COMP 110: Introduction to Programming Tyler Johnson Feb 23, 2009 MWF 11:00AM-12:15PM Sitterson 014.

COMP 110:Introduction to Programming

Tyler JohnsonFeb 23, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

Announcements

Lab 3 & 4 as well as Program 2 are graded


Submitting Assignments

There is a penalty of 1-day late (25%) for all resubmissions

You spend a lot of time on these assignments

Spend a few minutes making sure you’ve submitted correctly

Follow the instructions for checking your jar fileCheck the name of the file blackboard has received


Lab 3


Today in COMP 110

A few random things

Objects & References


If-Statement Example

What is the output?

int x = 7;int y = 5;

if(y > x)x = x + y;System.out.println("y > x");

System.out.println("y <= x");

y > x

y <= x

Output


If-Statement

Remember to include curly braces for if-statement bodies that include multiple statements

int x = 7;int y = 5;

if(y > x) {x = x + y;System.out.println("y > x");

}System.out.println("y <= x");


Local Variables

What is the output of greet()?

public class Example {

private String str = “hello”;

public void foo() { String str = “goodbye”; }

public void greet() { foo(); System.out.println(str); }}

hello

Output


Local Variables

What is the output of greet()?


private String str = “hello”;

public void foo() { str = “goodbye”; }

public void greet() { foo(); System.out.println(str); }}

9

goodbye

Output


Accessors & Mutators


private double data; //private, can not be accessed directly //from OUTSIDE the class

public double getData() { //this is an accessor return data; //this method is the ONLY way to access } //“data”

public void setData(double newData) { //this is a mutator data = newData; //this method is the ONLY way to } //change “data”

}


Accessors & Mutators

public static void main(String[] args) {Example e = new Example();

e.data = 6.5; //not allowed, data is private

e.setData(6.5); //ok, only way to set datadouble d = e.getData(); //ok, only way to get data

}


DecimalFormat

import java.text.DecimalFormat;DecimalFormat df = new DecimalFormat("0.00");double number = 12.3456;System.out.println(df.format(number));

Output: 12.35"0.00" is the pattern that the format method will use to format its output

Two digits after the decimal point, one digit before (but it will display all digits if more than one before)

Fractional portion will be rounded

12


DecimalFormat

DecimalFormat’s format method returns a StringBuffer, not a String, but you can still print out a StringBuffer

See Appendix 4 for more details

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Programming Demo

Grading ProgramTwo quizzes – 10 points eachA Midterm and Final – 100 points eachFinal Exam – 50%Midterm – 25%Quizzes – 25%

FunctionalityRead in a students score and display record


Programming Demo

ApproachInstance variable for each scoreMethod to read inputMethod to display student record• Including final score and final grade (A-F)• Use two helper methods

– One to calculate final score– Other to get final grade


Programming Demo

Programming


Objects & References

Section 5.3 in text


Variables of a Primitive Type

Variables of primitive type hold a value

int a = 6;double d = 6.55;boolean b = a > d;

We can sayThe value of a is 6The value of d is 6.55The value of b is false


Variables of a Class Type

What is the “value” of a variable of a class type?

Student jack = new Student(); //what is the value of jack?

Classes can have multiple data memberspublic class Student {

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

//…

}


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


Example: Books

public class Book {

private String name; private int page;

public void setPage(int page) {this.page = page;

}

public void setName(String name) {this.name = name;

}}


Example: Books

Assume we have a class named Book

Book jacksBook = new Book();Book samsBook = new Book();//each object refers to a different book

vs.Book jacksBook = new Book();Book samsBook = jacksBook; //samsBook refers to the same object as jacksBook


Objects in Memory

jacksBook

samsBook

?

?

Memory Book jacksBook;Book samsBook;

jacksBook = new Book();jacksBook.setName("Java");

samsBook = new Book();samsBook.setName("Java");

jacksBook.setPage(137);samsBook.setPage(253);

samsBook = jacksBook;samsBook.setPage(509);

jacksBook is now on p. 509!

??

??

??

Java?

Java?

Java?

Java?

Java137

Java253

Java137

Java253

Java509

2078

?

2078

1056

?

?

?

2078

1056

2078

2078

2078


Remember

Variables of a class type contain memory addresses

NOT objects themselves


== Operator on Objects

The == operator checks whether the values of two variables are the same

The value of class variable is a memory address

When using the == operator to compare two objects, you are checking whether they have the same address in memory


== vs. equals() for Strings Explained

String is a class typeWhat is the result of

String s1 = new String("Hello");String s2 = new String("Hello");boolean strEqual = (s1 == s2);

strEqual is false! Why?s1 and s2 store different addresses!


== vs. equals() for Strings explained

What is the results of

String s1 = new String("Hello");String s2 = new String("Hello");boolean strEqual = (s1.equals(s2));

strEqual is true! Why?String’s .equals() method checks if all the characters in the two Strings are the same


Defining an equals Method

Every class has a default .equals() methodReturns whether two objects of the class are “equal” in some senseDoes not necessarily do what you want

You decide what it means for two objects of a specific class type to be considered equal by writing your own .equals() method

Perhaps books are equal if the names and page numbers are equalPerhaps only if the names are equalPut this logic inside .equals() method


Writing the .equals() Method

public class Book {

private String name; private int page;

//…

//two books are equal if their name and pages are equal public boolean equals(Book book) { return (this.name.equals(book.name) && this.page ==

book.page); }}


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


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

Constructors

COMP 110: Introduction to Programming Tyler Johnson Feb 23, 2009 MWF 11:00AM-12:15PM Sitterson 014.

Documents

Transcript of COMP 110: Introduction to Programming Tyler Johnson Feb 23, 2009 MWF 11:00AM-12:15PM Sitterson 014.