ELC 310 Day 21

© 2004 Pearson Addison-Wesley. All rights reserved10-2 Agenda Questions? Capstone Proposals Overdue  4 Submitted  3 Accepted, 1 in negotiations  Proposal should be in correct format (see guidelines in WebCT) Capstone progress reports overdue Problem set 4 partially corrected  1 A, 1 B and 2 C’s  Not doing some of the problems really hurts your score Problem set 5 Parts A & B  Each worth 50 Points  Due November 22 and Dec 2 respectively Exam #3  November 22  Chap 7, 8 & 9 25 M/C  Available 7AM-7PM, Passwords will be ed Sunday Discussion on Exception Handling

Chapter 10 Exceptions

© 2004 Pearson Addison-Wesley. All rights reserved10-4 Exceptions Exception handling is an important aspect of object-oriented design Chapter 10 focuses on:  the purpose of exceptions  exception messages  the try-catch statement  propagating exceptions  the exception class hierarchy  GUI mnemonics and tool tips  more GUI components and containers

© 2004 Pearson Addison-Wesley. All rights reserved10-5 Outline Exception Handling The try-catch Statement Exception Propagation Exception Classes I/O Exceptions Tool Tips and Mnemonics Combo Boxes Scroll Panes and Split Panes

© 2004 Pearson Addison-Wesley. All rights reserved10-6 Exception Handling An exception is an object that defines an unusual or erroneous situation  Divide by zero  Index out of bounds  Can’t find a file  Security violation Exceptions are “thrown” and “caught”  Catching an exception allows you to recover from the problem An error is similar to an exceptions except that errors are generally unrecoverable There are 3 things you can do with an exception  Ignore it  Handle it where it happens  Handle at some other point in the program

© 2004 Pearson Addison-Wesley. All rights reserved10-7 Uncaught (ignored) Exceptions Uncaught exceptions cause program termination  Produces a message What exception occured Where the exception occurred Example  Zero.java Zero.java  Exception in thread "main" java.lang.ArithmeticException: / by zero ÏÏ§Ï at Zero.main(Zero.java:17)  Message includes Information about the exception –Method Exception.getmessage() Call stack trace –Method, file and line number –Method Exception.printStackTrace() Since the exceptions was not “caught”, the program terminated and the message was passed to the 0/S as output

© 2004 Pearson Addison-Wesley. All rights reserved10-8 Outline Exception Handling The try-catch Statement Exception Propagation Exception Classes I/O Exceptions Tool Tips and Mnemonics Combo Boxes Scroll Panes and Split Panes

© 2004 Pearson Addison-Wesley. All rights reserved10-9 try-catch Try-catch allows the program to catch and handle an exception when it is thrown  Fix the problem  try { block of code} catch (exception) { code to handle the exception} If the code in the try block does not produce an exception the catch block is skipped try Block catch handler No exception exception

© 2004 Pearson Addison-Wesley. All rights reserved10-10 Try-Catch Examples  ProductCodes.java ProductCodes.java  ProductCodes1.java ProductCodes1.java Since the exceptions are caught and handled, the processing of the program does not terminate till the user explicitly terminates the program The “finally” clause  Optional after a try-catch try Block catchhandler No exception exception Finally Block

© 2004 Pearson Addison-Wesley. All rights reserved10-11 Outline Exception Handling The try-catch Statement Exception Propagation Exception Classes I/O Exceptions Tool Tips and Mnemonics Combo Boxes Scroll Panes and Split Panes

© 2004 Pearson Addison-Wesley. All rights reserved10-12 Exception Propagation If an exception is not caught and handled, control moves to the method that called the method that threw the exception. You can handle the exception at the higher level  Called “propagating the exception”  The exception will propagate ‘up’ the stack till It is handled Or, it forces the program to terminate Example  Propagation.java Propagation.java  ExceptionScope.java ExceptionScope.java

© 2004 Pearson Addison-Wesley. All rights reserved10-13 Exception Propagation The exceptions moves up the stack of calling methods doIt helpMe helpMe(); obj.doIt(); main Exception thrown Exception thrown Exception Thrown Exception occurred

© 2004 Pearson Addison-Wesley. All rights reserved10-14 Outline Exception Handling The try-catch Statement Exception Propagation Exception Classes I/O Exceptions Tool Tips and Mnemonics Combo Boxes Scroll Panes and Split Panes

© 2004 Pearson Addison-Wesley. All rights reserved10-15 Exception Class Hierarchy Throwable class is parent of error and exception class You can define your own exceptions  Inheriting from an Exception class

© 2004 Pearson Addison-Wesley. All rights reserved10-16 Creating your own exception Example  CreatingExceptions.java CreatingExceptions.java  OutOfRangeException.java OutOfRangeException.java Checked and Unchecked Exceptions  Checked exceptions must be caught or explicitly thrown by listing in the “throws” clause  An unchecked exception requires no throws clause RunTimeException class (and it descendants) are the only unchecked exceptions in JAVA

© 2004 Pearson Addison-Wesley. All rights reserved10-17 Outline Exception Handling The try-catch Statement Exception Propagation Exception Classes I/O Exceptions Tool Tips and Mnemonics Combo Boxes Scroll Panes and Split Panes

© 2004 Pearson Addison-Wesley. All rights reserved10-18 I/O Exceptions I/O is JAVA is treated as a Stream  Ordered sequence of bytes from a source to a destination  Read from input streams  Write to output streams Three standard I/O streams  System.in > Input (keyboard)  System.out > Output (monitor)  System.err >Output for error messages (monitor) I/O streams throws IOexception  Checked exception Example  TestData.java test.dat TestData.java test.dat  TestData1.java test2.dat TestData1.javatest2.dat

ELC 310 Day 23

© 2004 Pearson Addison-Wesley. All rights reserved10-20 Agenda Questions? Capstone progress due Student Evaluation Problem set 5 Parts A Corrected  Good results Problem set 5 Part B  DUE Dec 2 Exam #3 Corrected  Poor results  1 A, 2 C’s and 1 D Discussion on  Tool tips and Mnemonics  Advanced GUI components Tool Tips and Mnemonics Combo Boxes Scroll Panes and Split Panes Next is Recursion, generally a difficult subject for CS students  Read ahead!

© 2004 Pearson Addison-Wesley. All rights reserved10-21 Tool tips and Mnemonics Tool tip  Short Line of text that appears when ever the cursor is in top of the Swing component  Works with all Swing Components (J*)  *.setTooTipText (“A message”); Mnemonic  Hot key (ALT-”a key”)  *.setMnemonic (‘C’);  Underlines letter Disabling components  *.setEnabled( false);  Grays out component Examples  Lightbulb.java Lightbulb.java  LightBulbPanel.java LightBulbPanel.java  LightBulbControls.java LightBulbControls.java

© 2004 Pearson Addison-Wesley. All rights reserved10-22 Combo boxes Combo Boxes create “Drop Down” menus Defined in JComboBox Class Two ways to add items to the drop down  Using the constructor  *.addItem(“the item”); Can also use ImageIcon Can be editable or uneditable  Default is uneditable New object  URL Url(“protocol”, “host”, ‘file” New interface  AudioClip *.loop(); *.play(); *.stop(); Example  JukeBox.java JukeBox.java  JukeBoxControls.Java JukeBoxControls.Java

© 2004 Pearson Addison-Wesley. All rights reserved10-23 Scroll Panes For Images and Information that is too large to fit in the assigned area Provides Scroll bars to navigate through the larger information or image Defined in JScrollPane class Example  TransmitMap.java TransmitMap.java

© 2004 Pearson Addison-Wesley. All rights reserved10-24 Split panes Panel that displays twp components separated by a movable divider bar  Horizontal Left, right  Vertical Top, bottom Defined in JSplitPane class Example  ListPanel.java ListPanel.java  PickImage.java PickImage.java

© 2004 Pearson Addison-Wesley. All rights reserved10-25 Summary Chapter 10 has focused on: the purpose of exceptions exception messages the try-catch statement propagating exceptions the exception class hierarchy GUI mnemonics and tool tips more GUI components and containers