COMP 110: Introduction to Programming Tyler Johnson Mar 2, 2009 MWF 11:00AM-12:15PM Sitterson 014.

COMP 110:Introduction to Programming

Tyler JohnsonMar 2, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

Announcements

Midterm on WedHanded back on Friday

Key to practice midterm has been posted

Program 3 due today by 5pm

I’ll be out of town March 16 & 18John Hansen will be lecturing


Midterm

No computers, notes, calculators etc.

You will be allowed a 1-page cheat-sheet


Questions?


Today in COMP 110

Review for Midterm


Hardware vs. Software

HardwarePhysical machineCPU, Memory

Software Set of instructions for the machine to execute


Memory

Holds data for the computer to processMain Memory (RAM – Random Access Memory)

Used for intermediate calculationsUsed to store the current program itself!Expensive

Auxiliary Memory (Secondary Memory)Disk drives, CDs, Flash drivesCheap


What is a Byte?

Data, such as numbers and keyboard characters are stored as series of bitsA bit is a digit with value 1 or 0Examples

00111010 is a byte with value 5801000001 is a byte with value 6501100001 is a byte with value 97

A byte is composed of 8 bitsJust large enough to store a keyboard character


Compiling & Running Java Programs

Java program

Java compiler

Bytecode program

Bytecode interpreter(Java JVM)

Machine code

Compiling a Java program

Running a Java program

Human-readable

Machine-readable

Java JVM-readable


Comments in Java

Two Types

Single-line comment• Begins with “//”

int i; //this is a single-line comment, write whatever you want

Multi-line comment• Everything within /* */

public class Program { /*

This is a multi-line comment. The compiler will completely ignore this text*/public static void main(String[] args) {


Algorithms and Pseudocode

Algorithm – a set of instructions for solving a problem

Pseudocode – combination of code and English used to express an algorithm before writing algorithm into code


Variables

Used to store data in a program

The data currently in a variable is its value

Value can change throughout a program

Name of variable is an identifier

Choose variable names that are meaningful!


How to Use Variables

Declare a variableint number;

Assign a value to the variablenumber = 37;

Change the value of the variablenumber = 513;


Keywords

Reserved words with predefined meaningsYou cannot name your variables keywordsif, else, return, new


Type

What kind of value the variable can holdTwo 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 typesbyte, short, int, longRepresent whole numbers such as 0, 5, 1883443

Floating-point types float, doubleRepresent numbers with some fractional component such as 1.01, 3932.123532, 0.0

CharactercharRepresents a single character such as ‘A’, ‘;’, ‘8’

BooleanbooleanRepresents 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 or False (0 to 1)


How to Name Variables

Use a combination ofLetters, digits (0-9), underscore (_)

First character cannot be a digit

Java is case sensitive “myVariable” & “myvariable” are considered different identifiers


Assignment Statements

Change a variable’s valueSyntax:

variable = expression;

Example:sleepNeeded = 8;sleepDesired = sleepNeeded * 2;

‘=’ sign is called the assignment operator


Assignment Compatibilities

byte » short » int » long » float » double

A value of any type in the list can be assigned to a type further down the list

Exampleint iVar = 7;double dVar = iVar; //this is legal


Type Casting

Type casting allows you to override assignment compatibilities

Exampledouble distance = 9.0;int points = (int)distance; //cast distance to an int

//distance is not changed in any way

//the value of points will be 9int x = (int)(6.0 + 5.5)


Type Casting

Type casting is not rounding!The effect of a type cast is truncation

Any fractional portion is discardedJust like when you divide two integers

Exampledouble bill = 25.75;int dollars = (int)bill ; //cast bill to an int//bill is not changed in any way//the value of dollars will be 25


Arithmetic Operators

Unary operators+, -, ++, --, !

Binary arithmetic operators*, /, %, +, -


Division Operator

The division operator (/) behaves differently depending on the data types used!Example

9 / 2 = 4 • Truncation when used with integers

9.0 / 2 = 4.5• Maintains fractions when used with real numbers


Remainder Operator

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

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


Specialized Assignment Operators

+=

a += 3; //same asa = a + 3;

-=

a -= 4; //same asa = a – 4;

*=

a *= 2; //same asa = a * 2;

/=

a /= 3; //same asa = a / 3;


Parentheses and Precedence

Expressions inside parentheses evaluated first

(cost + tax) * discountcost + (tax * discount)

Highest Precedence

First: the unary operators +, -, ++, --, !Second: the binary operators *, /, %Third: the binary operators +, -

Lowest Precedence


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 subtractionHardest to locate!


Strings

A string is a sequence of characters“Hello world!”“Enter a whole number from 1 to 99.”

String (capital S) is a class in Java, not a primitive type


String Indices

Strings consist of a sequence of characters

Each character has a position in the String

U N C i s G r e a t

0 1 2 3 4 5 6 7 8 9 10

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Positions

Characters


String Concatenation

We can concatenate two Strings together using the (+) operator

Example

String name = “Bob”;String greeting = “Hi ” + name;System.out.println(greeting);//prints “Hi Bob” to the screen


String Methods

string.length()string.equals(A_String)string.toLowerCase();

And others


Escape Characters

\" Double quote

\' Single quote

\\ Backslash

\n New line

\r Carriage return

\t Tab


Keyboard Input

Scanner kb = new Scanner(System.in);int num = kb.nextInt();double d = kb.nextDouble();


Boolean Expressions

(result == 0) is a boolean expression

Boolean expressions evaluate to either true or false

Examples10 > 5, (true)4 > 6, (false)Integers are whole numbers, (true)


If-Else Statement

An if-else statement allows us to make decisions in a program

int result = n % 2;

if(result == 0)System.out.println(“That number is even!”);

elseSystem.out.println(“That number is odd!”);


Java Comparison Operators

Math

Java Name

= == Equal to

≠ != Not equal to

> > Greater than

≥ >= Greater than or equal to

< < Less than

≤ <= Less than or equal toExample expressions:

variable <= 6myInt > 55 == 3


&&, || operators

AND

if ((temperature > 50) && (temperature < 75)) {// walk to school

}

OR

if (sunny || cloudy) {// walk to school

}


The ! Operator (NOT)

Boolean negation!false is true!true is false

Example

boolean cloudy;

…

if (!cloudy) {// walk to school if it’s not cloudy

}


Effect of Boolean Operators

A B A && B

A || B !A

true true true true false

true false false true false

false true false true true

false false false false true


The Type boolean

Can be either true or false

boolean sunny = true;boolean cloudy = false;

if (sunny || cloudy) {// walk to school

}

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Nested If Statements

if(balance >= 0) {if(INTEREST_RATE >= 0) balance = balance +

INTEREST_RATE*balance;else

System.out.println(“Negative Interest!”);}else

balance = balance – FEE;


Multi-Branch If Statements

if(score >=90)grade = ‘A’;

else if(score >=80)grade = ‘B’;

else if(score >=70)grade = ‘C’;

else if(score >=60)grade = ‘D’;

elsegrade = ‘F’;


Switch Statement Example

int year;…switch(year) {

case 1: System.out.println(“You are a freshman”); break;

case 2: System.out.println(“You are a sophomore”);

break;case 3: System.out.println(“You are a junior”);

break;case 4: System.out.println(“You are a senior”); break;default: System.out.println(“This is a default case”); break;

}


Enumerations

An enumeration allows us to give unique numeric values to a list of items

enum Flavor {Vanilla, Chocolate, Strawberry}

This statement assigns a unique numeric value to each of {Vanilla, Chocolate, Strawberry}


Floating-Point Numbers

Consider the number 1/3 = 0.333333

Computers have finite storage and cannot store infinitely repeating decimals

In a computer, the number 1/3 is stored as 0.333333, with part of the fraction missing or truncated

Since 0.333333 is < 1/3, this representation is inexact


Floating-Point Comparison

To compare two floating-point numbers, check if the difference is within some tolerance

float a,b;final float EPSILON = 1e-6; //10^-6…if(Math.abs(a – b) < EPSILON) //Math.abs() is absolute

valueSystem.out.println(“a and b are the same!”);

elseSystem.out.println(“a and b are different!”);


Loops

A portion of a program that repeats some action is called a loop

Evaluate

i <= n ?

Execute

Print i

i = i + 1;

Execute

End Program

Start

i = 1false

true

Loop Body

Stopping Condition

A loop that counts up to a certain number n


Types of Loops

Do-WhileWhen you want the body to be executed at least onceUseful for checking user input

ForMore convenient/readable when the number of iterations is known beforehand, e.g. stored in some counter variable

WhileSafest choice, can be used to create any kind of loopWhen it might be necessary for the loop to iterate zero times


While Example

A program that counts up to a certain number

int number;

… //number is set to some value, e.g. user input

int count = 1;while(count <= number) {

System.out.print(count + ", ");count++;

} 1, 2, 3, 4, ….

Output


Do-While Example

Read in a positive integer, if integer is negative, try again

int input;do {

System.out.println("Please enter a positive integer: ");input = keyboard.nextInt();

} while (input <= 0); //note the semicolon!

//input is guaranteed to be > 0 at this point


For Loop Example

Pseudocode for a possible for loop

Do the following for each value of count from 1-3:Display count

Java codeint count;

for(count = 1; count <= 3; count++)System.out.println(count);

1 2 3

Output


Infinite Loops

A loop that never ends is called an infinite loop

int count = 3;do {

System.out.println(count);count++;

} while(count >= 0);//execution will never reach this pointSystem.out.println("count after loop = " +

count);


Ending a Loop

Count-controlled loopsIf you know the number of loop iterationsfor (count = 0; count < iterations; count++)

User-controlled loopsAsk-before-iteratingSentinel value

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Nested Loops

Example

int sum = 0;int i = 0;

while(i < 10) {

for(int j = 0; j < 20; j++) sum = sum + j; //executes 10 * 20 times

i++; //executes 10 times, i = [0, 9]}


Classes, Objects, and Methods

Class: a definition of a kind of objectObject: an instance of a class

Contains instance variables (data) and methods

MethodsMethods that return a valueMethods that return nothing

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Classes

Car

- make: String- model: String- year: int- owner: String- location: String

+ accelerate(double pedalPressure): void+ brake(double pedalPressure): void+ sell(String newOwner): void+ start(): void

Class name

Attributes

Methods (actions)

A class is the definition of a kind of object

A UML Diagram


Creating Objects

Create an object called “jack” of class “Student”

Student jack = new Student();

Create an objectAssign memory address of object to variable


Instance Variables

The data members of a class are called instance variables

private String name;private int year;private double gpa; private String major;

private No direct access to the variables from outside the class


Methods

Two kinds of methods

Methods that return a value• Examples

– string.substring()– string.charAt()

Methods that return nothing• Example

– System.out.println()


Methods

public String getMajor() { return major;}

public void increaseYear() { year++;}

returns a String

returns nothing

return type


Calling Methods that Return Nothing

Syntaxobject.method();

Use them like Java statementsStudent jack = new Student();jack.year = 1;

jack.increaseYear(); //year = 2jack.increaseYear(); //year = 3

System.out.println("Jack’s class year is " + jack.year);

Jack’s class year is 3

Output


Methods that Return a Value

public String getClassYear() {

if(year == 1) return "Freshman"; else if(year == 2) return "Sophomore"; else if ...}

//add two numbers and return the resultpublic double getSum(double a, double b) {

return a+b;}


Calling Methods that Return a Value

Syntaxobject.method();

Use as a variable of the method’s return type

Student jack = new Student();jack.name = "Jack Smith";jack.major = "Computer Science";

String m = jack.getMajor();

System.out.println("Jack’s full name is " + jack.getName());System.out.println("Jack’s major is " + m);

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Local/Instance Variables

public class Student {

public String name; public int year; // ...

public void printInfo() { String info = name + ": " + year; System.out.println(info); }

public void increaseYear() { year++; }

public void decreaseYear() { year--; }}

• year and name are instance variables• can be used in any method in this class

• info is a local variable declared inside method printInfo()• can only be used inside method printInfo()


Methods with Parameters

Some methods need data as input in order to perform their function

Parameters can be used as (local) variables inside the method

public int square(int number) {

return number * number;}

Parameters go inside parentheses of method header


Methods with Multiple Parameters

Multiple parameters separated by commas

public double getTotal(double price, double tax) { return price + price * tax;}


Method Parameters and Arguments

Order, type, and number of arguments must match parameters specified in method heading

public String getMessage(int number, char c) { return number + ":" + character;

}

object.getMessage(7, 'c'); //okobject.getMessage(7, 'c', 7); //errorobject.getMessage(7.5, 'c'); //errorobject.getMessage(7, 6); //error


Public vs Private

public void increaseYear()public int year;

public: there is no restriction on how you can use the method or instance variable


Public vs Private

private double gpa;private int year;

private: can not directly use the method or instance variable’s name outside the class


Example

public class Student { public int year; private String major;}

Student jack = new Student();

jack.year = 1;

jack.major = “Computer Science”;

OK, year is public

Error!!! major is private


Information Hiding

A programmer using a method should only need to know what the method does, not how it does it

keyboard.nextInt()

Information hiding means hiding the details of the code from the programmer


Accessors and Mutators

How do you access private instance variables?Accessor methods (a.k.a. get methods, getters)

Allow you to look at data in private instance variables

Mutator methods (a.k.a. set methods, setters)

Allow you to change data in private instance variables


Example: Student

public class Student {

private String name; private int age;

public void setName(String studentName) { name = studentName; }

public void setAge(int studentAge) { age = studentAge; }

public String getName() { return name; }

public int getAge() { return age; }}

Accessors

Mutators


Encapsulation

Hiding details of a class that are not necessary to understand how objects of the class are used

Two partsInterfaceImplementation


Encapsulation

Implementation:

• Private instance variables• Private constants• Private methods• Bodies of public methods

Interface:

• Comments• Headings of public methods• Public named constants

Programmer Who Uses the Class:

• Create objects• Call methods


Call-by-Value

Parameters in Java are passed by value

The value of a variable is passed, not the variable itselfVariables passed to a method can never be changed

public class Example {

public void setVariable(int a) {a = 7;

}

public static void main(String[] args) {Example example = new Example();int a = 5;//a == 5example.setVariable(a); //does not change the value of a//a == 5

}}

Different Variables!


Parameters of a Primitive Type

public void increaseNum(int num) { num++;}

public void foo() { int x = 5; increaseNum(x); System.out.println(x);}

What is the output?5


Variables of a Class Type

The value of a variable of a class type is a memory address

The address of the object it refers to

Student jack = new Student(); //jack holds the address of the newly created

//object of the Student class

The address to this other location is called a reference to the object

Class types are also called reference types


Parameters of a Class Type

public void changeBook(Book book) { book = new Book("Biology");}

public void foo() { Book jacksBook = new Book("Java"); changeBook(jacksBook); System.out.println(jacksBook.getName());}

What is the output?Java


Parameters of a Class Type

public void changeBook(Book book) { book.setName("Biology");}

public void foo() { Book jacksBook = new Book("Java"); changeBook(jacksBook); System.out.println(jacksBook.getName());}

What is the output?Biology

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Wednesday

Midterm

COMP 110: Introduction to Programming Tyler Johnson Mar 2, 2009 MWF 11:00AM-12:15PM Sitterson 014.

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Transcript of COMP 110: Introduction to Programming Tyler Johnson Mar 2, 2009 MWF 11:00AM-12:15PM Sitterson 014.