1. COMPUTER 2430 Object Oriented Programming and Data Structures I

2. Exceptions

3. Class Stack
public class Stack { private Object[] items; private int top = 0; public Object pop() return items[-- top]; } ... What if the stack is empty (top == 0)? Crash!

4. Class Stack
public class Stack { private Object[] items; private int top = 0; /** Must call isEmpty before calling pop! */ public Object pop() return items[-- top]; } ... Can this avoid all crashes? NO!

5. Java Class Exception To avoid as many crashes as possible
Throws exception objects
Try and Catch

6. Throwing Exceptions
public class Stack { private Object[] items; private int top = 0; /** Should make sure it’s not empty before calling pop. An exception will be thrown if empty. */ public Object pop() if ( top == 0 ) throw new Exception(“Empty Stack!”); return items[-- top]; } ...

7. Throwing Exceptions
public class Stack { private Object[] items; private int top = 0; /** Should make sure it’s not empty before calling pop. An exception will be thrown if empty. */ public Object pop() throws Exception if ( top == 0 ) throw new Exception(“Empty Stack!”); return items[-- top]; } ...

8. Catching Exceptions
Stack myStack = new Stack(100); Object obj; ... try { obj = myStack.pop(); // public Object pop() throws Exception // statement NEXT } catch ( Exception e ) System.out.println( e.toString() ); Won’t crash! Method pop is aborted. Statement NEXT will not be executed.

9. Try-Catch-Finally
Stack myStack = new Stack(100); Object obj; ... try { obj = myStack.pop(); } catch ( Exception e ) System.out.println( e.toString() ); finally // Statements will always be executed, // even no exception is caught.
Quiz is coming?
Program due!

10. Method and Try-Catch-Finally
public void DoWork( Stack myStack ) { try obj = myStack.pop(); } catch ( Exception e ) System.out.println( e.toString() ); // public Object pop() throws Exception finally ... // Clean up DoWork() handles the Exception! No “throws” on the method header.
Quiz is coming?
Program due!

11. Method and Try-Catch-Finally
public void MoreWork() { Stack myStack; ... DoWork( myStack ); // public void DoWork( Stack myStack ) } Don’t need Try-Catch for Stack The exception is handled inside DoWork().
Quiz is coming?
Program due!

12. Method and Try-Catch-Finally
public void DoWork( Stack myStack ) throws Exception { ... obj = myStack.pop(); // public Object pop() throws Exception } DoWork() has no try-catch. Must throw Exception!
Quiz is coming?
Program due!

13. Try-Catch and Throws
public void MoreWork() { Stack myStack; ... DoWork( Stack myStack ); //public void DoWork( Stack myStack ) throws Exception } Must Try-Catch Or Throws Exception
Quiz is coming?
Program due!

14. Class Exception Exception ArrayIndexOutOfBoundsException
NullPointerException
Other Exceptions
User Exceptions
Sub-Exceptions

15. Multiple Exceptions
public void MyMethod() throws NullPointerException, ArrayIndexOutOfBoundsException { ... if ( ) throw new ArrayIndexOutOfBoundsException(“Out of bounds”); else if ( ) throw new NullPointerException(“No Object”); else }
Quiz is coming?
Program due!

16. Try-Catch-Finally
try { ... } catch (ArrayIndexOutOfBoundsException e ) System.out.println( e.toString() ); catch (NullPointerException e ) ... // More catch blocks can be here finally
Quiz is coming?
Program due!

17. Try-Catch-Finally
// Other exceptions will not be caught try { ... } catch (ArrayIndexOutOfBoundsException e ) System.out.println( e.toString() ); catch (NullPointerException e ) finally
Quiz is coming?
Program due!

18. Try-Catch-Finally
// Catch all exceptions! try { ... } catch (ArrayIndexOutOfBoundsException e ) catch (NullPointerException e ) catch (Exception e ) finally
Quiz is coming?
Program due!

19. User Defined Exceptions
public class StackEmptyException extends Exception { // could have data public StackEmptyException ( String s ) super(s); } // We do not do this in CS 2430.
Quiz is coming?
Program due!

20. Throw StackEmptyException
public class Stack { private Object[] items; private int top = 0; public Object pop() throws StackEmptyException if ( top == 0 ) throw new StackEmptyException (“Empty Stack!”); return items[-- top]; } ...

21. Catch StackEmptyException
public void DoWork( Stack myStack ) { try obj = myStack.pop(); } catch (StackEmptyException e ) // catch (Exception e ) System.out.println( e.toString() ); finally ... // Clean up
Quiz is coming?
Program due!

22. Throws StackEmptyException
public void DoWork( Stack myStack ) throws StackEmptyException { ... obj = myStack.pop(); }
Quiz is coming?
Program due!

23. Exceptions If a method throws an exception
The header must indicate that
public Object pop() throws Exception
If a method calls another method that throws an exception
Two options:
1. Try and Catch, or
2. The header must indicate that
public void DoWork(Stack myStack) throws Exception

24. Static Main()
public class Prog3 { ... public static void main( String [] args ) Scanner sc; try // throws an Exception if file not there sc = new Scanner( new File("Prog3_1.in") ); } catch (Exception ex) sc = new Scanner( System.in );
Quiz is coming?
Program due!

25. Parameter args of Main()
public class Prog3 { ... public static void main( String [] args ) String inFile = args[0]; String outFile = args[1]; } // Command line java Prog3 p3.in p3.out // args.length: 2 // args[0] : “p3.in” // args[1] : “p3.out”
Quiz is coming?
Program due!

26. Exercise
public class TryExceptions { public void f1() throws Exception System.out.println("In f1"); throw (new Exception("F1 Exception")); } ...
Quiz is coming?
Program due!

27. public class TryExceptions { public void f1() throws Exception System
public class TryExceptions { public void f1() throws Exception System.out.println("In f1"); throw (new Exception("F1 Exception")); } public void f2() try System.out.println("In f2"); f1(); catch ( Exception e ) System.out.println("Caught in f2");
Quiz is coming?
Program due!

28. public class TryExceptions{
public static void main(String [] arrgs)
TryExceptions tr = new TryExceptions();
int x = 1, y = 1;
while ( x < 4 && y < 4)
try
if ( x < 2 ) tr.f1();
tr.f2();
if ( x < 3 ) tr.f1();
System.out.println("At the bottom"); }
catch ( Exception e )
++ x;
System.out.println( "Caught in Main" );
++ y;
System.out.println( "X: " + x + " Y: " + y );
Quiz is coming?
Program due!

29. Schedule Lab 8 (5 points) Lab 7 (5points) Prog4 (20 points) Quiz 4
Due 11 pm, October 31
Lab 7 (5points)
Due 11 pm, November 2
Prog4 (20 points)
Due 11 pm, November 9
Quiz 4
Monday, November 5
Test 2
Wednesday, November 14