1. 1 Using Objects Chapter 3 Fall 2005 CS 101 Aaron Bloomfield

2. 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. 3 Values versus objects  Numbers Have values but they do not have behaviors  Objects Have attributes and behaviors  System.in References an InputStream  Attribute: keyboard  Behaviors: reading  System.out References an OutputStream  Attribute: monitor  Behaviors: printing

4. 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. 5 Using Rectangle objects  Let’s create some Rectangle objects  Rectangle creation: Rectangle r = new Rectangle (10, 20);  Objects have attributes (or properties): System.out.println (r.length); System.out.println (r.width);  Objects have behaviors (or methods): r.grow (10, 20) r.isEmpty() r.setLocation (5,4)

6. 6 Using String objects  Let’s create some String objects  String creation: String s = new String (“Hello world”);  Objects have attributes (or properties): But we can’t access them…  Objects have behaviors (or methods): s.substring(0,6) s.indexOf (“world”) s.toLowerCase()

7. 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. 8 So why bother with objects?  Let’s 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. 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”);  It’s just a shortcut that Java allows  The two lines are almost the same There is a minor difference between the two  Which we’ll get to later

10. 10 Visualizing objects  Class (type) name  Attributes (properties)  Methods (behaviors) + grow (int, int) : void + isEmpty ( ) : void + setLocation ( int, int ) : void + resize ( int, int ) : void +... Rectangle - width = 10 - height = 20 -...

11. 11 Date translation  Goal: to translate the date from American format to standard format

12. 12 End of lecture on 5 September 2005

13. 13 How well do we understand using objects?

14. 14 Sidewalk chalk guy Source: http://www.gprime.net/images/sidewalkchalkguy/ Source: http://www.gprime.net/images/sidewalkchalkguy/

15. 15 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 7 int x - double d ‘x’ char c

16. 16 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

17. 17 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 this lecture) Note that there is no “new” here

18. 18 0x0d4fe1a8  What’s happening in memory int j = 5; String s = “Hello world”;  Primitive types are never references; only objects References 2 5 int j Hello world String s Takes up 32 bits (4 bytes) of memory Takes up 32 bits (4 bytes) of memory Takes up  12 bytes of memory At memory location 0x0d4fe1a8 int j = 5; String s = “Hello world”;

19. 19 Other Java object types  String  Rectangle  Color  JFrame

20. 20 Representation peasPerPod message + length () : int + charAt ( int i ) : char + subString ( int m, int n ) : String + indexOf ( String s, int m ) : int +... String - text = "Don't look behind the door!" - length = 27 -...  Statements int peasPerPod = 8; String message = "Don't look behind the door!“

21. 21 Representation s + length () : int + charAt ( int i ) : char + subString ( int m, int n ) : String + indexOf ( String s, int m ) : int +... String - text = “I love CS 101" - length = 13 -... 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);

22. 22 Shorthand representation  Statements int peasPerPod = 8; String message = "Don't look behind the door!“

23. 23 Examples  Consider String a = "excellence“; String b = a;  What is the representation?

24. 24 Uninitialized versus null  Consider String dayOfWeek; Scanner inStream;  What is the representation?

25. 25 Uninitialized versus null  Consider String fontName = null; Scanner fileStream = null;  What is the representation? null fontName null fileStream fontName fileStream OR

26. 26 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

27. 27 The null reference  Consider: String s = “Hello world”; System.out.println (s.length());  What happens? Java prints out 11 s + length () : int + charAt ( int i ) : char + subString ( int m, int n ) : String + indexOf ( String s, int m ) : int +... String - text = “Hello world" - length = 11 -...

28. 28 Computer bugs

29. 29 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: …

30. 30 So what is a null reference good for?  Let’s say you had a method that returned a String when passed some parameters Normally it returns a valid String  But what if it can’t? How to deal with that?  Return a null reference

31. 31 References and memory  Most modern computers are 32-bit computers This means that a reference takes up 32 bits 2 32 = 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 That’s 16 billion Gb!

32. 32  Consider: String s1 = “first string”; String s2 = “second string”; s2 = s1; System.out.println (s2); length = 12 “first string” References 4 String s1 String s2 “second string” length = 13 What happens to this? String s1 = “first string”; String s2 = “second string”; s2 = s1; System.out.println (s2);

33. 33 Java’s 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

34. 34 Assignment  Consider String word1 = "luminous"; String word2 = "graceful"; word1 = word2;  Initial representation Garbage collection time! "luminous"word1 "graceful"word2

35. 35 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 Scanner:stdin "artiste"reponse

36. 36 String representation  Consider String alphabet = "abcdefghijklmnopqrstuvwxyz";  Standard shorthand representation  Truer representation

37. 37 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?

38. 38 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 + "."); }

39. 39 Program demo

40. 40 An optical illusion

41. 41 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.

42. 42 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

43. 43 End of lecture 7 September 2005  Well, that lecture sucked. Time to redo the last lecture’s worth of slides in a more coherent fashion

44. 44 Review  Variables of primitive types int, double, char, boolean, etc. Can assign a value to it Can read a value from it Can’t 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.

45. 45 Variable declaration  Consider:  int x = 5;  int x = 7;  Java won’t allow this You can only declare a variable once At the int x=7; line, Java already has a x spot in memory  It can’t have two

46. 46 String methods  length(): returns the String’s 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

47. 47 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.

48. 48 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

49. 49 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 + "."); }

50. 50 Program demo

51. 51 PhoneNumberFun.java import java.util.*; public class PhoneNumberFun { // main(): demonstrates a simple String manipulation public static void main(String[] args) { //... // determine the area code // determine the local number // arrange result // display input and result }

52. 52 Program demo

53. 53 Lots of piercings… This may be a bit disturbing…

54. 54 More String methods  trim() Returns the String without leading and trailing whitespace Whitespace is a space, tab, or return

55. 55 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) }

56. 56 Program demo

57. 57 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 It’s 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

58. 58 How well do we understand variables versus types?

59. 59 Classes vs. Objects  A class is a user-defined “thing” Examples: String, Scanner, Rectangle, etc.  We’ll start defining our own classes next chapter Classes are more complex than the primitive types A class is analogous to a type  It’s 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

60. 60 More on classes vs. objects

61. 61 How well do we understand classes versus objects?

62. 62 Medicine Medicine Physics Physics Public Health Public Health Chemistry Chemistry Engineering Engineering Literature Literature Psychology Psychology Economics Economics Peace Peace Biology Biology The 2004 Ig Nobel Prizes "The Effect of Country Music on Suicide.“ For explaining the dynamics of hula-hooping Investigating 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 It’s 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

63. 63 References

64. 64 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 7 int x - double d ‘x’ char c

65. 65 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

66. 66 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 this lecture) Note that there is no “new” here

67. 67 0x0d4fe1a8  What’s happening in memory int j = 5; String s = “Hello world”;  Primitive types are never references; only objects References 2 5 int j Hello world String s Takes up 32 bits (4 bytes) of memory Takes up 32 bits (4 bytes) of memory Takes up  12 bytes of memory At memory location 0x0d4fe1a8 int j = 5; String s = “Hello world”;

68. 68 Representation peasPerPod message + length () : int + charAt ( int i ) : char + subString ( int m, int n ) : String + indexOf ( String s, int m ) : int +... String - text = "Don't look behind the door!" - length = 27 -...  Statements int peasPerPod = 8; String message = "Don't look behind the door!“

69. 69 Representation s + length () : int + charAt ( int i ) : char + subString ( int m, int n ) : String + indexOf ( String s, int m ) : int +... String - text = “I love CS 101" - length = 13 -... 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);

70. 70 Shorthand representation  Statements int peasPerPod = 8; String message = "Don't look behind the door!“

71. 71 Examples  Consider String a = "excellence“; String b = a;  What is the representation?

72. 72  Consider: String s1 = “first string”; String s2 = “second string”; s2 = s1; System.out.println (s2); length = 12 “first string” References 3 String s1 String s2 “second string” length = 13 What happens to this? String s1 = “first string”; String s2 = “second string”; s2 = s1; System.out.println (s2);

73. 73 Java’s 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

74. 74 End of lecture on 12 September 2005  Mary Lou also gave her talk today  Want to start a few slides back to review references

75. 75 Warn your grandparents! Historically, this class has been lethal to grandparents of students in the class Historically, this class has been lethal to grandparents of students in the class –More often grandmothers This happens most around test time This happens most around test time –Although occasionally around the times a big assignment is due

76. 76 Uninitialized versus null  Consider String dayOfWeek; Scanner inStream;  What is the representation?

77. 77 Uninitialized versus null  Consider String fontName = null; Scanner fileStream = null;  What is the representation? null fontName null fileStream fontName fileStream OR

78. 78 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

79. 79 The null reference  Consider: String s = “Hello world”; System.out.println (s.length());  What happens? Java prints out 11 s + length () : int + charAt ( int i ) : char + subString ( int m, int n ) : String + indexOf ( String s, int m ) : int +... String - text = “Hello world" - length = 11 -...

80. 80 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: …

81. 81 What happens in Windows…

82. 82 So what is a null reference good for?  Let’s say you had a method that returned a String when passed some parameters Normally it returns a valid String  But what if it can’t? How to deal with that?  Return a null reference

83. 83 References and memory  Most modern computers are 32-bit computers This means that a reference takes up 32 bits 2 32 = 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 That’s 16 billion Gb!

84. 84 Why speling is not so important… I cdnuolt blveieetaht I cluod aulaclty uesdnatnrd waht I was rdanieg. The phaonmneal pweor of thehmuan mind. Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht thefrist and lsat ltteer be in the rghit pclae. The rset can be a taotl mses andyou can sitll raed it wouthit a porbelm. Tihs is bcuseae the huamn mnid deosnot raed ervey lteter by istlef, but the wrod as a wlohe. Amzanig huh? yaeh and I awlyas thought slpeling was ipmorantt.

85. 85 Assignment  Consider String word1 = "luminous"; String word2 = "graceful"; word1 = word2;  Initial representation Garbage collection time! "luminous"word1 "graceful"word2

86. 86 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 Scanner:stdin "artiste"reponse

87. 87 String representation  Consider String alphabet = "abcdefghijklmnopqrstuvwxyz";  Standard shorthand representation  Truer representation

88. 88 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?

89. 89  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); More String methods "-12"s1"3.14"s2 "a"s3 v1 -12 v2 3.14 v3 ‘a’

90. 90 Just in time for Valentine’s Day…

91. 91 Bittersweets: Dejected sayings I MISS MY EX I MISS MY EX PEAKED AT 17 PEAKED AT 17 MAIL ORDER MAIL ORDER TABLE FOR 1 TABLE FOR 1 I CRY ON Q I CRY ON Q U C MY BLOG? U C MY BLOG? REJECT PILE REJECT PILE PILLOW HUGGIN PILLOW HUGGIN ASYLUM BOUND ASYLUM BOUND DIGNITY FREE DIGNITY FREE PROG FAN PROG FAN STATIC CLING STATIC CLING WE HAD PLANS WE HAD PLANS XANADU 2NITE XANADU 2NITE SETTLE 4LESS SETTLE 4LESS NOT AGAIN NOT AGAIN

92. 92 Bittersweets: Dysfunctional sayings RUMORS TRUE RUMORS TRUE PRENUP OKAY? PRENUP OKAY? HE CAN LISTEN HE CAN LISTEN GAME ON TV GAME ON TV CALL A 900# CALL A 900# P.S. I LUV ME P.S. I LUV ME DO MY DISHES DO MY DISHES UWATCH CMT UWATCH CMT PAROLE IS UP! PAROLE IS UP! BE MY YOKO BE MY YOKO U+ME=GRIEF U+ME=GRIEF I WANT HALF I WANT HALF RETURN 2 PIT RETURN 2 PIT NOT MY MOMMY NOT MY MOMMY BE MY PRISON BE MY PRISON C THAT DOOR? C THAT DOOR?

93. 93 Final variables  Consider final String POEM_TITLE = “Appearance of Brown"; final String WARNING = “Weather ball is black";  What is the representation?

94. 94 Final variables object type constant In general, these attributes can be modified through member methods The reference cannot be modified once it is established Value

95. 95 Rectangle The dimensions of the new Rectangle The upper-left-hand corner of the new Rectangle int x = 3; int y = 4; int width = 5; int height = 2; Rectangle r = new Rectangle(x, y, width, height);

96. 96 Final variables  Consider final String LANGUAGE = "Java"; The reference cannot be modified once it is established "Java" LANGUAGE

97. 97  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); Rectangle

98. 98 stu  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; + length () : int + subString ( int m, int n ) : String + indexOf ( String s, int m ) : int + indexOf ( String s ) : int +... String -... String method usage 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; “Groundhog Day"“May Day" - text = “Halloween" - length = 9 “Halloween" - text = “Groundhog Day" - length = 13 - text = “May Day" - length = 7 x10y4 “Hallow" v

99. 99 stu  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; String method usage “Groundhog Day"“May Day"“Halloween"x10y4 “Hallow" v s = t; u = null; Java error: cannot assign a value to final variable u

100. 100 s Rectangle method usage r  Consider: Rectangle r = new Rectangle(); final Rectangle s = new Rectangle (1, 2, 3, 4); r.setWidth(5); r.setHeight(6); s.setWidth (7); r = new Rectangle (8,9,10,11); s = new Rectangle (12,13,14,15); + setWidth ( int w ) + setHeight ( int wh ) + setX ( int x ) + setY ( int y ) +... Rectangle + setWidth ( int w ) + setHeight ( int wh ) + setX ( int x ) + setY ( int y ) +... Rectangle - width = 7 - height = 2 - x = 3 - y = 4 + setWidth ( int w ) + setHeight ( int wh ) + setX ( int x ) + setY ( int y ) +... Rectangle - width = 8 - height = 9 - x = 10 - y = 11 - width = 1 - height = 2 - x = 3 - y = 4 Rectangle r = new Rectangle(); final Rectangle s = new Rectangle (1, 2, 3, 4); r.setWidth(5); r.setHeight(6); s.setWidth (7); r = new Rectangle (8,9,10,11); s = new Rectangle (12,13,14,15); - width = 0 - height = 0 - x = 0 - y = 0 - width = 5 - height = 0 - x = 0 - y = 0 - width = 5 - height = 6 - x = 0 - y = 0

101. 101 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();

102. 102 Scanner usage examples  Consider: Scanner stdin = new Scanner (System.in); int x = stdin.nextInt(); double d = stdin.nextDouble(); String s = stdin.next(); Scanner: stdin “hello world” sd3.5x5 Scanner stdin = new Scanner (System.in); int x = stdin.nextInt(); double d = stdin.nextDouble(); String s = stdin.next();

103. 103 Today’s demotivators

104. 104 End of lecture on 14 Sep 2005  We also did the first few slides of chapter 5  The remaining slides in this set were done on 19 Sep 2005

105. 105 Overloading  We’ve 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 4 th character on Another version: s.substring (3,6)  This version will return a string from the 3 rd to the 5 th character 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

106. 106 More on overloading  Consider the ‘+’ operator It can mean integer addition: 3+5 = 8 It can mean floating-point addition: 3.0+5.0 = 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

107. 107 More on more on overloading  Consider the valueOf() method in the String class s.valueOf (3)  The parameter is an int s.valueOf (3.5)  The parameter is a double s.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

108. 108 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 won’t use this terminology much, but the book uses it

109. 109 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 won’t use this terminology much, but the book uses it

110. 110 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);

111. 111 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 no parameters (the ‘default’ constructor)  The constructor is always the first method called when creating (or ‘constructing’) an object c1 Circle - radius = 0.0 - PI = 3.14159… - … + Circle() + Circle (double r) + …

112. 112 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 1 double parameters (the ‘specific’ constructor)  The constructor is always the first method called when creating (or ‘constructing’) an object c1 Circle - radius = 0.0 - PI = 3.14159… - … + Circle() + Circle (double r) + … Circle - radius = 2.0 - PI = 3.14159… - … + Circle() + Circle (double r) + …

113. 113 Constructor varieties  The default constructor usually sets the attributes of an object to default values But that’s not why it’s called default (we’ll 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 We’ll get to why it’s called a specific constructor later The specific constructor takes in one or more parameters There can be more than one (via overloading)

114. 114 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