What kind of value the variable can hold• Two kinds of types

– Primitive type - indecomposable values (single number or letter)• int, double, char, boolean

– Class type - objects with both data and methods• Scanner, System


Four Kinds of Primitive Types

• Integer types– byte, short, int, long– Represent whole numbers such as 0, 5, 1883443

• Floating-point types– float, double– Represent numbers with some fractional component

such as 1.01, 3932.123532, 0.0

• Character– char– Represents a single character such as ‘A’, ‘;’, ‘8’

• Boolean– boolean– Represents a single bit (on or off, true or false, 1 or 0)


Java Primitive Types

Type Name

Kind of Value

Memory Used

Range of Values

byte Integer 1 byte -128 to 127

short Integer 2 bytes -32,768 to 32,768

int Integer 4 bytes -2,147,483,648 to 2,147,483,647

long Integer 8 bytes -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807

float Floating-point 4 bytes ±3.40282347 x 10+38 to ±1.40239846 x 10-45

double Floating-point 8 bytes ±1.79769313486231570 x 10308 to ±4.94065645841246544 x

10-324

char Character 2 bytes 0 to 65,535

boolean boolean 1 bit True of False (0 to 1)


Java Primitive Types: Range

doubl e

fl oat

l ong

i nt

short

byte


Variables and Memory

• When declaring a variable, a certain amount of memory is assigned based on the declared primitive type

int age;

double length;

char letter;

main memory


Variable Declaration

• Why is it important to tell the computer the type of the data?– Different types of data take up different

amounts of space in memory– Different types of data may allow different

operations• We can add two numbers, but does it make sense

to add two letters?


Variable Declarations

• For now, declare all your variables inside main

public class ExampleProgram{ public static void main(String[] args) {

int count;double average;…

}}


How to Name Variables

•Use a combination of– Letters, digits (0-9), underscore (_)

•First character cannot be a digit

•Java is case sensitive– “myVariable” & “myvariable” are considered

different identifiers


Example Variable Names

• Legal names– inputStream, my_Grade, my_Grade2

• Illegal names– Bright*, Power-Hour, 7eleven

• Generally first word is lowercase, following words start uppercase:– myCat, numStudents, newYorkCity


Variable Names Exercise

• Name

– weezer– michael.bolton– ben_folds– n1n– kenny-G– 311– 2pac

• Legal?

– Yes– No– Yes– Yes– No– No– No


Keywords

•Reserved words with predefined meanings– if, else, return, new, …– see

http://java.sun.com/docs/books/tutorial/java/nutsandbolts/_keywords.html

•You cannot use a keyword as a variable name– int if;

• will not compile

– int ifThen;• Is ok


Assignment Statements

•Change a variable’s value•Syntax:

– variable = expression;

•Example:– hoursPerDay= 24;– hoursPerWeek= hoursPerDay*7;

•‘=’ sign is called the assignment operator


Behind the Assignment Statement

• variable = expression;– CPU calculates the value of the expression– Writes the value to the memory location of the variable

• hoursPerWeek= hoursPerDay* 7;– Get the current value of hoursPerDay from its memory

location– Calculate hoursPerDay* 7– Assign the result to the memory location of

hoursPerWeek


Assignment Statement

• A variable can occur on both sides of an assignment statement

• Example– count = count + 10;

• increases variable count by 10


Variable Initialization

• You need to give variables an initial value before you use them– Otherwise you may observe unexpected

behavior

int count; //variable declarationcount = 1; //variable assignment



• You can also initialize variables directly in the declaration

• For exampleint myVar; //this is a declarationmyVar = 0; //this is an assignment

• Is equivalent toint myVar = 0; //declaration AND assignment



• Multiple variables can be declared and initialized simultaneously

int grade0 = 95, grade1 = 89, grade2 = 71;


Arithmetic Operators

• The usual suspects– Addition (+)

• a + b

– Subtraction (-)• a - b

– Multiplication (*)• a * b

– Division (/)• a / b• NOTE: (9/2) = 4 and (9.0/2) = 4.5


Remainder Operator

• The remainder or modulo operator (%) gives the remainder when one whole number is divided by another

• Example– i = 10%4; //the value of i will be 2– j = 6%3; //the value of j will be 0– k = 17%6; //the value of k will be 5


Remainder Operator

• The remainder operator can be used to determine if a number is even or odd

• Given any integer n, n%2 will produce either 0 or 1

– If n%2 equals 0, n is even

– If n%2 equals 1, n is odd


Parentheses and Precedence

• Parentheses can be used to indicate the order in which operations should be performed

• Example– What is the value of “result”?

int a = 3, b = 1, c = 2, result = 0;

result = (a + b) * c;

result = a + (b * c);

result = 8

result = 5


Review

• You should now– Have a better understanding of objects and

classes

– Have a good idea of what an algorithm is

– Be able to write a basic Java program that performs simple arithmetic


Testing & Debugging

• It’s easy to make mistakes when programming– These mistakes are called bugs

• The process of eliminating mistakes in a program is called debugging


Programming Errors

• Syntax Error – Failure to follow the rules of the language– E.g. missing semi-colon

• Run-time Error – An error that causes the program to halt and produce an error message– E.g. Program crashes

• Logic Error – When a program fails to produce the correct result– E.g accidentally using addition when you meant to

use subtraction– Hardest to locate!


Vending Machine Change

• Problem– Given an amount of change in cents,

determine the number of quarters, dimes, nickels, and pennies that equals that amount of change

• Example:– 67 cents would be dispensed as

• 2 quarters, 1 dimes, 1 nickel, and 2 pennies


Programming Demo

• Vending Machine Change

• Steps– Pseudocode

– Programming

– Testing/Debugging


Designing the Algorithm

• How do we know that 67 cents is 2 quarters, 1 dimes, 1 nickel and 2 pennies?– First we used as many quarters as we could (2)

• That leaves 17 cents

– Second we used as many dimes as we could (1)• Still left with 7 cents

– Third we used as many nickels as we could (1)• That leaves 2 cents

– Lastly we used the remaining amount as pennies (2)


Vending Machine Change

• Pseudocode– Ask user for the amount of change– Determine the amount in quarters– Subtract the value in quarters from amount– Determine the amount in dimes– Subtract the value in dimes from amount– Determine the amount in nickels– Subtract the amount in nickels from amount– Set the number of pennies to the remaining

amount– Output the result


Friday

• Recitation– Lab 1

• Bring– Laptops (fully charged)

• New students see me after class

comp 110: spring 20091 announcements lab 0 was due today (noon) lab 1 is on friday bring laptops new...

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