Intro to Computer Science II. Exceptions

Transcription

1 Intro to Computer Science II Exceptions

2 Admin Exam review Break from Quizzes lab questions?

3 JScrollPane JScrollPane Another swing class Allows a scrollable large component. JList? Constructors See next Page

4 JScrollPane JScrollPane Another swing class Allows a scrollable large component. JList? Constructors See next Page

5 JscrollPane II Constructors JScrollPane(Component view) Creates a JScrollPane that displays the contents of the specified component, where both horizontal and vertical scrollbars appear whenever the component's contents are larger than the view. JScrollPane(Component view, int vsbpolicy, int hsbpolicy) Creates a JScrollPane that displays the view component in a viewport whose view position can be controlled with a pair of scrollbars. JScrollPane(int vsbpolicy, int hsbpolicy) Creates an empty (no viewport view) JScrollPane with specified scrollbar policies.

6 Scroll Policies VERTICAL_SCROLLBAR_AS_NEEDED HORIZONTAL_SCROLLBAR_AS_NEEDED The default. The scroll bar appears when the viewport is smaller than the client and disappears when the viewport is larger than the client. VERTICAL_SCROLLBAR_ALWAYS HORIZONTAL_SCROLLBAR_ALWAYS Always display the scroll bar. The knob disappears if the viewport is large enough to show the whole client. VERTICAL_SCROLLBAR_NEVER HORIZONTAL_SCROLLBAR_NEVER Never display the scroll bar. Use this option if you don't want the user to directly control what part of the client is shown, or if you want them to use only non-scroll-bar techniques (such as dragging).

7 JScrollPane size Size of JscrollPane by default big enough to hold initial contents without scrolling Want to set size 1) call set size on JscrollPane 2) call setvisiblerowcount(int visiblerowcount) on the JList inside the JScrollPane 2 is preferred.

8 Traditional Error handling Traditional approach: Method returns error code Problem: Forget to check for error code Failure notification may go undetected Problem: Calling method may not be able to do anything about failure Program must fail too and let its caller worry about it Many method calls would need to be checked Instead of programming for success x.dosomething() you would always be programming for failure: if (!x.dosomething()) return false;

9 More Modern Error handling The error code was king in C and fortran and more C++ popularized exceptions Java et. al. ran with them recoverable, un-ignorable error. you've seen them java.lang.nullpointerexception java.lang.arrayoutofboundsexception exception handler gets them rather than caller.

10 Throwing an Exception Throw an exception object to signal an exceptional condition Example: IllegalArgumentException: illegal parameter value IllegalArgumentException exception = new IllegalArgumentException("Amount exceeds balance"); throw exception; throw new IllegalArgumentException("Amount exceeds balance");

11 When exception thrown When an exception is thrown, method terminates immediately similar to a return in the middle of method except: Execution continues with an exception handler

12 Example Exception public class BankAccount{ public void withdraw(double amount) throws IllegalArgumentException{ if (amount > balance){ IllegalArgumentException exception = new IllegalArgumentException("Amount exceeds balance"); throw exception; } balance = balance - amount; }... }

13 The built in java exception hierarchy

14 Wake up sleepyheads! How should you modify the deposit method to ensure that the balance is never negative? Suppose you construct a new bank account object with a zero balance and then call withdraw(10). What is the value of balance afterwards?

15 Syntax: throwing Exceptions Syntax: Example: Purpose: throw exceptionobject; throw new IllegalArgumentException(); To throw an exception and transfer control to a handler for this exception type

16 Checked Exceptions Compiler checks some exceptions have to either deal with them or explicitly state that you don't Are due to external circumstances that the programmer cannot prevent Majority occur when dealing with input and output For example, IOException

17 Unchecked Exceptions Unchecked Exceptions Extend the class RuntimeException or Error They are the programmer's fault Examples of runtime exceptions: NumberFormatException IllegalArgumentException NullPointerException Example of error (an exception that extends): OutOfMemoryError

18 Checked vs. Unchecked Categories aren't perfect: Scanner.nextInt throws unchecked InputMismatchException which extends RuntimeException Programmer cannot prevent users from entering incorrect input

19 Exceptions and files Deal with checked exceptions principally when programming with files and streams e.g, use a Scanner to read a file String filename =...; FileReader reader = new FileReader(filename); Scanner in = new Scanner(reader); But, FileReader constructor can throw a FileNotFoundException

20 Example of using throws public void read(string fname) throws FileNotFoundException { FileReader reader=new FileReader(fname); Scanner in = new Scanner(reader);... }

21 Dealing with Exceptions When calling a method that throws an exception Two choices: Handle the exception Tell compiler that you want method to be terminated when the exception occurs Use throws specifier so method can throw a checked exception example on next slide Better to declare exception thrown than to handle it incompetently

22 Throws Multiple For multiple exceptions: public void read(string filename) throws IOException, ClassNotFoundException Keep in mind inheritance hierarchy: If method can throw an IOException and FileNotFoundException, only use IOException since FileNotFoundException is-a IOException

23 Throws Syntax Syntax Example: Purpose: accessspecifier returntype methodname(parametertype parametername,...) throws ExceptionClass, ExceptionClass,... public void read(scanner in) throws IOException To indicate the checked exceptions that this method can throw

24 how much of this made sense? Suppose a method calls the FileReader constructor and the read method of the FileReader class, which can throw an IOException. Which throws specification should you use? Why is a NullPointerException not a checked exception?

25 Catching Exceptions Oh yeah you might want to handle them exception handler with try/catch statement try block contains statements that may cause an exception catch clause contains handler for an exception type Example next slide

26 try{ String filename =...; FileReader reader = new FileReader(filename); Scanner in = new Scanner(reader); String input = in.next(); int value = Integer.parseInt(input);...} catch (FileNotFoundException e){.} catch (IOException exception){ exception.printstacktrace();} catch (NumberFormatException exception){ System.out.println("Input was not a number"); }

27 Syntax: Try try{ statement statement... } catch (ExceptionClass exceptionobject){ statement statement...} catch (ExceptionClass exceptionobject){ statement statement...}

28 Have we covered the back row yet? Suppose the file with the given file name exists and has no contents. Trace the flow of execution in the try block in this section. Is there a difference between catching checked and unchecked exceptions?

29 finally finally is done no matter what Exception terminates current method Example: Danger: Can skip over essential code reader = new FileReader(filename); Scanner in = new Scanner(reader); readdata(in); reader.close(); // May never get here Must execute reader.close() even if exception happens Use finally clause for code that must be executed "no matter what"

30 Finally II Executed when try block is exited in any of three ways: After last statement of try block After last statement of catch clause, if this try block caught an exception When an exception was thrown in try block and not caught Recommendation: don't mix catch and finally clauses in same try block put the finally blocks all at the end

31 Finally syntax Syntax try{ statement statement... } finally{ statement statement... }

32 Example of finally clause Finally Example FileReader reader = null; try { reader= new FileReader(filename); Scanner in = new Scanner(reader); readdata(in); } finally { reader.close(); // if an exception occurs, finally clause is also // executed before exception is passed to its handler }

33 I wish I asked my question first! Why was the reader variable declared outside the try block? Suppose the file with the given name does not exist. Trace the flow of execution of the code segment in this section.

34 JFileChooser Recall FileReader can throw file not found exception minimize with file chooser premade dialog for choosing files uses system dialog if available JFileChooser chooser = new JFileChooser(); call showopendialog(null); //param is parent window return is whether use chose ok or cancel call getselectedfile() to get file.

35 Reading Assignment. Read Chapter 10 To page 685 (I haven't yet covered object serialization) [and I probably won't]