Course Status Polymorphism Containers Exceptions Midterm Review. CS Java. Introduction to Java. Andy Mroczkowski

Transcription

1 CS Java Introduction to Java Andy Mroczkowski uamroczk@cs.drexel.edu Department of Computer Science Drexel University February 11, 2008 / Lecture 4

2 Outline Course Status Course Information & Schedule Assignments Questions Polymorphism Polymorphism Re-visisted Pitfalls Constructors & Polymorphism Other Containers Overview ArrayList HashMap Generics Exceptions Concepts Catching Exceptions Throwing Exceptions Runtime Exceptions Restrictions and Pitfalls Midterm Review

3 Course Information Instructor Andy Mroczkowski Office Location: UC 147 Office Hours: Monday 6-7 & evenings by appointment Course CS Java Web: uamroczk/cs190 Book: Thinking in Java, 4th Edition Previous edition available for free at wwww.mindview.net/books/tij/ Course materials adapted from Nadya Belov

4 Schedule 1 Jan 7 Jan 14 Jan 21 Jan 28 Feb 4 Feb 11 Feb 18 Mar 3 Feb 25 Mar 10 Mar 17 Course Overview, Introduction to Java No Class No Class (University Holiday) Object Oriented Concepts, Control Statements Access Control, Intialization & Cleanup, Reuse, Interfaces, Containers, Exceptions, Polymorphism, Midterm Revie Midterm, Inner Classes, Strings File I/O, Threading Graphical User Interfaces (AWT, Swing) Networking, Special Topics (Encryption, Reflection) Final 1 Also subject to change

5 Grades Assignment 3 is graded Grades reported in comments in BB Vista See me about grading issues

6 Assignment 4 Questions? Push back due date to 22 Feb

7 Assignment 4 Questions? Push back due date to 22 Feb

8 Midterm Monday Feb 18th

9 Midterm Monday Feb 18th

10 Questions Difference between: 2 String[] args String args[] String[] args is preferred String args[] is discouraged 2

11 Questions Difference between: 2 String[] args String args[] String[] args is preferred String args[] is discouraged 2

12 Questions Do super calls have to occur first in the constructor? Yes.

13 Questions Do super calls have to occur first in the constructor? Yes.

14 Questions Why would you want several things to point to the same object? Singleton Only one instance for the entire VM

15 Questions Why would you want several things to point to the same object? Singleton Only one instance for the entire VM

16 Questions Why would you want several things to point to the same object? Singleton Only one instance for the entire VM

17 Class Participation! Let s write a Singleton! Hooray!

18 Class Participation! Let s write a Singleton! Hooray!

19 Singleton - Implementation p u b l i c class Singleton { protected Singleton ( ) { } s t a t i c p r i v a t e Singleton _instance = n u l l ; s t a t i c p u b l i c Singleton getinstance ( ) { i f (! _instance ) { _instance = new Singleton ( ) ; } r e t u r n _instance ; }

20 Polymorphism

21 Upcasting Cast an object to something "up" the inheritance tree, to make it more generic Why?

22 Upcasting Cast an object to something "up" the inheritance tree, to make it more generic Why?

23 Upcasting Cast an object to something "up" the inheritance tree, to make it more generic Why?

24 Instruments (again) interface Instrument void play() String what() void adjust() Wind void play() String what() void adjust() implements implements Percussion void play() String what() void adjust() implements Stringed void play() String what() void adjust() extends Woodwind void play() String what() void adjust() extends Brass void play() String what() void adjust()

25 Instruments (again) public class Music2 { public s t a t i c void tune ( Wind i ) { i. play ( Note. MIDDLE_C ) ; } public s t a t i c void tune ( Stringed i ) { i. play ( Note. MIDDLE_C ) ; } public s t a t i c void tune ( Brass i ) { i. play ( Note. MIDDLE_C ) ; } } public s t a t i c void main ( String [ ] args args ) { Wind f l u t e = new Wind ( ) ; Stringed v i o l i n = new Stringed ( ) ; Brass frenchhorn = new Brass ( ) ; tune ( f l u t e ) ; tune ( v i o l i n ) ; tune ( frenchhorn ) ; }

26 Instruments (again) (improved) public class Music2 { public s t a t i c void tune ( Instrument i ) { i. play ( Note. MIDDLE_C ) ; } } public s t a t i c void main ( String [ ] args args ) { Wind f l u t e = new Wind ( ) ; Stringed v i o l i n = new Stringed ( ) ; Brass frenchhorn = new Brass ( ) ; tune ( f l u t e ) ; tune ( v i o l i n ) ; tune ( frenchhorn ) ; }

27 How does this work? Magic! Actually, late binding Java doesn t decide which method to call untill run time.

28 How does this work? Magic! Actually, late binding Java doesn t decide which method to call untill run time.

29 How does this work? Magic! Actually, late binding Java doesn t decide which method to call untill run time.

30 How does this work? Magic! Actually, late binding Java doesn t decide which method to call untill run time.

31 Calling Overridden Methods What if I want to call a method that has been overridden directly? You can t, sorry.

32 Calling Overridden Methods What if I want to call a method that has been overridden directly? You can t, sorry.

33 Pitfalls Overriding private methods private methods are automatically final static methods don t behave polymorphically must explicitly say Class.method

34 Constructors & Polymorphism Order of Initialization 1. Parent classes Constructors are called, recursively Default Constructor if no super() call 2. Members are initialized in order of declaration 3. Constructor s body is executed

35 Inheritance - Constructors class C i r c l e { private double r a d i u s ; public C i r c l e ( double r ) { r a d ius = r ; } private C i r c l e ( ) { } } class Sphere extends C i r c l e { Sphere ( double r ) { super ( r ) ; } } public double getvolume ( ) ;

36 Polymorphism inside Constructors What happens when we call a polymorphic method inside a constructor? The overridden version is used. Is this dangerous? It can be. Lesson: Be careful in your constructors.

37 Polymorphism inside Constructors What happens when we call a polymorphic method inside a constructor? The overridden version is used. Is this dangerous? It can be. Lesson: Be careful in your constructors.

38 Polymorphism inside Constructors What happens when we call a polymorphic method inside a constructor? The overridden version is used. Is this dangerous? It can be. Lesson: Be careful in your constructors.

39 Polymorphism inside Constructors What happens when we call a polymorphic method inside a constructor? The overridden version is used. Is this dangerous? It can be. Lesson: Be careful in your constructors.

40 Downcasting interface Instrument void play() String what() void adjust() Wind void play() String what() void adjust() implements implements Percussion void play() String what() void adjust() implements Stringed void play() String what() void adjust() extends Woodwind void play() String what() void adjust() extends Brass void play() String what() void adjust() What happens we we cast down the inheritance tree?

41 Downcasting All casts in Java are checked There are no unsafe casts

42 New in Java 5 - Covariant Return Types Overridden method may return a different type, as long as it is derived from the return type of the method it is overriding

43 Containers Collections & Maps Iterators Common Examples Generics

44 Containers Collections & Maps Iterators Common Examples Generics

45 Collections & Maps Collection Sequence of elements with one or more restrictions List, Array, Stack, Queue Map Look up elements by key aka. Dictionary, Hash Table

46 Iterators Moves through a list and select each object in correct order Again, information hiding Provided by the list object

47 Iterator Example n0 n1 n2 n3 n4 n5

48 ArrayList An ArrayList holds a list of ordered Object objects. Basic Operations Add elements Retrieve elements by index Remove elements by index Get the number of elements Get all elements

49 ArrayList An ArrayList holds a list of ordered Object objects. Basic Operations Add elements Retrieve elements by index Remove elements by index Get the number of elements Get all elements

50 ArrayList Creating and Adding Elements import java. u t i l. A r r a y L i s t ; class A r r a y L i s t T e s t { public s t a t i c void main ( S t r i n g [ ] args ) { A r r a y L i s t autobots = new A r r a y L i s t ( ) ; } } autobots. add ( " Jazz " ) ; autobots. add ( " Bumblebee " ) ; autobots. add ( " Wheeljack " ) ;

51 ArrayList Accessing an Element { } S t r i n g a1 = autobots. get ( 0 ) ; S t r i n g a2 = autobots. get ( 1 ) ; Is there a problem?

52 ArrayList Accessing an Element { } S t r i n g a1 = autobots. get ( 0 ) ; S t r i n g a2 = autobots. get ( 1 ) ; Is there a problem?

53 ArrayList Accessing an Element - Fixed { } S t r i n g a1 = ( S t r i n g ) autobots. get ( 0 ) ; S t r i n g a2 = ( S t r i n g ) autobots. get ( 1 ) ;

54 ArrayList Iterating Over All Elements import java. u t i l. I t e r a t o r ; { } I t e r a t o r i = autobots. i t e r a t o r ( ) ; while ( i. hasnext ( ) ) { System. out. p r i n t l n ( i. next ( ) ) ; }

55 HashMap An HashMap holds a number of Object objects that are associated with an Object key. Basic Operations Add elements Retrieve elements by key Remove elements by key Get the number of elements Get all the keys Get all the values

56 HashMap An HashMap holds a number of Object objects that are associated with an Object key. Basic Operations Add elements Retrieve elements by key Remove elements by key Get the number of elements Get all the keys Get all the values

57 HashMap Creating and Adding Elements import java. u t i l. HashMap ; import java. u t i l. A r r a y L i s t ; class HashMapTest { public s t a t i c void main ( S t r i n g [ ] args ) { } } HashMap transformers = new HashMap ( ) ; transformers. put ( new A r r a y L i s t ( ), " autobots " ) ; transformers. put ( new A r r a y L i s t ( ), " decepticons " ) ;

58 HashMap Accessing an Element A r r a y L i s t goodguys = ( A r r a y L i s t ) transformers. get ( " autobots " ) ; A r r a y L i s t badguys = ( A r r a y L i s t ) transformers. get ( " decepticons " ) ;

59 HashMap Removing an Element transformers. remove ( " decepticons " ) ; transformers. remove ( " dinobots " ) ;

60 HashMap Removing an Element transformers. remove ( " decepticons " ) ; transformers. remove ( " dinobots " ) ; We didn t insert "dinobots" - is that ok?

61 Make containers type-safe New in Java 5 Example Generics A r r a y L i s t < String > autobots = new A r r a y L i s t < String > ( ) ; autobots. add ( " Jazz " ) ; autobots. add ( " Bumblebee " ) ; autobots. add ( " Wheeljack " ) ; S t r i n g a1 = autobots. get ( 0 ) ; S t r i n g a2 = autobots. get ( 1 ) ;

62 Make containers type-safe New in Java 5 Example Generics A r r a y L i s t < String > autobots = new A r r a y L i s t < String > ( ) ; autobots. add ( " Jazz " ) ; autobots. add ( " Bumblebee " ) ; autobots. add ( " Wheeljack " ) ; S t r i n g a1 = autobots. get ( 0 ) ; S t r i n g a2 = autobots. get ( 1 ) ;

63 Exceptions

64 Exceptions Concepts Catching Exceptions finally Throwing Exceptions Constructors & Interfaces Restrictions & Pitfalls

65 Exceptions - Concepts Exceptional Condition Separate error handling from normal logic Handle errors at the appropriate context

66 The Return Code Way FILE i f p, ofp ; char mode = " r " ; i f p = fopen ( " i n f i l e ", mode ) ; i f ( i f p == NULL) { f p r i n t f ( s t d e r r, "Can t open i n p u t f i l e i n f i l e! \ n " ) ; e x i t ( 1 ) ; } ofp = fopen ( " o u t f i l e ", "w" ) ; i f ( ofp == NULL) { f p r i n t f ( s t d e r r, "Can t open output f i l e %s! \ n ", o u t f i l e ) ; e x i t ( 1 ) ; }

67 The Return Exception Way t r y { BufferedReader i n = new BufferedReader (new FileReader ( " i n f i l e " ) ) ; BufferedReader i n = new B u f f e r e d W r i t e r (new F i l e W r i t e r ( " o u t f i l e " ) ) ; } catch ( IOException e ) { / / a l l error handling here e. p r i ntstacktrace ( ) ; }

68 Causing an Exception A r r a y L i s t < String > autobots = new A r r a y L i s t < String > ( ) ; autobots. add ( " Jazz " ) ; autobots. add ( " Bumblebee " ) ; autobots. add ( " Wheeljack " ) ; S t r i n g a1 = autobots. get ( 0 ) ; S t r i n g a2 = autobots. get ( 1 ) ; S t r i n g a3 = autobots. get ( 5 ) ; / /???

69 Causing an Exception - Results $ java ArrayListTest Jazz Bumblebee Wheeljack Exception in thread "main" java.lang.indexoutofboundsexception: Index: 5, Size: 3 at java.util.arraylist.rangecheck(arraylist.java:546) at java.util.arraylist.get(arraylist.java:321) at ArrayListTest.main(ArrayListTest.java:19)

70 What kinds of exceptions are out there? ClassNotFoundException CloneNotSupportedException FontFormatException IllegalAccessException InstantiationException InterruptedException IOException NoSuchFieldException NoSuchMethodException RuntimeException TimeoutException UnsupportedAudioFileException To name a few

71 What kinds of exceptions are out there? ClassNotFoundException CloneNotSupportedException FontFormatException IllegalAccessException InstantiationException InterruptedException IOException NoSuchFieldException NoSuchMethodException RuntimeException TimeoutException UnsupportedAudioFileException To name a few

72 What kinds of exceptions are out there? ClassNotFoundException CloneNotSupportedException FontFormatException IllegalAccessException InstantiationException InterruptedException IOException NoSuchFieldException NoSuchMethodException RuntimeException TimeoutException UnsupportedAudioFileException To name a few

73 Catching Exceptions

74 try and catch Code that may throw exceptions must be wrapped in a try block Exceptions are caught in catch block Basic Structure t r y { / / code t h a t may throw an exception } catch ( SomeException e1 ) { / / handle SomeException } catch ( OtherException e2 ) { / / handle OtherException }

75 Matching Exceptions Match in order First match, not necessarily best match

76 Matching Exceptions - Example t r y { S t r i n g a3 = autobots. get ( 5 ) ; / /??? } catch ( Exception e1 ) { e1. printstacktrace ( ) ; } catch ( IndexOutOfBoundsException e2 ) { System. e r r. p r i n t l n ( " Index out of bounds " ) ; }

77 finally Cleanup Always executed even after return

78 finally Cleanup Always executed even after return

79 finally Cleanup Always executed even after return

80 Stack Traces Exception in thread "main" java.lang.indexoutofboundsexception: Index: 5, Size: 3 at java.util.arraylist.rangecheck(arraylist.java:546) at java.util.arraylist.get(arraylist.java:321) at ArrayListTest.main(ArrayListTest.java:19)

81 Throwing Exceptions You can throw your own Exceptions with: thow and throws

82 Throwing Exceptions - Example class SimpleException extends Exception { } public class I n h e r i t i n g E x c e p t i o n s { public void f ( ) throws SimpleException { System. out. p r i n t l n ( " Throw SimpleException from f ( ) " ) ; throw new SimpleException ( ) ; } public s t a t i c void main ( String [ ] args ) { InheritingExceptions ie = new InheritingExceptions ( ) ; t r y { i e. f ( ) ; } catch ( SimpleException e ) { System. out. p r i n t l n ( " Caught i t! " ) ; } } }

83 Throwing Exceptions - Output Throw SimpleException form f() Caught it!

84 Exception Specification All the exceptions thrown by a method must be specified Example void f o o ( ) throws TimeoutException, MissingFileException, PurpleMonkeyDishwasherException What about specifying exceptions it doesn t actually throw?

85 Exception Specification All the exceptions thrown by a method must be specified Example void f o o ( ) throws TimeoutException, MissingFileException, PurpleMonkeyDishwasherException What about specifying exceptions it doesn t actually throw?

86 Exception Specification All the exceptions thrown by a method must be specified Example void f o o ( ) throws TimeoutException, MissingFileException, PurpleMonkeyDishwasherException What about specifying exceptions it doesn t actually throw?

87 Re-throwing Exceptions Wrapping Throw as uncaught zuh?

88 Re-throwing Exceptions Wrapping Throw as uncaught zuh?

89 Re-throwing Exceptions Wrapping Throw as uncaught zuh?

90 Constructors & Interfaces Constructors can throw exceptions Why is this cool? Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Interfaces and abstract methods can specify exceptions Concrete methods in abstract classes can throw them

91 Constructors & Interfaces Constructors can throw exceptions Why is this cool? Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Interfaces and abstract methods can specify exceptions Concrete methods in abstract classes can throw them

92 Constructors & Interfaces Constructors can throw exceptions Why is this cool? Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Interfaces and abstract methods can specify exceptions Concrete methods in abstract classes can throw them

93 Constructors & Interfaces Constructors can throw exceptions Why is this cool? Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Interfaces and abstract methods can specify exceptions Concrete methods in abstract classes can throw them

94 Constructors & Interfaces Constructors can throw exceptions Why is this cool? Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Interfaces and abstract methods can specify exceptions Concrete methods in abstract classes can throw them

95 Runtime Exceptions ArithmeticException BufferOverflowException BufferUnderflowException ClassCastException IllegalArgumentException NegativeArraySizeException NoSuchElementException NullPointerException To name a few

96 Runtime Exceptions ArithmeticException BufferOverflowException BufferUnderflowException ClassCastException IllegalArgumentException NegativeArraySizeException NoSuchElementException NullPointerException To name a few

97 Runtime Exceptions Do not need to be explicitly caught Although you can catch them if you want to Uncaught runtime exceptions print the stack trace and exit

98 Runtime Exceptions Do not need to be explicitly caught Although you can catch them if you want to Uncaught runtime exceptions print the stack trace and exit

99 Restrictions and Pitfalls When you override a method, you may only throw exceptions specified by the base class Constructors Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Exceptions in interfaces The Lost Exception

100 Restrictions and Pitfalls When you override a method, you may only throw exceptions specified by the base class Constructors Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Exceptions in interfaces The Lost Exception

101 Restrictions and Pitfalls When you override a method, you may only throw exceptions specified by the base class Constructors Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Exceptions in interfaces The Lost Exception

102 Restrictions and Pitfalls When you override a method, you may only throw exceptions specified by the base class Constructors Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Exceptions in interfaces The Lost Exception

103 Restrictions and Pitfalls When you override a method, you may only throw exceptions specified by the base class Constructors Can throw anything they want, regardless of the base class Cannot catch any exceptions thrown by the base class Exceptions in interfaces The Lost Exception

104 The Lost Exception class WorldWillExplodeException extends Exception { } class T r i v i a l E x c e p t i o n extends Exception { } public class LostMessage { void f ( ) throws WorldWillExplodeException { throw new WorldWillExplodeException ( ) ; } void dispose ( ) throws T r i v i a l E x c e p t i o n { throw new TrivialException ( ) ; } } public s t a t i c void main ( String [ ] args ) { t r y { LostMessage lm = new LostMessage ( ) ; t r y { lm. f ( ) ; } f i n a l l y { lm. dispose ( ) ; } } catch ( Exception e ) { System. out. p r i n t l n ( e ) ; } }

105 Midterm Format Similar to Quiz Find the bug Write a simple program What does this program do? True or False

106 Java Basics + Virtual Machine + Class file + Garbage collection - Java History Midterm Review OOP Concepts + Why OO? (Information hiding, Re-use, Encapsulation) + Inheritance + Polymorphism

107 Midterm Review Operators + if - else and switch + for and while loops + mathematical operators (+ - * /) + relational operators (< > ==!=) + boolean operators (&&!) + increment operators (++ ) + new operator + special string operators - bitwise operators - ternary operator - operator precedence

108 Midterm Review Loops + for + while Access Control + scope + packages + access specifiers (default, public, private, protected)

109 Initialization & Cleanup + constructors + new operator + finalize method - clone Re-use + Inheritance + Composition + final keyword - Delegation Midterm Review

110 Midterm Review Interfaces + Abstract classes + Interfaces + implements vs extends

111 Midterm Review Polymorphism Containers Exceptions

112 Class Participation 2! Let s do some True-False! Hooray!

113 Class Participation 2! Let s do some True-False! Hooray!