COMP 110: Introduction to Programming Tyler Johnson Feb 16, 2009 MWF 11:00AM-12:15PM Sitterson 014.

COMP 110:Introduction to Programming

Tyler JohnsonFeb 16, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

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Program 2 due Wednesday by midnight

Lab 4Part 2 is now extra credit


Lab 2


Questions?


Today in COMP 110

Classes & MethodsRead Section 5.1 in text


Classes

Java programs consists of objects of various class types interacting with one another

You have already been using classes

System, String, Scanner, Math


Classes

A class is a definition of a kind of object

It describes a general template or blueprint for creating objects of that class

A class “Car” is a definition of what a “Car” is and what it can do


Classes & Objects

Objects of the class Car might have the following attributes

MakeModelYearOwnerLocation

All objects of the class Car have these attributes, but with potentially different values


Classes & Objects

Class CarMakeModelYearOwnerLocation

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

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


Classes & Objects

Important: classes do not have data; individual objects have data

Classes specify what kind of data objects have

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Class CarMakeModelYearOwnerLocation

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


Classes & Objects

Both the “camry” and “ford” objects satisfy the definition of the class “Car”

They are said to be instances of the “Car” class

When we create an object of a certain class, we are said to instantiate it


Methods

Classes also define the actions that objects of its type can performThese are called methods

All objects of the same class have the same methods

The class “Car” might have the following methods

accelerate()brake()sell()start()


String Methods

string.length()string.charAt(index)string.substring(index1, index2)string.toLowerCase()string.equals(A_String)


UML

A UML class diagram can be useful in describing classes

UML stands for Universal Modeling Language


UML

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 UML class diagram for the class “Car”


Defining Classes

You have already been defining your own classes!

Every program you have written defines a class

//define the class TipCalculatorpublic class TipCalculator {

…}


Class Files & Separate Compilation

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

ClassName ClassName.java

The class “Car” must be saved in a file called “Car.java”


Class Files & Separate Compilation

What happens when you compile a .java file?

.java file gets compiled into a .class file• .class file contains Java bytecode• Car.java is compiled into Car.class

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


The Student Class

Student

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

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


Defining the Class Student

public class Student {

public String name; public int year; public double GPA; public String major; // ...

public String getMajor() { return major; }

public void increaseYear() { year++;

}// …

}

Class name

Data(instance variables)

Methods


Instance Variables

The data members of a class are called instance variables

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

public No restrictions on how these instance variables are used (more details later)


Creating Objects

Declaring/Initializing Primitives

int i = 0;double area = 10.5;

Declaring/Creating Objects

Scanner keyboard = new Scanner(System.in);


Creating Objects

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

Student jack = new Student();

Create an objectAssign memory address of object to variable


Using Instance Variables: Inside the Class Definition


public String name; public int year; public double GPA; public String major; // ...

public String getMajor() { return major; }


}// …

}


Using public 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 sam = new Student(); sam.name = "Samantha Smart"; sam.major = "Biology";

System.out.println(sam.name + " is majoring in " + sam.major);}

“jack.name” and “sam.name” are two different variables because they belong to different objects


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


Defining Methods that Return Nothing

Method heading: keywordspublic: 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() { year++;}


Method printData

As 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);}


Methods

All method definitions must appear inside the definition of the class to which they belong!


// … public String getMajor() { return major; }


}// …

}


Methods

When you use a method such as

keyboard.nextInt()

you are said to call or invoke the method


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


Defining Methods that Return a Value

Method heading: keywordspublic: no restriction on how to use the method (more details later)Type: the type of value the method returns

Method body: statements executedMust be inside a pair of bracesMust have a return statement

public String getMajor() { return major;}


The Return Statement

A method that returns a value must have at least one return statement

Terminates the method, and returns a value to the caller

Syntax:return Expression;

Expression can be any expression that produces a value of the return type

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Methods that Return a Value

public String getClassYear() {

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


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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Can also be used in methods that return nothing

Terminates the method

Syntax:return;

public void increaseYear() {

if(year >= 4) return; //exit the method

year++;}

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Typically, you want to write your methods to contain only one return statement

It will make your programs easier to read


The Main Method

The main method is a method like any other

public static void main(String[] args) {// …

}

The main method is not invoked by you, but invoked automatically by the system when you run a program


The Main Method

Classes that have a main method can be run as a program

Not all classes have a main methodSome are used inside the main method of other classes


Local Variables

Variables declared inside a method are called local variables

As opposed to instance variables, which are declared in the class definition


public double gpa; //an instance variable

public static void main(String[] args) { int size; //a local variable // …

}}


Local/Instance variables

Instance variablesDeclared in a class• Can be used anywhere in the class that declares the

variable, including inside the class’ methods

Local variablesDeclared in a method• Can only be used inside the method that declares the

variable


Programming Demo

Counter classA class used to count things

Define a class that records a count as a non-negative whole numberInclude methods for the following

Reset counter to 0Increment the countDecrement the countGet the current countDisplay the current count


Counter Class

No methods should allow the count to become negative

Write a program to test the class


Programming Demo

Programming


Wednesday

More about Classes

COMP 110: Introduction to Programming Tyler Johnson Feb 16, 2009 MWF 11:00AM-12:15PM Sitterson 014.

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Transcript of COMP 110: Introduction to Programming Tyler Johnson Feb 16, 2009 MWF 11:00AM-12:15PM Sitterson 014.