COMP 110COMP 110ClassesClasses

Luv KohliOctober 1, 2008

MWF 2-2:50 pmSitterson 014

AnnouncementsAnnouncementsLab 4 due

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Questions?Questions?

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Today in COMP 110Today in COMP 110Briefly go over Program 2

Classes

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Program 2Program 2

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1 2 . 7 + 3 . 30 1 2 3 4 5 6 7 8 9

Operand 1 Operand 2Operator

Program 2Program 2

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1 2 . 7 + 3 . 30 1 2 3 4 5 6 7 8 9

Assume the String above is in the variable calculation.

int firstSpace = calculation.indexOf(‘ ’);int lastSpace = calculation.lastIndexOf(‘ ’);

double operand1 = Double.parseDouble(calculation.substring(0, firstSpace));double operand2 = Double.parseDouble(calculation.substring(lastSpace + 1));char operator = calculation.charAt(firstSpace + 1);

Program 2Program 2Now you can determine which calculation to

perform using a switch or if/else

double result = 0.0;

switch (operator)

{

case ‘+’:

result = operand1 + operand2;

break;

case ‘-’:

result = operand1 – operand2;

break;

// and so on

}

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Exponent ^Exponent ^double result = 0.0;

switch (operator)

{

case ‘^’:

result = 1.0;

for (int count = 0; count < (int) operand2; count++)

result *= operand1;

break;

case ‘-’:

result = operand1 – operand2;

break;

// and so on

}

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== generally bad for floating-point== generally bad for floating-point

Floating-point numbers are imprecise◦After doing many computations, value may

not be exactly the same◦5.0 * 3.0 might be different from

1.0 + 2.0 + 3.0 + 4.0 + 5.0Okay for this assignment for the divide by

0 extra credit

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num++ vs. ++numnum++ vs. ++numnum++ is NOT num + 1num++ does num = num + 1So does ++num, BUT, there is a difference

int num1 = 5;

System.out.println(num1++);

// outputs num1 (5), then increments num1

int num2 = 5;

System.out.println(++num2);

// increments num2, then outputs num2 (6)

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ClassClassA class is the definition of a kind of object

◦A blueprint for constructing specific objects

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Class Name: Automobile

Data: amount of fuel speed license plate

Methods (actions): accelerate: How: Press on gas pedal. decelerate: How: Press on brake pedal.

Objects, InstantiationObjects, Instantiation

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Object Name: patsCar

amount of fuel: 10 gallonsspeed: 55 miles per hourlicense plate: “135 XJK”

Object Name: suesCar

amount of fuel: 14 gallonsspeed: 0 miles per hourlicense plate: “SUES CAR”

Object Name: ronsCar

amount of fuel: 2 gallonsspeed: 75 miles per hourlicense plate: “351 WLF”

Instantiations, or instances, of the class Automobile

UML (Universal Modeling Language)UML (Universal Modeling Language)

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Automobile

- fuel: double- speed: double- license: String

+ accelerate(double pedalPressure): void+ decelerate(double pedalPressure): void

Class name

Data

Methods (actions)

ObjectsObjectsImportant: classes do not have data;

individual objects have dataClasses specify what kind of data objects

have

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Class files and separate compilationClass files and separate compilation

Each Java class definition goes in its own, SEPARATE .java file

ClassName save the file as ClassName.java

Student.java includes the class Student

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Class files and separate compilationClass files and separate compilation

What happens when you compile a .java file?◦ .java file gets compiled into a .class file

Contains Java bytecode Same filename except for .class instead of .java (note to self: show this)

You can compile a Java class before you have a program that uses it

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class Studentclass Student

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Class Name: Student

- Name- Year- GPA- Major- Credits- GPA sum

+ getName+ getMajor+ printData+ increaseYear How: increase year by 1+ calcGpa How: average grades

class Studentclass Student

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Class Name: Student

- name: String- year: int- gpa: double- major: String- credits: int- gpaSum: double

+ getName(): String+ getMajor(): String+ printData(): void+ increaseYear(): void+ calcGpa(double grade): void

Defining a classDefining a classpublic class Student{ public String name; public int classYear; public double GPA; public String major; // ...

public String getMajor() { return major; }

public void increaseYear() { classYear++; }}

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Class name

Data(instance variables)

Methods

Creating an objectCreating an objectCreate an object jack of class StudentStudent jack = new Student();

Scanner keyboard = new Scanner(System.in);

Create an object keyboard of class Scanner

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Create an objectReturn memory address of object

Assign memory address of object to variable

Instance variablesInstance variablesData defined in the class are called

instance variables

public String name; public int classYear; public double GPA; public String major;

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public: no restrictions on how these instance variables are used (more details later – public is actually a bad idea here)

type: int, double, String…

variables

Using instance variables: inside the class definitionUsing instance variables: inside the class definition

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public class Student{ public String name; public int classYear; public double GPA; public String major; // ...

public String getMajor() { return major; }

public void increaseYear() { classYear++; }}

Using Using publicpublic instance variables outside a class instance variables outside a class

public static void main(String[] args){ Student jack = new Student(); jack.name = “Jack Smith”; jack.major = “Computer Science”;

System.out.println(jack.name + “ is majoring in ” + jack.major);

Student apu = new Student(); apu.name = “Apu Nahasapeemapetilon”; apu.major = “Biology”;

System.out.println(apu.name + “ is majoring in ” + apu.major);}

jack.name and apu.name are two different instance variables because they belong to different objects

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MethodsMethodsTwo kinds of methods

◦Methods that return a value Examples: String’s .substring() method,

String’s .indexOf() method, etc.

◦Methods that return nothing Example: System.out.println()

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MethodsMethods

public String getMajor(){ return major;}

public void increaseYear(){ classYear++;}

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returns a String

returns nothing

return type

Defining methods that return nothingDefining methods that return nothing

Method heading: keywords◦public: no restriction on how to use the

method (more details later)◦void: the method returns nothing

Method body: statements executed when the method is called (invoked)◦Must be inside a pair of braces

public void increaseYear(){ classYear++;}

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Method printDataMethod printDataAs usual, inside a block (defined by

braces), you can have multiple statements

public void printData(){ System.out.println(“Name: ” + name); System.out.println(“Major: ” + major); System.out.println(“GPA: ” + gpa);}

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Calling methods that return nothingCalling methods that return nothing

object, followed by dot, then method name, then ()

Use them as Java statementsStudent jack = new Student();jack.classYear = 1;

jack.increaseYear();

System.out.println(“Jack’s class year is ” + jack.classYear);

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Defining methods that return a valueDefining methods that return a value

Method heading: keywords◦public: no restriction on how to use the

method (more details later)◦Type: the type of value the method returns

Method body: statements executed◦Must be inside a pair of braces◦Must have a return statement

public String getMajor(){ return major;}

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returnreturn statement statementA method that returns a value must have

at least one return statementTerminates the method, and returns a

value to the callerSyntax:

◦return Expression;Expression can be any expression that

produces a value of type specified by the return type in the method heading

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Methods that return a valueMethods that return a valuepublic String getClassYear(){ if (classYear == 1) return “Freshman”; else if (classYear == 2) return “Sophomore”; else if ...}

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Calling methods that return a valueCalling methods that return a value

object, followed by dot, then method name, then () (same as before)

Use them as a value of the type specified by the method’s return type

Student jack = new Student();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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returnreturn statement statementCan also be used in methods that return

nothingTerminates the methodSyntax:

◦return;

public void increaseYear(){ if (classYear >= 4) return; classYear++;}

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FridayFridayMore about classes

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