COMP 110: Introduction to Programming Tyler Johnson Feb 18, 2009 MWF 11:00AM-12:15PM Sitterson 014.

COMP 110:Introduction to Programming

Tyler JohnsonFeb 18, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

Announcements

Program 2 due tonight by midnight

Lab 4 due tomorrow by midnight

Midterm Wed, Mar 4th


Program 1


Questions?


Today in COMP 110

Classes & Methods


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


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


Simple Example


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()


Simple Example



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

public void increaseYear() { year++; info = “My info string”; // ERROR!!! }

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

The compiler will not recognize the variable info inside of method increaseYear()


More about Local Variables

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

public static void main(String[] args) {

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

String info = “Hello there!”; System.out.println(info);

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);}

Variable info in main method not affected by variable info in printInfo method in class Student


Named Constants in Classes

We can declare named constants as instance variables

public class Circle {

public double radius; //normal instance variablepublic final double PI = 3.14159; //named constant

//…}


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


Calling a Method with Parameters



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

public void setClassYear(int classYear) { year = classYear; }}


Calling a Method with Parameters


Student jack = new Student(); jack.setName(“Jack Smith”); jack.setClassYear(3);}

Arguments


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

+ = ???


Automatic Type Casting

public class SalesComputer {

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

SalesComputer sc = new SalesComputer();double total = sc.getTotal(“19.99”, Color.RED);double total = sc.getTotal(19.99);double total = sc.getTotal(19.99, 0.065);int price = 50;total = sc.getTotal(price, 0.065);Automatic typecasting


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!


The Keyword this

Within a class definition, this is a name for the receiving object

Frequently omitted, but understood to be there


public int year;

public void increaseYear() { this.year++; //use of this not necessary in this case}

}

19


The Keyword this


Student jack = new Student();jack.increaseYear();

Student sam = new Student();sam.increaseYear();

}

public class Student() {

public int year;

public void increaseYear() {this.year++;

}}

this indicates the receiving object(The object the method was called on)


The Keyword this

When is the keyword this useful?When we need to differentiate between local/instance variables w/ the same name


public int year;

public void setClassYear(int year) { this.year = year; }}


Calling Methods within Methods

It’s possible to call methods within other methods

If calling a method of the same class, no need to specify receiving object

public void method1() {method2(); //no object needed, current object assumed

}


The Keyword this

Why don’t we need to specify an object when calling between methods of the same class?

Use of this keyword is implied

public void method1() { method2();}

public void method1() { this.method2();}

===


Calling Methods within Methods

public class Example {

public void method1() {System.out.println(“method1!");method2(); //no object needed, current object assumedSystem.out.println(“done with method2!");

}

public void method2() {System.out.println(“method2!");

}

public static void main(String[] args) {Example example = new Example(); //create object of class

Exampleexample.method1();System.out.println("Finished!");

}}


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


More about Private

Hides instance variables and methods inside the class/object

Invisible to external users of the class

Users cannot access private class members directly

Information hiding


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


Example: Rectangle

public class Rectangle {

public int width; public int height; public int area;

public void setDimensions(int newWidth, int newHeight) {

width = newWidth; height = newHeight; area = width * height; }

public int getArea() { return area; }}


Example: Rectangle

Rectangle box = new Rectangle();box.setDimensions(10, 5); //calculates areaSystem.out.println(box.getArea());// Output: 50

box.width = 6; //access instance variable directly

System.out.println(box.getArea());

// Output: 50, but wrong answer!


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


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


Private Methods

Why make methods private?Helper methods that will only be used from inside a class should be private

• External users have no need to call these methods

Encapsulation


Encapsulation

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

Two partsInterfaceImplementation


Class Interface

A class interface tells programmers all they need to know to use the class in a programA class interface includes

Headings for public methods• public Return_Type Method_Name(Parameters)

Named constants• public static final Var_Name = Constant;

Comments for using public methods• /* This method does the following */


Implementation

The implementation of a class consists of the private elements of the class definition

Private instance variables and constants• private Type Var_Name;

Private methods• private Return_Type Method_Name()

Bodies of public methods• public void myMethod() {

//this is the implementation}


Encapsulation

Implementation should not affect behavior described by interface

Two classes can have the same behavior but different implementations


Two Implementations of Rectangle


private int width; private int height; private int area;

public void setDimensions( int newWidth, int

newHeight) {

width = newWidth; height = newHeight; area = width * height; }

public int getArea() { return area; }}


private int width; private int height;

public void setDimensions( int newWidth, int newHeight)

{ width = newWidth; height = newHeight; }

public int getArea() { return width * height; }}


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


Guidelines

Comments before class definitionClass header

Declare instance variables as privateUse get & set methods to access instance variablesDeclare helper methods as private/**/ for interface comments and // for implementation comments


In-Class Exercise

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

Turn in for class participation credit


Friday

Recitation

BringLaptops (fully charged)Textbook

COMP 110: Introduction to Programming Tyler Johnson Feb 18, 2009 MWF 11:00AM-12:15PM Sitterson 014.

Documents

Transcript of COMP 110: Introduction to Programming Tyler Johnson Feb 18, 2009 MWF 11:00AM-12:15PM Sitterson 014.