Introduction to Computer Programming

1
Introduction to Computer Programming

• Error Handling

2
What to do with errors?

• Return something the caller will understand
• Public static int myMethod(int x)
• if (x lt 0) return -1
• else do what you want the method to do
• Throw Exception
• Public static int myMethod(int x)
• if (x lt 0)
• throw new IllegalArgumentException(x is
  negative and must be 0 or greater)
• else do what you want the method to do

3
What is an Exception?

• Java has objects called Exceptions that carry
  information about errors.
• IllegalArgumentException is one type
• When code throws an exception, it stops
  processing the method without a proper return.
• When an exception is a Checked type, it also
  stops the program from running if noone deals
  with it.

4
Make an Exception Happen

• import java.util.Scanner
• public class Scan
• public static int getNumb()
• Scanner rdr new Scanner(System.in)
• System.out.println("Enter a number from 1 to
  10")
• int entry rdr.nextInt()
• return entry

5
What happens to the calling program when an
Exception happens?

• If the caller does not handle it
• Crash
• Like your Scanner programs that received letters
  instead of numbers
• If the caller handles it
• Does not crash
• Does not look at return value

6
How to code Exception Handling

• Try/Catch block
• public static void main()
• int x
• try
• x getNumb()
• catch (Exception e)
• System.out.println("X was never
  filled")
• System.out.println("Every line will print
  here")

7
You Try a Try/Catch Block

• Given this method
• public static int times6(int x)
• if (x lt 0)
• throw new IllegalArgumentException(x "is
  negative and must be 0 or greater")
• else return 6 x
• Code a caller method that takes in 1 number and
  uses it to call times6.
• When the method times6 returns normally, print
  the returned number.
• When the method times6 throws an exception, print
  oops!
• Always print The End

8
Answer to Try/Catch practice

• public static void caller (int a)
• try
• int y times6(a)
• System.out.println(y)
• catch (Exception e)
• System.out.println("oops!")
• System.out.println("goodbye")

9
Getting the message back

• When you receive an exception
• Get the message text with
• Exception object . getmessage()
• Example
• catch (IllegalArgumentException e)
• System.out.println("oops!"
  e.getMessage())

10
Checking Types

• You can throw different types of exceptions
• Test them in order using elseif logic
• catch (IllegalArgumentException e)
• System.out.println("oops!"
  e.getMessage())
• catch(Exception e)
• System.out.println("oops2 ")

11
Summary

• You can stop executing a method with a throw
  command.
• Create an exception to throw new
  IllegalArgumentException(tell about the error)
• You can catch the error in the calling program
  using a try /catch block.
• Try
• Catch