Programming Languages and Techniques (CIS120)

Transcription

1 Programming Languages and Techniques (IS120) Lecture 30 April 6, 2015 ExcepEons hapter 27

2 Iterator quiz public static void iteratorexn() { List<String> strs = new LinkedList<String>(); strs.add(""); strs.add("is 120"); } 1 Iterator<String> iter = strs.iterator(); System.out.println(iter.next()); System.out.println(iter.next()); System.out.println(iter.next()); 3 interface Iterator<E> { public boolean hasnext(); public E next(); public void delete(); // optional } What does code do? 2 4

3 ExcepEons Dealing with the unexpected

4 Why do methods fail? Some methods make requirements iterator must have "next" element, treeset element must be comparable, argument must be non- null Interfaces may be imprecise Some Iterators don't support the "remove" operaeon External components of a system might fail Try to open a file that doesn't exist Resources might be exhausted Program uses all of the computer's disk space These are all excep%onal circumstances how do we deal with them?

5 Ways to handle failure Return an error value (or default value) e.g. Math.sqrt returns NaN ("not a number") if given input < 0 e.g. Many Java libraries return null e.g. file reading method returns - 1 if no more input available aller must check return value Use with cau%on easy to introduce nasty bugs! Use an informaeve result e.g. in Oaml we used opeons to signal poteneal failure e.g. in Java, we can create a special class like opeon Passes responsibility to caller, but caller forced to do the proper check Use excepeons Available both in Oaml and Java Any caller (not just the immediate one) can handle the situaeon If an excepeon is not caught, the program terminates

6 ExcepEons An excepeon is an object represeneng an abnormal condieon Its internal state describes what went wrong e.g. NullPointerExcepEon, IllegalArgumentExcepEon, IOExcepEon an define your own excepeon classes Throwing an excepeon is an emergency exit from the current context The excepeon propagates up the invocaeon stack unel it either reaches the top of the stack, in which case the program aborts with the error, or the excepeon is caught atching an excepeon lets callers take appropriate aceons to handle the abnormal circumstances

7 Example from HW 7 void loadimage (String filename) { try { Picture p = new Picture(fileName); // could fail //... code to display the new picture in the window // executes only if the picture is successfully created. } catch (IOException ex) { // Use the GUI to send an error message to the user // using a dialog window JOptionPane.showMessageDialog( frame, // parent of dialog window // error message to display "annot load file\n" + ex.getmessage(), "Alert", // title of dialog JOptionPane.ERROR_MESSAGE // type of dialog ); }

8 Quiz: atching the ExcepEon class { public void foo() {.bar(); System.out.println("here in foo"); } public void bar() { try {.baz(); } catch (Exception e) { System.out.println("caught"); } System.out.println("here in bar"); } } public void baz() { throw new RuntimeException(); } What gets printed if we do (new ()).foo();? 1. here in foo here in bar caught 2. here in bar caught here in foo 3. caught here in bar here in foo 4. something else

9 Workspace (new ()).foo(); Stack Heap

10 Workspace (new ()).foo(); Stack Heap

11 ().foo(); Workspace Stack Heap Allocate a new instance of in the heap.

12 ().foo(); Workspace Stack Heap

13 Workspace.bar(); Stack Heap Save a copy of the current workspace in the stack, leaving a hole, wriden _, where we return to. Push the pointer, followed by arguments (in case none) onto the stack.

14 Workspace.bar(); Stack Heap

15 Workspace Stack Heap try { baz(); } catch (Exception e) { System.out.Println ( caught ); } here in bar );

16 Workspace Stack Heap try { baz(); } catch (Exception e) { System.out.Println ( caught ); } here in bar ); When execueng a try/catch block, push onto the stack a new workspace that contains all of the current workspace except for the try { } code. Replace the current workspace with the body of the try.

17 Workspace.baz(); Body of the try. Everything else. When execueng a try/catch block, push onto the stack a new workspace that contains all of the current workspace except for the try { } code. Replace the current workspace with the body of the try. Stack catch (RuntimeException e) { System.out.Println ( caught ); } here in bar ); Heap

18 Workspace.baz(); onenue execueng as normal. Stack catch (RuntimeException e) { System.out.Println ( caught ); } here in bar ); Heap

19 Workspace Stack Heap throw new RuntimeException(); The top of the stack is off the bodom of the page catch (RuntimeException e) { System.out.Println ( caught ); } here in bar );

20 Workspace Stack Heap throw new RuntimeException(); catch (RuntimeException e) { System.out.Println ( caught ); } here in bar );

21 Workspace Stack Heap throw (); catch (RuntimeException e) { System.out.Println ( caught ); } here in bar ); Runtime Exception

22 Workspace Stack Heap throw (); Discard the current workspace. Then, pop saved workspace frames off the stack, looking for the most recently pushed one that contains a try/catch block whose catch clause declares a supertype of the excepeon being thrown. If no matching catch is found, abort the program with an error. catch (RuntimeException e) { System.out.Println ( caught ); } here in bar ); Runtime Exception

23 Workspace Stack Heap Discard the current workspace. Then, pop saved workspace frames off the stack, looking for the most recently pushed one that contains a try/catch block whose catch clause declares a supertype of the excepeon being thrown. If no matching catch is found, abort the program with an error. catch (RuntimeException e) { System.out.Println ( caught ); } here in bar ); Try/atch for ( )? Runtime Exception No!

24 Workspace Stack Heap When a matching catch block is found, add a new binding to the stack for the excepeon variable declared in the catch. Then replace the workspace with catch body and the rest of the saved workspace. onenue execueng as usual. catch (RuntimeException e) { System.out.Println ( caught ); } here in bar ); RuntimeEx ception Try/atch for ( )? Yes!

25 Workspace Stack Heap { System.out.Println ( caught ); } here in bar ); When a matching catch block is found, add a new binding to the stack for the excepeon variable declared in the catch. Then replace the workspace with catch body and the rest of the saved workspace. onenue execueng as usual. e RuntimeEx ception

26 Workspace Stack Heap { System.out.Println ( caught ); } here in bar ); onenue execueng as usual. e Runtime Exception

27 Workspace Stack Heap { ; } here in bar ); onenue execueng as usual. e Runtime Exception onsole caught