Using Objects

1
Using Objects

• Chapter 3
• Fall 2006
• CS 101
• Aaron Bloomfield

2
Getting classy

• Purpose of this chapter
• Gain experience creating and manipulating objects
  from the standard Java types
• Why
• Prepares you for defining your own classes and
  creating and manipulating the objects of those
  classes

3
Values versus objects

• Numbers
• Have values but they do not have behaviors
• In particular, each has only ONE value (or
  attribute)
• Objects
• Have attributes and behaviors
• An object can have multiple values (or attributes)

4
Using objects

• First, we create an object
• Scanner stdin new Scanner (System.in)
• Most object creation lines look like this
• Then we use the object
• stdin.nextInt()
• stdin.nextDouble()
• Note that we could have called the object foo,
  bar, or anything
• stdin is just what we chose to call it

5
Using Rectangle objects

• Lets create some Rectangle objects
• Rectangle creation
• Rectangle r new Rectangle (10, 20)
• Objects have attributes (or properties)
• System.out.println (r.width)
• System.out.println (r.height)
• Objects have behaviors (or methods)
• r.grow (10, 20)
• r.isEmpty()
• r.setLocation (5,4)

6
Using String objects

• Lets create some String objects
• String creation
• String s new String (Hello world)
• Objects have attributes (or properties)
• But we cant access them
• Objects have behaviors (or methods)
• s.substring(0,6)
• s.indexOf (world)
• s.toLowerCase()

7
The lowdown on objects

• Objects are things that have properties
  (attributes) and behaviors (methods)
• We first create one or more objects
• We then manipulate their properties and call
  their methods

8
So why bother with objects?

• Lets say you want to do a lot of String
  manipulation
• Once you create a String object, all the
  manipulation methods are contained therein
• Sun already wrote the methods for us
• So we can use String objects instead of writing
  our own code to get the substring, indexOf, etc.

9
More on Strings

• Strings are used very often
• As a shortcut, you can use
• String s Hello world
• instead of
• String s new String (Hello world)
• Its just a shortcut that Java allows
• The two lines are almost the same
• There is a minor difference between the two
• Which well get to later

10
Visualizing objects

• Class (type) name
• Attributes (properties)
• Methods (behaviors)

11
How well do we understand using objects?
12
For Valentines Day
13
Bittersweets Dejected sayings

• I MISS MY EX
• PEAKED AT 17
• MAIL ORDER
• TABLE FOR 1
• I CRY ON Q
• U C MY BLOG?
• REJECT PILE
• PILLOW HUGGIN

• ASYLUM BOUND
• DIGNITY FREE
• PROG FAN
• STATIC CLING
• WE HAD PLANS
• XANADU 2NITE
• SETTLE 4LESS
• NOT AGAIN

14
Bittersweets Dysfunctional sayings

• RUMORS TRUE
• PRENUP OKAY?
• HE CAN LISTEN
• GAME ON TV
• CALL A 900
• P.S. I LUV ME
• DO MY DISHES
• UWATCH CMT

• PAROLE IS UP!
• BE MY YOKO
• UMEGRIEF
• I WANT HALF
• RETURN 2 PIT
• NOT MY MOMMY
• BE MY PRISON
• C THAT DOOR?

15
Review

• Variables of primitive types
• int, double, char, boolean, etc.
• Can assign a value to it
• Can read a value from it
• Cant do much else!
• Objects
• String, Rectangle, etc.
• Have many parts
• Rectangle has width, length, etc.
• Like a complex type
• Have methods
• String has length(), substring(), etc.

16
String methods

• length() returns the Strings length (duh!)
• String s hello world
• String t goodbye
• System.out.println (s.length())
• System.out.println (t.length())
• Prints 11 and 7
• Note that calling s.length() is different than
  calling t.length()!
• Both return the length
• But of different Strings

17
More String methods

• Consider
• String weddingDate "August 21, 1976"
• String month weddingDate.substring(0, 6)
• System.out.println("Month is " month ".")
• What is the output?
• Month is August.

18
More String methods

• Consider
• String fruit "banana"
• String searchString "an"
• int n1 fruit.indexOf(searchString, 0)
• int n2 fruit.indexOf(searchString, n1 1)
• int n3 fruit.indexOf(searchString, n2 1)
• System.out.println("First search " n1)
• System.out.println("Second search " n2)
• System.out.println("Third search " n3)
• What is the output?
• First search 1
• Second search 3
• Third search -1

19
End of lecture on 6 September 2006
20
String program examples
21
Program WordLength.java

• public class WordLength
• public static void main(String args)
• Scanner stdin new Scanner(System.in)
• System.out.print("Enter a word ")
• String word stdin.next()
• int wordLength word.length()
• System.out.println("Word " word " has
  length "
• wordLength ".")

22
Program demo

• WordLength.java

23
More String methods

• trim()
• Returns the String without leading and trailing
  whitespace
• Whitespace is a space, tab, or return

24
Date translation

• Goal to translate the date from American format
  to standard format

25
DateTranslation.java

• // Convert user-specified date from American to
  standard format
• import java.util.
• class DateTranslation
• // main() application entry point
• static public void main(String args)
• // produce a legend (Step 1)
• // prompt the user for a date in American
  format (Step 2)
• // acquire the input entered by the user (Step
  3)
• // echo the input back (Step 4)
• // get month entered by the user (Step 5)
• // get day entered by the user (Step 6)
• // get year entered by the user (Step 7)
• // create standard format version of input
  (Step 8)
• // display the translation (Step 9)

26
Program demo

• DateTranslation.java

27
Todays demotivators
28
Classes vs. Objects
29
Variables vs. Types

• The type is the recipe or template for how to
  create a variable
• Examples int, double, char, boolean, etc.
• There are only 8 primitive types
• There are only a few things you can do with a
  type
• Declare a variable
• int x
• Use it as a cast
• x (int) 3.5
• There is only one of each type
• The variable is the actual instance of a type in
  memory
• Its a spot in memory where you store a value
• You choose the name width, x, thatThemThereValue,
  etc.
• You can have as may variables as you want but
  only one type!
• Like the difference between a recipe and a bunch
  of cookies

30
How well do we understand variables versus types?
31
Classes vs. Objects

• A class is a user-defined thing
• Examples String, Scanner, Rectangle, etc.
• Well start defining our own classes later this
  semester
• Classes are more complex than the primitive types
• A class is analogous to a type
• Its just more complex and user-defined
• There can be only one class of each name
• An object is an instance of a class
• There is only one String class, but you can have
  100 String objects
• A object is analogous to a variable
• It just is a reference instead
• A class is a template used for creating objects

32
More on classes vs. objects
33
How well do we understand classes versus objects?
34
Lots of piercings

• This may be a bit disturbing

35
References
36
Java and variables

• Consider
• int x 7
• double d
• char c x
• The variable name is the actual spot in memory
  where the value is stored
• Note that d does not have a value

37
What is a reference

• A reference is a memory address
• References are like pointers in C/C
• But they are not the exact same thing!
• C has references also (in addition to pointers)
• You may hear me call them pointers instead of
  references
• All objects in Java are declared as references

38
References 1

• Consider
• int j 5
• String s Hello world
• Java translates that last line into
• String s new String (Hello world)
• (Not really, but close enough for now)

Note that there is no new here
39
References 2

• Whats happening in memory
• int j 5
• String s Hello world
• Primitive types are never references only objects

• int j 5
• String s Hello world

String s
Takes up 32 bits (4 bytes) of memory
Takes up 32 bits (4 bytes) of memory
At memory location 0x0d4fe1a8
Takes up ?12 bytes of memory
40
Representation

• Statements
• int peasPerPod 8
• String message "Don't look behind the door!

41
Representation
String s I love CS 101 int l
s.length() char c s.charAt (3) String t
s.subString(1,2) int t s.indexOf (t, 0)
A period means follow the reference
42
Shorthand represntation

• ConsiderString s Hello world
• Takes up a lot of space on my slides
• So well use a shorthand representation

43
Examples

• Consider
• String a "excellence
• String b a
• What is the representation?

44
End of lecture on 11 September 2006
45
References 3

• Consider
• String s1 first string
• String s2 second string
• s2 s1
• System.out.println (s2)

• String s1 first string
• String s2 second string
• s2 s1
• System.out.println (s2)

What happens to this?
46
Javas garbage collection

• If an object in memory does not have a reference
  pointing to it, Java will automagically delete
  the object
• This is really cool!
• In C/C, you had to do this by yourself

47
An optical illusion
48
The null reference
49
Uninitialized versus null

• Consider
• String dayOfWeek
• Scanner inStream
• What is the representation?

50
Uninitialized versus null

• Consider
• String fontName null
• Scanner fileStream null
• What is the representation?

OR
51
The null reference

• Sometimes you want a reference to point to
  nothing
• Use the null reference
• String s null
• The null reference is equivalent to a memory
  address of zero (0x00000000)
• No user program can exist there

52
The null reference

• Consider
• String s Hello world
• System.out.println (s.length())
• What happens?
• Java prints out 11

53
The null reference

• Consider
• String s null
• System.out.println (s.length())
• This is called accessing (or following) a null
  pointer/reference
• What happens?
• Java java.lang.NullPointerException
• C/C Segmentation fault (core dumped)
• Windows

54
What happens in Windows
55
So what is a null reference good for?

• Lets say you had a method that returned a String
  when passed some parameters
• Normally it returns a valid String
• But what if it cant? How to deal with that?
• Return a null reference

56
References and memory

• Most modern computers are 32-bit computers
• This means that a reference takes up 32 bits
• 232 4 Gb
• This means that a 32-bit machine cannot access
  more than 4 Gb of memory!
• Well, without doing some tricks, at least
• Most machines come with 1 Gb memory these days
• Will come with 4 Gb in a year or so
• 64-bit machines will have a maximum of 16
  exabytes of memory
• Giga, Tera, Peta, Exa
• Thats 16 billion Gb!

57
The 2004 Ig Nobel Prizes

• Medicine
• Physics
• Public Health
• Chemistry
• Engineering
• Literature
• Psychology
• Economics
• Peace
• Biology

"The Effect of Country Music on Suicide. For
explaining the dynamics of hula-hooping Investigat
ing the scientific validity of the Five-Second
Rule The Coca-Cola Company of Great Britain For
the patent of the combover The American Nudist
Research Library Its easy to overlook things
even a man in a gorilla suit. The Vatican, for
outsourcing prayers to India The invention of
karaoke, thereby providing an entirely new way
for people to learn to tolerate each other For
showing that herrings apparently communicate by
farting
58
Using object examples
59
Assignment

• Consider
• String word1 "luminous"
• String word2 "graceful"
• word1 word2
• Initial representation

Garbage collection time!
60
Using objects

• Consider
• Scanner stdin new Scanner(System.in)
• System.out.print("Enter your account name ")
• String response stdin.next()
• Suppose the user interaction is
• Enter your account name artiste

61
String representation

• Consider
• String alphabet "abcdefghijklmnopqrstuvwxyz"
• Standard shorthand representation
• Truer representation

62
String representation

• Consider
• String alphabet "abcdefghijklmnopqrstuvwxyz"
• char c1 alphabet.charAt(9)
• char c2 alphabet.charAt(15)
• char c3 alphabet.charAt(2)
• What are the values of c1, c2, and c3? Why?

63
More String methods

• Consider
• int v1 -12
• double v2 3.14
• char v3 'a'
• String s1 String.valueOf(v1)
• String s2 String.valueOf(v2)
• String s3 String.valueOf(v3)

• int v1 -12
• double v2 3.14
• char v3 'a'
• String s1 String.valueOf(v1)
• String s2 String.valueOf(v2)
• String s3 String.valueOf(v3)

64
Final variables

• Consider
• final String POEM_TITLE Appearance of Brown"
• final String WARNING Weather ball is black"
• What is the representation?

65
Final variables

• Consider
• final String LANGUAGE "Java"

66
Todays demotivators
67
Rectangle
68
Rectangle

• Consider
• final Rectangle BLOCK new Rectangle(6, 9, 4,
  2)
• BLOCK.setLocation(1, 4)
• BLOCK.resize(8, 3)

• final Rectangle BLOCK new Rectangle(6, 9, 4,
  2)
• BLOCK.setLocation(1, 4)
• BLOCK.resize(8, 3)

69
String method usage

• Consider
• String s "Halloween"
• String t "Groundhog Day"
• String u "May Day"
• String v s.substring(0,6)
• int x t.indexOf ("Day", 0)
• int y u.indexOf ("Day")
• s t
• u null

• String s "Halloween"
• String t "Groundhog Day"
• String u "May Day"
• String v s.substring(0,6)
• int x t.indexOf ("Day", 0)
• int y u.indexOf ("Day")
• s t
• u null

70
String method usage

• Consider
• String s "Halloween"
• String t "Groundhog Day"
• final String u "May Day"
• String v s.substring(0,6)
• int x t.indexOf ("Day", 0)
• int y u.indexOf ("Day")
• s t
• u null

• s t
• u null

Java error cannot assign a value to final
variable u
71
Rectangle method usage

• Consider
• Rectangle r new Rectangle()
• final Rectangle s new
• Rectangle (3, 4, 1, 2)
• r.setWidth(5)
• r.setHeight(6)
• s.setWidth (7)
• r new Rectangle (10,11,8,9)
• s new Rectangle (12,13,14,15)

• Rectangle r new Rectangle()
• final Rectangle s new
• Rectangle (3, 4, 1, 2)
• r.setWidth(5)
• r.setHeight(6)
• s.setWidth (7)
• r new Rectangle (10,11,8,9)
• s new Rectangle (12,13,14,15)

72
Scanner review

• To initialize a Scanner object
• Scanner stdin new Scanner (System.in)
• Scanner stdin Scanner.create (System.in)
• This one will not work!
• To read an int from the keyboard
• stdin.nextInt()
• To read a double from the keyboard
• stdin.nextDouble()
• To read a String from the keyboard
• stdin.next()

73
Scanner usage examples

• Consider
• Scanner stdin new Scanner (System.in)
• int x stdin.nextInt()
• double d stdin.nextDouble()
• String s stdin.next()

• Scanner stdin new Scanner (System.in)
• int x stdin.nextInt()
• double d stdin.nextDouble()
• String s stdin.next()

74
Beware!!!
75
End of lecture on 13 September 2006
76
Overloading
77
Overloading

• Consider the operator
• It can mean integer addition 35 8
• It can mean floating-point addition 3.05.0
  8.0
• It can mean string concatenation foo bar
  foobar
• The operator has multiple things it can do
• a.k.a. the operator is overloaded

78
More on overloading

• Weve seen a number of methods
• In the String class substring(), charAt(),
  indexOf(), etc.
• In the Rectangle class setLocation(),
  translate()
• Consider the substring() method in the String
  class
• One version s.substring(3)
• This will return a string from the 4th character
  on
• Another version s.substring (3,6)
• This version will return a string from the
  character at index 3 up to (but not including!)
  the character at index 6
• There are multiple versions of the same method
• Differentiated by their parameter list
• The substring method can take one OR two
  parameters
• This is called overloading

79
More on more on overloading

• Consider the valueOf() method in the String class
• String.valueOf (3)
• The parameter is an int
• String.valueOf (3.5)
• The parameter is a double
• String.valueOf (3)
• The parameter is a char
• There are multiple versions of this method
• Differentiated by their parameter list
• Thus, the valueOf() method is overloaded

80
More on methods
81
Accessors

• Some methods allow us to find out information
  about an object
• In the Rectangle class getWidth(), getHeight()
• These methods are called accessors
• They allow us to access attributes of the object
• An accessor is a method that allows us to find
  out attributes of object
• Usually start with get in the method name
• I wont use this terminology much, but the book
  uses it

82
Mutators

• Some methods allow us to set information about
  the object
• In the Rectangle class setLocation(),
  setBounds()
• These methods are called mutators
• They allow us to change (or mutate) the
  attributes of an object
• A mutator is a method that allows us to set
  attributes of object
• Usually start with set in the method name
• I wont use this terminology much, but the book
  uses it

83
Constructors

• A constructor is a special method called ONLY
  when you are creating (or constructing) and
  object
• The name of the constructor is ALWAYS the exact
  same name as the class
• Scanner stdin new Scanner (System.in)
• String foo new String (hello world)
• There can be overloaded constructors
• Rectangle r new Rectangle()
• Rectangle s new Rectangle (1, 2, 3, 4)

84
Calling the Circle constructor

• To create a Circle object
• Circle c1 new Circle()
• This does four things
• Creates the c1 reference
• Creates the Circle object
• Makes the c1 reference point to the Circle
  object
• Calls the constructor with noparameters (the
  default constructor)
• The constructor is always the first method called
  when creating (or constructing) an object

85
Calling the Circle constructor

• To create a Circle object
• Circle c1 new Circle(2.0)
• This does four things
• Creates the c1 reference
• Creates the Circle object
• Makes the c1 reference point to the Circle
  object
• Calls the constructor with 1double parameters
  (the specificconstructor)
• The constructor is always the first method called
  when creating (or constructing) an object

86
Constructor varieties

• The default constructor usually sets the
  attributes of an object to default values
• But thats not why its called default (well get
  to that later)
• The default constructor ALWAYS takes in zero
  parameters
• Thus, there can be only one
• A specific constructor sets the attributes of the
  object to the passed values
• Well get to why its called a specific
  constructor later
• The specific constructor takes in one or more
  parameters
• There can be more than one (via overloading)

87
Method types review

• With the exception of constructors, these names
  are purely for human categorization
• Accessor allows one to access parts of the
  object
• Mutator allows one to change (mutate) a part of
  an object
• Constructor used to create a object
• Default constructor takes in no parameters
• Specific constructor takes in one or more
  parameters
• Facilitator
• Any method that is not one of the above

88
Todays demotivators
89
Java documentation
90
Java documentation
91
Java packages

• Group similar classes together
• Packages we will use
• java.lang automatically imported by Java
• Contains the clases needed by the Java language
• java.util contains Scanner, Vector, etc.
• Contains various utility classes
• java.text we will use it later in the semester
• Contains classes used to manipulate text
• Any package (other than java.lang) must be
  imported to use the classes within it

92
Not going over remaining slides in this set
93
Example last semesters HW J2
94
Last semesters HW J2

• Found online at http//www.cs.virginia.edu/asb/te
  aching/cs101-fall05/hws/hwj2/index.html
• The HW listed 10 steps to be performed
• Used the StringBuffer class
• Which can be found at http//java.sun.com/j2se/1.5
  .0/docs/api/java/lang/StringBuffer.html
• Strings are immutable
• Meaning that once you create a String, you can
  never change it
• There are no mutator methods
• You can change what the String reference points
  to, but not the String itself

95
Preliminaries

• import java.util.
• public class StringBufferManipulator
• public static void main (String args)
• // Preliminaries
• System.out.println ("StringBuffer
  manipulator\n")
• Scanner stdin new Scanner (System.in)
• // Code for steps 1 to 10 will go here

96
Step 1

• The user needs to enter two strings one long
  string (say, 10 or so characters at a minimum)
  and a shorter string that is contained within the
  longer string. 
• This input should be obtained via the nextLine()
  method, as using the next() method will not read
  in a string that contains spaces.
• // Step 1
• System.out.println ("Enter a long string")
• String longString stdin.nextLine()
• System.out.print ("\nEnter a shorter string
  within )
• System.out.println (the long string")
• String shortString stdin.nextLine()
• System.out.println ()

97
Step 2

• Create a StringBuffer object from the longer
  string -- this is the StringBuffer that you will
  manipulate for the rest of the homework.  There
  are two ways to do this create a default
  constructred StringBuffer, and append() the long
  string to that, or use the StringBuffer with the
  appropriate specific constructor.
• // Step 2
• StringBuffer buffer new StringBuffer(longString
  )

98
Step 3

• Include, as a comment in your program, the code
  for creating the StringBuffer in the other way
  from step 2.
• // Step 3
• // StringBuffer buffer new StringBuffer()
• // buffer.append(longString()

99
Step 4

• Find the position of the small string within the
  StringBuffer, and save that position.
• // Step 4
• int pos buffer.indexOf(shortString)

100
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101
Step 5

• Delete the small string from the StringBuffer,
  and print out the result.
• // Step 5
• int shortLength shortString.length()
• buffer.delete (pos, posshortLength)
• System.out.println (buffer)

102
Step 6

• Insert "CS101" into the position of the
  StringBuffer where the small string was
  originally found (from step 3), and print out the
  result
• // Step 6
• buffer.insert (pos, "CS101")
• System.out.println (buffer)

103
Step 7

• Remove the last word from the string. You can
  assume that everything from the last space (found
  via lastIndexOf()) to the end of the String is
  the last word. Print out the result.
• // Step 7
• pos buffer.lastIndexOf(" ")
• int bufferLength buffer.length()
• buffer.delete(pos, bufferLength)
• System.out.println (buffer)

104
Step 8

• Append " rocks" to the end of the StringBuffer,
  and print out the result. Note that there is a
  space before the work 'rocks'.
• // Step 8
• buffer.append (" rocks")
• System.out.println (buffer)

105
Step 9

• Delete the character at position n/2, where n is
  the length of the StringBuffer.  Print out the
  result.
• // Step 9
• int n buffer.length()
• buffer.deleteCharAt (n/2)
• System.out.println (buffer)

106
Step 10

• Reverse the StringBuffer, and print out the
  result.
• // Step 10
• buffer.reverse()
• System.out.println (buffer)

107
Program demo

• StringBufferManipulator.java