COMP200 EXCEPTIONS. OOP using Java, based on slides by Shayan Javed

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1 1 1 COMP200 EXCEPTIONS OOP using Java, based on slides by Shayan Javed

2 Exception Handling 2

3 3 Errors Syntax Errors Logic Errors Runtime Errors

4 4 Syntax Errors Arise because language rules weren t followed.

5 5 Syntax Errors Arise because language rules weren t followed. Detected by the compiler! javac for Java! g++ for C++

6 6 Logic Errors Program compiles and runs, but results are wrong.

7 7 Logic Errors Program compiles and runs, but results are wrong. Detected and fixed through testing.

8 8 Logic Errors Program compiles and runs, but results are wrong. Detected and fixed through testing. Arise because logic coded by the programmer was incorrect.

9 9 Logic Errors Program compiles and runs, but results are wrong. Detected and fixed through testing. Arise because logic coded by the programmer was incorrect.! Example wrote c = a - b instead of c = a + b

10 10 Runtime Errors Occur when program is running environment detects it and can t carry it out

11 11 Runtime Errors Occur when program is running environment detects it and can t carry it out Examples of Code Errors:! Divide by zero

12 12 Runtime Errors Occur when program is running environment detects it and can t carry it out Examples of Code Errors:! Divide by zero! Array out of bounds

13 13 Runtime Errors Occur when program is running environment detects it and can t carry it out Examples of Code Errors:! Divide by zero! Array out of bounds! Accessing a null pointer (reference)

14 14 Runtime Errors Occur when program is running environment detects it and can t carry it out Examples of Code Errors:! Divide by zero! Array out of bounds! Accessing a null pointer (reference)! Integer overflow

15 15 Runtime Errors Occur when program is running environment detects it and can t carry it out Examples of Code Errors:! Divide by zero! Array out of bounds! Accessing a null pointer (reference)! Integer overflow Programs crash when such exceptions are not handled

16 16 Exception An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program s instructions.

17 17 Exception Handling in Java Mechanism for handling exceptions by detecting and responding to them in a systematic, uniform and reliable manner.

18 18 Exception Handling in Java Mechanism for handling exceptions by detecting and responding to them in a systematic, uniform and reliable manner. Any exceptions not handled within the Java program are caught by the Java Runtime Environment (JRE)

19 19 Exception A method in Java throws Exceptions! Something went wrong

20 20 Exception A method in Java throws Exceptions! Something went wrong Exceptions are Objects! Every Exception is a subclass of the Exception class

21 21 Unchecked Exceptions/Errors ClassNotFoundException IOException ArithmeticException Exception AWTException NullPointerException RuntimeException IndexOutOfBoundsException Object Throwable Several more classes IllegalArgumentException LinkageError Several more classes VirtualMachineError Error AWTError Several more classes

22 22 System Errors ClassNotFoundException IOException ArithmeticException Exception AWTException NullPointerException RuntimeException IndexOutOfBoundsException Object Throwable Several more classes IllegalArgumentException LinkageError Several more classes VirtualMachineError Error AWTError Several more classes

23 23 System Errors Thrown by the Java Virtual Machine (JVM)

24 24 System Errors Thrown by the Java Virtual Machine (JVM) Represented by the Error class

25 25 System Errors Thrown by the Java Virtual Machine (JVM) Represented by the Error class Describes internal system errors

26 26 System Errors Thrown by the Java Virtual Machine (JVM) Represented by the Error class Describes internal system errors Rarely occur if they do you can t do much other than terminating

27 27 Runtime Exceptions (Unchecked) ClassNotFoundException IOException ArithmeticException Exception AWTException NullPointerException RuntimeException IndexOutOfBoundsException Object Throwable Several more classes IllegalArgumentException LinkageError Several more classes VirtualMachineError Error AWTError Several more classes

28 28 Checked Exceptions ClassNotFoundException IOException ArithmeticException Exception AWTException NullPointerException RuntimeException IndexOutOfBoundsException Object Throwable Several more classes IllegalArgumentException LinkageError Several more classes VirtualMachineError Error AWTError Several more classes

29 29 Checked Exceptions ClassNotFoundException IOException ArithmeticException Exception AWTException NullPointerException RuntimeException IndexOutOfBoundsException Object Throwable Several more classes IllegalArgumentException Need to explicitly deal with Checked Exceptions: Error LinkageError VirtualMachineError Several more classes try and catch them, or throw them AWTError Several more classes

30 30 Exception Handling Keywords:! try some code, catch any Exceptions

31 31 Exception Handling Keywords:! try some code, catch any Exceptions! or throw an Exception

32 32 Exception Handling Keywords:! try some code, catch any Exceptions! or throw an Exception! finally execute some code

33 33 Exception Handling Java forces you to deal with checked Exceptions

34 34 Exception Handling Java forces you to deal with checked Exceptions Two ways to deal with them: void p1 () { try { riskymethod(); } catch (Exception ex) {... } } (a)

35 35 Exception Handling Java forces you to deal with checked Exceptions Two ways to deal with them: void p1 () { try { riskymethod(); } catch (Exception ex) {... } } void p1 () throws Exception { } riskymethod(); (a) (b)

36 36 Catching Exceptions Thrown Exceptions have to be eventually caught somewhere in your code

37 37 Exception Information So an Exception has been caught what can we do with it? try { // Statements which throw Exceptions } catch (Exception exception) { } // ALL Exceptions

38 38 Exception Information Some useful methods in the Throwable class:

39 39 Exception Information Some useful methods in the Throwable class: String tostring():! Returns a short description of the Exception

40 40 Exception Information Some useful methods in the Throwable class: String tostring():! Returns a short description of the Exception String getmessage():! Returns a detailed description of the Exception

41 41 Exception Information Some useful methods in the Throwable class: String tostring():! Returns a short description of the Exception String getmessage():! Returns a detailed description of the Exception void printstacktrace():! Prints the stacktrace information on the console

42 42 Exception Information Some useful methods in the Throwable class: void printstacktrace():! Prints the stacktrace information on the console Sample output: java.lang.nullpointerexception at MyClass.method2(MyClass.java:9) at MyClass.method1(MyClass.java:6) at MyClass.main(MyClass.java:3)

43 43 Exception Information Example: java.io.printwriter output = null; try { output = new java.io.printwriter( text.txt ); output.println( Welcome to Java ); output.close(); } catch (java.io.ioexception ex){ System.out.println(ex.toString()); ex.printstacktrace() ; }

44 44 Problems Example: java.io.printwriter output = null; try { output = new java.io.printwriter( text.txt ); output.println( Welcome to Java ); output.close(); } catch (java.io.ioexception ex){ System.out.println(ex.toString()); ex.printstacktrace() ; } Must execute output.close() even if Exception occurs

45 45 Solution Use the finally clause for code that must be executed no matter what

46 46 Solution Use the finally clause for code that must be executed no matter what try { // Statement(s) which throw Exceptions } catch (Exception1 exception1) { // Handles Exceptions of type Exception1 } catch (Exception exception) { // Handles Exceptions of type Exception } finally { } // code executed whether there is an Exception or not

47 47 Solution Example: java.io.printwriter output = null; try { output = new java.io.printwriter( text.txt ); output.println( Welcome to Java ); } catch (java.io.ioexception ex){ ex.printstacktrace() ; } finally { if (output!= null) output.close(); }

48 48 The finally block Executed when try block is exited in these ways:

49 49 The finally block Executed when try block is exited in these ways:! After last statement of try block

50 50 The finally block Executed when try block is exited in these ways:! After last statement of try block! After last statement of the catch block (if an Exception was caught)

51 51 The finally block Executed when try block is exited in these ways:! After last statement of try block! After last statement of the catch block (if an Exception was caught)! When an Exception is thrown in try but NOT caught

52 52 The finally block Executed when try block is exited in these ways:! After last statement of try block! After last statement of the catch block (if an Exception was caught)! When an Exception is thrown in try but NOT caught Executed even if there is a return statement prior to reaching the finally block

53 53 Throwing Exceptions If written code could encounter a runtime error:

54 54 Throwing Exceptions If written code could encounter a runtime error: It creates an Exception object and throws it

55 55 Throwing Exceptions If written code could encounter a runtime error: It creates an Exception object and throws it and must also declare it in the method description

56 56 Throwing Exceptions If written code could encounter a runtime error: It creates an Exception object and throws it and must also declare it in the method description! Only if the Exception is a checked Exception! Optional for unchecked

57 57 Throwing Exceptions Example: public void setradius(double newradius) { if (newradius >= 0) radius = newradius; else throw new IllegalArgumentException( "Radius cannot be negative"); }

58 58 Throwing Exceptions Example: OPTIONAL public void setradius(double newradius) throws IllegalArgumentException { } if (newradius >= 0) else radius = newradius; throw new IllegalArgumentException( "Radius cannot be negative");

59 59 Throwing Exceptions } try { Circle c1 = new Circle(5); c1.setradius(-5); catch (IllegalArgumentException ex) { System.out.println(ex); }

60 60 Throwing Exceptions } try { Circle c1 = new Circle(5); c1.setradius(-5); catch (IllegalArgumentException ex) { System.out.println(ex); } Output: Radius cannot be negative

61 61 Creating Custom Exceptions Create custom Exception classes if predefined classes not sufficient

62 62 Creating Custom Exceptions Create custom Exception classes if predefined classes not sufficient To create a custom class:! class should extend Exception

63 63 Creating Custom Exceptions Create custom Exception classes if predefined classes not sufficient To create a custom class:! class should extend Exception! Good practice to add: A default (empty) constructor A constructor with one String parameter

64 64 Creating Custom Exceptions public class InvalidRadiusException extends Exception { } private double radius; public InvalidRadiusException() { } super( Invalid radius! ); public InvalidRadiusException(double radius) { } super("invalid radius! ); this.radius = radius; public double getradius() { } return radius;

65 65 Creating Custom Exceptions Example: public void setradius(double newradius) throws IllegalArgumentException { } if (newradius >= 0) else radius = newradius; throw new IllegalArgumentException( "Radius cannot be negative");

66 66 Creating Custom Exceptions Example: public void setradius(double newradius) throws InvalidRadiusException { } if (newradius >= 0) else radius = newradius; throw new InvalidRadiusException(radius);

67 67 Creating Custom Exceptions } try { Circle c1 = new Circle(5); c1.setradius(-5); catch (IllegalArgumentException ex) { System.out.println(ex); }

68 68 Creating Custom Exceptions } try { Circle c1 = new Circle(5); c1.setradius(-5); catch (InvalidRadiusException ex) { System.out.println( Invalid Radius: + ex.getradius()); }

69 69 Creating Custom Exceptions } try { Circle c1 = new Circle(5); c1.setradius(-5); catch (InvalidRadiusException ex) { System.out.println( Invalid Radius: + ex.getradius()); } Output: Invalid Radius: -5.0

70 70 When to create Custom Exceptions Use the exception classes in the API whenever possible.

71 71 When to create Custom Exceptions Use the exception classes in the API whenever possible. Write your own custom exception class if you answer yes to one of the following:

72 72 When to create Custom Exceptions Use the exception classes in the API whenever possible. Write your own custom exception class if you answer yes to one of the following:! Do you need an exception type that isn t represented by Java?

73 73 When to create Custom Exceptions Use the exception classes in the API whenever possible. Write your own custom exception class if you answer yes to one of the following:! Do you need an exception type that isn t represented by Java?! Would it help users if they can differentiate your exceptions from those thrown by classes from other vendors?

74 74 When to create Custom Exceptions Use the exception classes in the API whenever possible. Write your own custom exception class if you answer yes to one of the following:! Do you need an exception type that isn t represented by Java?! Would it help users if they can differentiate your exceptions from those thrown by classes from other vendors?! Do you want to pass more than just a string to the exception handler?

75 75 When to use Exceptions Use if the event is exception and truly an error

76 76 When to use Exceptions Use if the event is exception and truly an error Do not use it to deal with simple, expected situations Example: try { System.out.println(refVar.toString()); } catch (NullPointerException ex) { System.out.println("refVar is null"); }

77 77 When to use Exceptions Use if the event is exception and truly an error Do not use it to deal with simple, expected situations Example: try { System.out.println(refVar.toString()); } catch (NullPointerException ex) { System.out.println("refVar is null"); } Replace with: if (refvar!= null) System.out.println(refVar.toString()); else System.out.println("refVar is null");

Chapter 12 Exception Handling

Chapter 12 Exception Handling 1 Motivations Goal: Robust code. When a program runs into a runtime error, the program terminates abnormally. How can you handle the runtime error so that the program can