1. Advanced Programming in Java
Sadegh Aliakbary
Sharif University of Technology
Spring 2012

2. Agenda Error handling mechanisms Exception handling framework
Benefits of exception handling framework
Exception handling in Java
Spring 2012
Sharif University of Technology

3. Watch This Method
public static Integer getYear(String day){ String yearString = day.substring(0,4); int year = Integer.parseInt(yearString); return year; } public static void main(String[] args) { String day = "2010/11/29"; Integer year = getYear(day); System.out.println(year);
Spring 2012
Sharif University of Technology

4. Exceptions What is wrong with it?
What if day parameter is not a day representation?
day = “salam!”
What if day parameter is malformed?
Day = “29 Nov 2010”
What if day parameter is empty?
String s = "";
What if day parameter is null?
These occasions are called Exception
Spring 2012
Sharif University of Technology

5. Handling Exceptions What to do with exceptions? Exit the program
Printing the error on console
Returning a special value
e.g. -1
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Sharif University of Technology

6. Important Note
Sometimes the method can’t handle the exception effectively
What should a method do when an exception occurs?
Exit the program?
Suppose you are in a desktop application
Excel, Word, a game, …
Print on console?
edu site
A game
Spring 2012
Sharif University of Technology

7. Returning a Special Value
We can return a special value to report an exception
E.g.
return null;
return -1;
return 0;
return “”;
Why not?
Spring 2012
Sharif University of Technology

8. Why not? There is no special value There are many exceptions Ambiguity
Need for documentation
Combination of program code and exception code
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Sharif University of Technology

9. There is no special value
public static int minimum(int[] nums ){ int m = Integer.MAX_VALUE; for (int i : nums) { m = Math.min(m, i); } return m; int[] array = {1,2,-1}; int minimumFound = minimum(array);
Spring 2012
Sharif University of Technology

10. Exception Handling
Exception Handling is a framework for handling exceptions
;-)
It simplifies code
Separates business code and exception code
Spring 2012
Sharif University of Technology

11. What is an Exception? Exceptional event
Error that occurs during runtime
Cause normal program flow to be disrupted
Examples ?
Divide by zero errors
Accessing the elements of an array beyond its range
Invalid input
Hard disk crash
Opening a non-existent file
Heap memory exhausted
Spring 2012
Sharif University of Technology

12. Default Exception Handling
Provided by Java runtime
Prints out exception description
Prints the stack trace
Hierarchy of methods where the exception occurred
Causes the program to terminate
Spring 2012
Sharif University of Technology

13. Example 17 class DivByZero { 18 public static void main(String a[]) {
19 System.out.println(3/0);
20 }
21 }
Exception in thread "main" java.lang.ArithmeticException: / by zero
at exception.Test2.main(Test2.java:19)
Note: Exception is a runtime concept
This code has no syntax error (No compile-time error)
Spring 2012
Sharif University of Technology

14. What Happens When an Exception Occurs?
When an exception occurs within a method
The method creates an exception object
And hands it off to the runtime system
This job is called “throwing an exception”
Exception object contains
information about the error
its type
the state of the program when the error occurred
Exception line of code
Spring 2012
Sharif University of Technology

15. What Happens When an Exception Occurs (2)?
The runtime system searches the call stack for a method that contains an exception handler
When an appropriate handler is found
The runtime system passes the exception to the handler
The exception handler catches the exception
What if the runtime system can not find an exception handler?
Uses the default exception handler
Spring 2012
Sharif University of Technology

16. `
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17. Spring 2012
Sharif University of Technology

18. Exception Handling in Java
public static Integer getYear(String day) { String yearString = day.substring(0, 4); int year = Integer.parseInt(yearString); return year; } public static void main(String[] args) { Scanner scanner = new Scanner(System.in); System.out.print("Enter a well-formed date: "); String date = scanner.next(); Integer year = getYear(date); System.out.println(year);
Spring 2012
Sharif University of Technology

19. getYear()
public static Integer getYear(String day) throws Exception { if (day == null) throw new Exception("null value"); if (day.length() == 0) throw new Exception("empty value"); if (!matchesDateFormat(day)) throw new Exception("malformed value"); String yearString = day.substring(0, 4); int year = Integer.parseInt(yearString); return year; } private static boolean matchesDateFormat(String input) { return input.matches("\\d\\d\\d\\d/\\d\\d/\\d\\d");
Spring 2012
Sharif University of Technology

20. main()
public static void main(String[] args) { Scanner scanner = new Scanner(System.in); boolean ok = false; while (ok == false) { System.out.print("Enter a well-formed date: "); String date = scanner.next(); try { Integer year = getYear(date); System.out.println(year); ok = true; } catch (Exception e) { System.out.println(e.getMessage()); }
Spring 2012
Sharif University of Technology

21. Exception Handling Keywords
throw
throws a new exception
throws
Declares exception throw
If a method may throw an exception, it should declare it
try
Start a block with exception handling
catch
Catch the exception
Spring 2012
Sharif University of Technology

22. Benefits of Exception Handling Framework
Separating Error-Handling code from “regular” business logic code
Propagating errors up the call stack
Grouping and differentiating error types
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23. Example
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24. Separating Error-Handling Code
Consider pseudocode method
It reads an entire file into memory
readFile {
open the file;
determine its size;
allocate that much memory;
read the file into memory;
close the file; }
Spring 2012
Sharif University of Technology

25. Traditional Programming
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26. With Exception Handling Framework
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Sharif University of Technology

27. Note
You should still write code for detecting, reporting and handling exceptions
Exception handling framework is not responsible for these jobs!
It only helps you organize the work more effectively
Spring 2012
Sharif University of Technology

28. Propagating Errors Up the Call Stack
Traditional approach
Each method should explicitly forward the exception
Use a special return code
Using return type for reporting exceptions
Smells bad!
New approach
Automatic
Beautiful!
Spring 2012
Sharif University of Technology

29. Grouping and Differentiating Error Types
All exceptions thrown within a program are objects
The grouping or categorizing of exceptions is a natural outcome of the class hierarchy
Spring 2012
Sharif University of Technology

30. Spring 2012
Sharif University of Technology

31. Example
class MultipleCatch { public static void main(String args[]) { try { int den = Integer.parseInt(args[0]); System.out.println(3/den); } catch (ArithmeticException exc) { System.out.println(“Divisor was 0.”); } catch (ArrayIndexOutOfBoundsException exc2) { System.out.println(“Missing argument.”); } System.out.println(“After exception.”);
Spring 2012
Sharif University of Technology

32. Nested Tries
class NestedTryDemo { public static void main(String args[]){ try { int a = Integer.parseInt(args[0]); int b = Integer.parseInt(args[1]); System.out.println(a/b); } catch (ArithmeticException e) { System.out.println(“Div by zero error!"); } } catch (ArrayIndexOutOfBoundsException) { System.out.println(“Need 2 parameters!");
Spring 2012
Sharif University of Technology

33. Bad Use of Exceptions Don’t Use Exception instead of If-else
Use exceptions for exceptions!
Spring 2012
Sharif University of Technology

34. Writing Your Own Exceptions
Your class should extend Exception class
Exception subclasses could be thrown and caught
Steps to follow
Create a class that extends Exception class
Customize the class
Members and constructors may be added to the class
Exception classes are usually simple classes
With no (or few) methods and properties
Spring 2012
Sharif University of Technology

35. Example
class HateStringExp extends Exception { /* some code */ } String input = "invalid input"; try { if (input.equals("invalid input")) { throw new HateStringExp(); System.out.println("Accept string."); } catch (HateStringExp e) { System.out.println("Hate string!”);
Spring 2012
Sharif University of Technology

36. getYear(), revisited
public static Integer getYear(String day) throws Exception { if (day == null) throw new NullPointerException(); if (day.length() == 0) throw new EmptyValueException(); if (!matchesDateFormat(day)) throw new MalformedValueException(); String yearString = day.substring(0, 4); int year = Integer.parseInt(yearString); return year; } private static boolean matchesDateFormat(String input) { return input.matches("\\d\\d\\d\\d/\\d\\d/\\d\\d");
Spring 2012
Sharif University of Technology

37. Finally Contains the code for cleaning up after a try or a catch try {
//..
} catch (ExceptionType e) {
//…
} ...
} finally {
<code to be executed before the try block ends> }
Contains the code for cleaning up after a try or a catch
Spring 2012
Sharif University of Technology

38. Finally (2) Block of code is always executed
Despite of different scenarios:
Normal completion
Forced exit occurs using a return, a continue or a break statement
Caught exception thrown
Exception was thrown and caught in the method
Uncaught exception thrown
Exception thrown was not specified in any catch block in the method
Spring 2012
Sharif University of Technology

39. Spring 2012
Sharif University of Technology

40. public static int myMethod(int n) { try { switch (n) { case 1: System
public static int myMethod(int n) { try { switch (n) { case 1: System.out.println("One"); return 1; case 2: System.out.println("Two"); throwMyException(); case 3: System.out.println("Three"); } return 4; } catch (Exception e) { System.out.println("catch"); return 5; } finally { System.out.println("finally"); return 6;
class MyException extends Exception {}
Quiz!
private static void throwMyException()
throws MyException {
throw new MyException(); }
int a = myMethod(1);
System.out.println("myMethod(1)=" + a);
a = myMethod(2);
System.out.println("myMethod(2)=" + a);
a = myMethod(3);
System.out.println("myMethod(3)=" + a);
Spring 2012
Sharif University of Technology

41. Result: One finally myMethod(1)=6 Two catch myMethod(2)=6 Three
Spring 2012
Sharif University of Technology

42. Unchecked Exceptions private static void function(String[] args) {
int den = Integer.parseInt(args[0]);
System.out.println(3 / den); }
public static void main(String[] args) {
function(args);
The method function() may throw exceptions
But it has not declared it with throws keyword
Why?
Because some exceptions are unchecked
such as ArithmeticException and ArrayIndexOutOfBoundsException
Spring 2012
Sharif University of Technology

43. Checked and Unchecked Exceptions
Java compiler checks
the program should catch or list the occurring exception
If not, compiler error will occur
Unchecked exceptions
Not subject to compile-time checking for exception handling
Built-in unchecked exception classes
Error
RuntimeException
Their subclasses
Unchecked exceptions only relax compiler
The runtime behavior is the same
Spring 2012
Sharif University of Technology

44. Exception Class Hierarchy
Spring 2012
Sharif University of Technology

45. Exception Classes and Hierarchy
Multiple catches should be ordered from subclass to superclass
Or else, Compile error: Unreachable catch block…
class MultipleCatchError {
public static void main(String args[]){
try {
int a = Integer.parseInt(args [0]);
int b = Integer.parseInt(args [1]);
System.out.println(a/b);
} catch (ArrayIndexOutOfBoundsException e) {
//..
} catch (Exception ex) { }
Spring 2012
Sharif University of Technology

46. Exceptions & Inheritance
Suppose method f() overrides parent’s method
f() in child class can not throw more exceptions than those of f() in Parent class
Less or equal exceptions in throws declaration
These mistakes bring compiler error
Why?
Polymorphic method invocations may cause failure in catching some exceptions
Spring 2012
Sharif University of Technology

47. Example (1) class Parent{ void f(){} } class Child extends Parent{
void f()throws Exception{}
Result?
Compiler Error
Spring 2012
Sharif University of Technology

48. Example (2) class Parent{ void f()throws ArithmeticException{} }
class Child extends Parent{
void f()throws ArithmeticException, IOException{}
Result?
Compiler Error
Spring 2012
Sharif University of Technology

49. Example (3) class Parent{ void f()throws ArithmeticException{} }
class Child extends Parent{
void f()throws Exception{}
Result?
Compiler Error
Spring 2012
Sharif University of Technology

50. Example (4) class Parent{ void f()throws Exception{} }
class Child extends Parent{
void f()throws ArithmeticException{}
Result?
No Error
Spring 2012
Sharif University of Technology

51. Conclusion f() in child class can not throw more exceptions
Less or equal exceptions in throws declaration
f() in child class can not throw more general exceptions
f() in child class can throw more specific exceptions
Reason:
Prevent uncaught exceptions in polymorphic invocations
Spring 2012
Sharif University of Technology

52. Quiz!
Spring 2012
Sharif University of Technology

53. Spring 2012
Sharif University of Technology

54. Output: a() in D a() in C b() in D BadFormatException Good Bye!
interface A { void a(Object o); } interface B { void b(String s); class C implements A { public void a(Object o) { System.out.println("a() in C"); public void b(String s) { System.out.println("b() in C"); class D extends C implements A, B { System.out.println("a() in D"); super.a(o); System.out.println("b() in D"); try{ int parseInt = Integer.parseInt(s); }catch(Exception e){//NumberFormatException throw new BadFormatException();
class BadFormatException extends RuntimeException { }
public class Quiz {
public static void main(String[] args) {
A a1 = new C();
B b1 = new D();
C c1 = new C();
C c2 = new D();
Object o = new String("Twenty Two");
try {
f(c2);
a1.a(o);
c1.a((String)o);
b1.b((String)o);
c2.a(o);
} catch (NumberFormatException e) {
System.out.println("NumberFormatException");
} catch (BadFormatException e) {
System.out.println("BadFormatException");
} catch (Exception e) {
System.out.println("Exception");
}finally{
System.out.println("Good Bye!");
public static void f(A a) {
a.a(a);
Output:
a() in D
a() in C
b() in D
BadFormatException
Good Bye!
Spring 2012
Sharif University of Technology

55. Further Reading Throwable and Error classes Assertions
assert name!=null;
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Sharif University of Technology

56. References http://www.javapassion.com/javase/javaexceptions.pdf
Spring 2012
Sharif University of Technology

57. Spring 2012
Sharif University of Technology