COP 3330

1
COP 3330

• Notes 3/7

2
Todays Topics

• Exceptions
• Abstract Classes

3
Exceptions

• Exceptions are abnormal events that occur during
  execution
• An unhandled exception is considered to be a
  runtime error
• Generating an exception is a comparatively slow
  operation, so don't use them to deal with normal
  events in performance critical code!

4
Exceptions

• Exceptions in Java are objects that inherit from
  Exception, which inherits from Throwable
• Exceptions have some of useful methods that can
  be called (most notably printStackTrace)

5
Exception Handling

• try-catch Use a try-catch construct to deal with
  exceptions that you know how to cope with
• Example
• try
• Scanner fin new Scanner(new File("inventory.txt
  "))
• // continue on and read the file
• catch(FileNotFoundException e)
• System.out.println("Could not read
  inventory.txt")

6
Exception Handling

• throws Declare that your method throws an
  exception if the exception needs to be handled
  elsewhere
• Example from Rational class
• public int intValue() throws ArithmeticException
• // This may generate a divide-by-zero
• return numerator / denominator
• Bad example
• public static void main(String args) throws
  Exception

7
Try-catch

• Syntax of try-catch
• try
• Action
• catch( ExceptionType identifier)
• Handler

8
Try-catch

• Multiple catch clauses are permissible
• Example
• try
• Scanner fin new Scanner(new
  File("input.txt"))
• while(fin.hasNext())
• System.out.println(fin.nextInt()/fin.nextInt())
  
• catch(FileNotFoundException e)
• System.out.println("Could not find input.txt")
• catch(InputMismatchException e)
• System.out.println("File contained
  non-integers")
• catch(NoSuchElementException e)
• System.out.println("File contained an odd of
  ints")
• catch(ArithmeticException e)
• System.out.println("Divide by zero")

9
Finally Clause

• The finally clause of a try-catch is executed
  regardless of if there are exceptions or not
  (even uncaught exceptions)

10
Finally Clause

• Example
• int a new int10
• Scanner fin null
• try
• fin new Scanner(new File("example.txt"))
• for(int i0ilt10 i)
• ai fin.nextInt()
• catch(FileNotFoundException e)
• System.out.println("File not found")
• finally
• if(fin ! null)
• fin.close()

11
Exception Handling

• throws Declare that your method throws an
  exception if the exception needs to be handled
  elsewhere
• Example from Rational class
• public int intValue() throws ArithmeticException
• // This may generate a divide-by-zero
• return numerator / denominator
• Bad example (Legal code, just bad idea)
• public static void main(String args) throws
  Exception

12
Interpreting a Stack Trace

• Example (from a modified version of assignment 2
  solution)
• Exception in thread "main" java.lang.NullPointerEx
  ception
• at evacuation.Evacuee.compareTo(Evacuee.java76)
• at evacuation.Evacuee.compareTo(Evacuee.java1)
• at java.util.PriorityQueue.fixUp(Unknown Source)
• at java.util.PriorityQueue.offer(Unknown Source)
• at java.util.PriorityQueue.add(Unknown Source)
• at evacuation.EvacueeQueue.add(EvacueeQueue.java
  27)
• at evacuation.Evacuation.main(Evacuation.java58)

13
Interpreting a Stack Trace

• What that means is that the program crashed when
  something was added to the EvacueeQueue
• The problem occurred when the EvacueeQueue tried
  to compare two Evacuees
• The problem occurred when the comparison between
  two Evacuee tried to call a method on an Object
  that was null

14
Abstract Classes

• Abstract classes are part way between an
  interface and class
• Abstract classes are classes so theyre extended
  rather than implemented
• Abstract classes cannot be instantiated
• Invalid example Number x new Number()
• Valid example Number x new Integer(5)

15
Abstract Classes

• Abstract classes can have data members
  (interfaces can only have static constants)
• Abstract classes can have fully fledged methods
• Abstract classes can have abstract methods

16
Abstract Methods

• Abstract methods do not have a body
• Abstract methods have only a prototype
• A class that extends an abstract class MUST
  implement the abstract methods it inherits

17
Abstract Classes

• Example
• public abstract class GeometricObject
• private Color color
• public Color getColor()
• return color
• public void setColor(Color c)
• color c
• public abstract Rectangle getBoundingBox()