Chapter 5. Exceptions. CSC 113 King Saud University College of Computer and Information Sciences Department of Computer Science. Dr. S.

Transcription

1 Chapter 5 Exceptions CSC 113 King Saud University College of Computer and Information Sciences Department of Computer Science Dr. S. HAMMAMI

2 Objectives After you have read and studied this chapter, you should be be able to to Improve the the reliability of of code by by incorporating exception-handling and and assertion mechanisms. Write methods that that propagate exceptions. Implement the the try-catch blocks for for catching and and handling exceptions. Write programmer-defined exception classes. Distinguish the the checked and and unchecked, or or runtime, exceptions.

3 Introduction to Exception Handling No No matter matter how how well well designed a a program is, is, there there is is always always the the chance chance that that some some kind kind of of error error will will arise arise during during its its execution. A well-designed program should should include include code code to to handle handle errors errors and and other other exceptional conditions when when they they arise. arise. Sometimes the the best best outcome can can be be when when nothing nothing unusual happens However, the the case case where where exceptional things things happen must must also also be be prepared for for Java Java exception handling facilities are are used used when when the the invocation of of a method may may cause cause something exceptional to to occur occur

4 Introduction to Exception Handling Java library software (or (or programmer-defined code) provides a mechanism that signals when something unusual happens This This is is called throwing an an exception In In another place in in the program, the programmer must provide code that deals with the exceptional case This This is is called handling the the exception

5 Definition An exception represents an error condition that can occur during the normal course of of program execution. When an exception occurs, or or is is thrown, the normal sequence of of flow is is terminated. The exception-handling routine is is then executed; we say the thrown exception is is caught.

6 Not Catching Exceptions The avgfirstn() method expects that N > If If N = 0, 0, a divide-by-zero error occurs in in avg/n. /** * Precondition: N > 0 * Postcondition: avgfirstn() equals the average of (1+2+ +N) */ public double avgfirstn(int N) duuble sum = 0; for (int k = 1; k <= N; k++) sum += k; return sum/n; // What if N is 0?? // avgfirstn() Bad Design: Doesn t guard against divide-by-0.

7 class class AgeInputVer1 AgeInputVer1 private private int intage; public public void void setage(string setage(strings) s) age age = Integer.parseInt(s); Integer.parseInt(s); public public int intgetage() return return age; age; Not Catching Exceptions public public class class AgeInputMain1 AgeInputMain1 public public static static void void main( main( String[] String[] args args )) AgeInputVer1 AgeInputVer1 P = new new AgeInputVer1( AgeInputVer1( ); ); P.setAge("nine"); P.setAge("nine"); System.out.println(P.getAge()); Error message for invalid input Exception Exception in in thread thread "main" "main" java.lang.numberformatexception: java.lang.numberformatexception: For For input input string: string: "nine" "nine" at at java.lang.numberformatexception.forinputstring(unknown java.lang.numberformatexception.forinputstring(unknown Source) Source) at at java.lang.integer.parseint(unknown java.lang.integer.parseint(unknown Source) Source) at at java.lang.integer.parseint(unknown java.lang.integer.parseint(unknown Source) Source) at at AgeInputVer1.setAge(AgeInputVer1.java:5) AgeInputVer1.setAge(AgeInputVer1.java:5) at at AgeInputMain1.main(AgeInputMain1.java:8) AgeInputMain1.main(AgeInputMain1.java:8)

8 Not Catching Exceptions class class AgeInputVer1 AgeInputVer1 private private int intage; public public void void setage(string setage(strings) s) age age = Integer.parseInt(s); Integer.parseInt(s); public public int intgetage() return return age; age; public public class class AgeInputMain2 AgeInputMain2 public public static static void void main( main( String[] String[] args args )) AgeInputVer1 AgeInputVer1 P = new new AgeInputVer1( AgeInputVer1( ); ); P.setAge("9"); P.setAge("9"); System.out.println(P.getAge()); 9

9 Java s Exception Hierarchy Unchecked exceptions: belong to a subclass of RuntimeException and are not monitored by the compiler.

10 Some Important Exceptions Class ArithmeticException ArithmeticException ArrayIndexOutOfBounds- ArrayIndexOutOfBounds- FileNotFoundException FileNotFoundException IndexOutOfBoundsException IndexOutOfBoundsException NullPointerException NullPointerException NumberFormatException NumberFormatException StringIndexOutOfBoundsException StringIndexOutOfBoundsException Description Division Division by by zero zero or or some some other other kind kind of of arithmetic arithmetic problem problem An An array array index index is is less less than than zero zero or or Exception Exception greater greater than than or or equal equal to to the the array's array's length length Reference Reference to to a a unfound unfound file file IllegalArgumentException IllegalArgumentException Method Method call call with with improper improper argument argument An An array array or or string string index index out out of of bounds bounds Reference Reference to to an an object object which which has has not not been been instantiated instantiated Use Use of of an an illegal illegal number number format, format, such such as as when when calling calling a a method method A String String index index less less than than zero zero or or greater greater than than or or equal equal to to the the String's String's length length

11 Catching an Exception try catch class class AgeInputVer2 AgeInputVer2 private private int int age age public public void void setage(string setage(string s) s) try try age age = Integer.parseInt(s); catch catch (NumberFormatException e) e) System.out.Println( age System.out.Println( age is is invalid, invalid, Please Please enter enter digits digits only"); only"); public public int int getage() getage() return return age; age; We are catching the number format exception, and the parameter e represents an instance of the NumberFormatException class

12 Catching an Exception To accomplish this repetition, we will put the whole try-catch statement in side a loop: import import java.util.scanner; java.util.scanner; class class AgeInputVer3 AgeInputVer3 private private int int age; age; public public void void setage(string setage(string s) s) String String m =s; =s; Scanner Scanner input input = = new new Scanner(System.in); Scanner(System.in); boolean boolean ok ok = = true; true; while while (ok) (ok) try try age age = = Integer.parseInt(m); Integer.parseInt(m); ok ok = = false; false; catch catch (NumberFormatException (NumberFormatException e) e) System.out.println( age System.out.println( age is is invalid, invalid, Please Please enter enter digits digits only"); only"); m = = input.next(); input.next(); public public int int getage() getage() return return age; age; This This statement statement is is executed executed only only if if no no exception exception is is thrown thrown by by parseint. parseint.

13 try-catch Control Flow

14 The Exception Class: Getting Information There are two methods we can call to get information about the thrown exception: getmessage printstacktrace Simple: only constructor methods.

15 try... The Exception Class: Getting Information catch (NumberFormatException e) We are catching the number format exception, and the parameter e represents an instance of the NumberFormatException class System.out.println(e.getMessage()); System.out.println(e.printStackTrace()); For input string: "nine" java.lang.numberformatexception: For input string: "nine" at java.lang.numberformatexception.forinputstring(unknown Source) at java.lang.integer.parseint(unknown Source) at java.lang.integer.parseint(unknown Source) at AgeInputVer1.setAge(AgeInputVer1.java:11) at AgeInputMain1.main(AgeInputMain1.java:8)

16 try-throw-catch Mechanism throw thrownew new ExceptionClassName(PossiblySomeArguments); When When an an exception exception is is thrown, thrown, the the execution execution of of the the surrounding surrounding try tryblock is is stopped stopped Normally, Normally, the the flow flow of of control control is is transferred transferred to to another another portion portion of of code code known known as as the the catch catchblock The The value value thrown thrown is is the the argument argument to to the the throw throwoperator, operator, and and is is always always an an object object of of some some exception exception class class The The execution execution of of a a throw throwstatement is is called called throwing throwing an an exception exception

17 try-throw-catch Mechanism A throw throw statement statement is is similar similar to to a a method method call: call: throw throw new new ExceptionClassName(SomeString); In In the the above above example, example, the the object object of of class class ExceptionClassName ExceptionClassNameis is created created using using a a string string as as its its argument argument This This object, object, which which is is an an argument argument to to the the throw throw operator, operator, is is the the exception exception object object thrown thrown Instead Instead of of calling calling a a method, method, a a throw throw statement statement calls calls a a catch catch block block

18 try-throw-catch Mechanism When When an an exception exception is is thrown, thrown, the the catch catch block block begins begins execution execution The The catch catch block block has has one one parameter parameter The The exception exception object object thrown thrown is is plugged plugged in in for for the the catch catch block block parameter parameter The The execution execution of of the the catch catch block block is is called called catching catching the the exception, exception, or or handling handling the the exception exception Whenever Whenever an an exception exception is is thrown, thrown, it it should should ultimately ultimately be be handled handled (or (or caught) caught) by by some some catch catch block block

19 try-throw-catch Mechanism When When a try tryblock is is executed, two two things things can can happen: No No exception is is thrown thrown in in the the try try block block The The code code in in the the try tryblock is is executed to to the the end end of of the the block block The The catch catchblock is is skipped The The execution continues with with the the code code placed placed after after the the catch catchblock An An exception is is thrown thrown in in the the try try block block and and caught caught in in the the catch catch block block The The rest rest of of the the code code in in the the try try block block is is skipped Control Control is is transferred to to a following catch catch block block (in (in simple simple cases) cases) The The thrown thrown object object is is plugged in in for for the the catch catch block block parameter The The code code in in the the catch catch block block is is executed The The code code that that follows follows that that catch catch block block is is executed (if (if any) any)

20 public class CalcAverage public double avgfirstn(int N ) double sum = 0; try if (N <=0) throw new Exception("ERROR: Can't average 0 elements"); for (int k = 1; k <= N; k++) sum += k; return sum/n; public class CalcAverage public double avgfirstn(int N ) double sum = 0; try if (N <0) throw new Exception("ERROR: Can't average negative elements"); for (int k = 1; k <= N; k++) sum += k; return sum/n; catch(arithmeticexception e) System.out.println(e.getmessage()); System.out.println( N=Zero is an valid denomiattor, please try again "); catch (Exception e) System.out.println e.getmessage() + Please enter positive integer );

21 Multiple catch Blocks A try tryblock can can potentially throw any any number of of exception values, and and they they can can be be of of differing types In In any any one one execution of of a try tryblock, at at most most one one exception can can be be thrown (since a throw statement ends ends the the execution of of the the try try block) However, different types of of exception values can can be be thrown on on different executions of of the the try tryblock Each Each catch block can can only only catch values of of the the exception class class type type given in in the the catch block heading Different types of of exceptions can can be be caught by by placing more more than than one one catch block after after a try tryblock Any Any number of of catch blocks can can be be included, but but they they must must be be placed in in the the correct order

22 Multiple catch Blocks

23 Multiple catch Control Flow

24 Multiple catch Control Flow: Example import import java.util.*; java.util.*; //InputMismatchException; //InputMismatchException; //import //import java.util.arithmeticexception; java.util.arithmeticexception; public public class class DividbyZero2 DividbyZero2 public public static static void void main main (String (String args[]) args[]) //throws //throws ArithmeticException ArithmeticException Scanner Scanner input input = = new new Scanner(System.in); Scanner(System.in); boolean boolean done=false; done=false; do do try try System.out.print("Please System.out.print("Please enter enter an an integer integer number number : : "); "); int int a a =input.nextint(); =input.nextint(); System.out.print("Please System.out.print("Please enter enter an an integer integer number number : : "); "); int int b b =input.nextint(); =input.nextint(); int int c=a/b; c=a/b; System.out.println("a System.out.println("a ="+ ="+ a a +" +" b= b= "+b+ "+b+ " " amd amd quotient quotient ="+c); ="+c); done=true; done=true; catch (InputMismatchException var1 ) catch (InputMismatchException var1 ) System.out.println("Exception :"+ var1); System.out.println("Exception :"+ var1); System.out.println("please try again: "); System.out.println("please try again: "); catch(arithmeticexception var2) catch(arithmeticexception var2) System.out.println("\nException :"+ var2); System.out.println("\nException :"+ var2); System.out.println("Zero is an valid denomiattor"); System.out.println("Zero is an valid denomiattor"); while(!done); while(!done); Please enter an integer number : car Please enter an integer number : car Exception :java.util.inputmismatchexception Exception :java.util.inputmismatchexception You must enter an integer value, please try again: You must enter an integer value, please try again: Please enter an integer number : 14 Please enter an integer number : 14 Please enter an integer number : 0 Please enter an integer number : 0 Exception :java.lang.arithmeticexception: / by zero Exception :java.lang.arithmeticexception: / by zero Zero is an valid denomiattor, please try again Zero is an valid denomiattor, please try again Please enter an integer number : 7 Please enter an integer number : 7 Please enter an integer number : 3 Please enter an integer number : 3 a =7 b= 3 amd quotient =2 a =7 b= 3 amd quotient =2

25 Pitfall: Catch the More Specific Exception First When catching multiple exceptions, the order of of the catch blocks is is important When an an exception is is thrown in in a try block, the catch blocks are examined in in order The first one that matches the type of of the exception thrown is is the one that is is executed

26 The finally Block

27 try-catch-finally Control Flow

28 try-catch-finally Control Flow If If the the try-catch-finally blocks are are inside a method definition, there are are three possibilities when the the code is is run: run: The Thetry tryblock runs runs to to the the end, end, no no exception is is thrown, thrown, and and the the finally block block is is executed An An exception is is thrown thrown in in the thetry tryblock, caught caught in in one one of of the the catch catchblocks, and and the thefinally block block is is executed An An exception is is thrown thrown in in the thetry tryblock, there there is is no no matching catch catchblock in in the the method, the thefinally block block is is executed, and and then then the the method invocation ends ends and and the the exception object object is is thrown thrown to to the the enclosing method

29 Propagating Exceptions Throwing an Exception in a Method Sometimes it it makes sense to to throw an an exception in in a method, but but not not catch it it in in the the same method Some Some programs that that use use a method should should just just end end if if an an exception is is thrown, thrown, and and other other programs should should do do something else else In In such such cases, cases, the the program using using the the method should should enclose the the method invocation in in a try try block, block, and and catch catch the the exception in in a catch catch block blockthat follows follows In In this this case, case, the the method itself itself would would not not include include try tryand catch catchblocks However, it it would would have have to to include include a throws throwsclause

30 Declaring Exceptions in a throws Clause If If a method can can throw an an exception but but does not not catch it, it, it it must provide a warning This This warning is is called called a throws throwsclause The The process of of including an an exception class class in in a throws throws clause clause is is called called declaring the the exception throws throws AnException //throws clause clause The The following states states that that an an invocation of of amethod could could throw throw AnException public public void void amethod() throws throws AnException

31 Declaring Exceptions in a throws Clause If If a method can throw more than one type of of exception, then separate the exception types by by commas public void void amethod() throws AnException,AnotherException If If a method throws an an exception and does not catch it, it, then the method invocation ends immediately

32 Exception propagation and throws clause Method Method getdepend() may may throw throw a a number number format format exception when when converting a a string string to to an an integer, integer, but but it it does does not not catch catch this this exception. The The call call to to getdepend() occurs occurs in in the the try try block block of of method method main(), main(), so so main() main() handles handles the the exception in in its its catch catch block. block. If If main() main() did did not not have have a a catch catch block block for for number number format format exceptions, the the exception would would be be handled handled by by the the JVM. JVM. // // postcondition: Returns int value of a numeric data string. // postcondition: Throws an exception Returns int if string value is of not a numeric numeric. data string. // Throws an exception if string is not numeric. public public static static int int getdepend() getdepend() throws throws NumberFormatException NumberFormatException String String numstr numstr = = jnputnext(); jnputnext(); return return Integer.parseInt(numStr); Integer.parseInt(numStr); // // postcondition: postcondition: Calls Calls getdepend() getdepend() and and handles handles its its exceptions. exceptions. public public static static void void main(string[] main(string[] args) args) int int children children = = 1; 1; // // problem problem input, input, default default is is 1 1 try try children = getdepend(); children = getdepend(); catch catch (NumberFormatException (NumberFormatException ex) ex) // // Handle Handle number number format format exception. exception. System.out.println("Invalid System.out.println("Invalid integer integer + + ex); ex);

33 Exception Thrower When a method may throw an exception, either directly or or indirectly, we call the method an exception thrower. Every exception thrower must be one of of two types: catcher. propagator.

34 Types of Exception Throwers An exception catcher is is an exception thrower that includes a matching catch block for the thrown exception. An exception propagator does not contain a matching catch block. A method may be a catcher of of one exception and a propagator of of another.

35 Sample Call Sequence Method A calls calls method B, B, Method B calls calls method C, C, Method C calls calls method D. D. Every time a method is executed, the method s name is placed on top of the stack.

36 Sample Call Sequence When an an exception is is thrown, the the system searches down the the stack from from the the top, top, looking for for the the first first matching exception catcher. Method D throws an an exception, but but no nomatching catch block exits exits in in the the method, so so method D is is an an exception propagator. The system then then checks method C. C. C is is also also an an exception propagator. Finally, the the system locates the the matching catch block in in method B, B, and and therefore, method B is is the the catcher for for the the exception thrown by by method D. D. Method A also also includes the the matching catch block, but but it it will will not not be be executed because the the thrown exception is is already caught by by method B and and method B does does not not propagate this this exception. void C() throws Exception. void D() throws Exception.

37 Example Consider Consider the the Fraction Fraction class. class. The The setdenominator setdenominatormethod method of of the the Fraction Fraction class class was was defined defined as as follows: follows: Throwing Throwing an an exception exception is is a a much much better better approach. approach. Here s Here s the the modified modified method method that that throws throws an an IlleglArgumentException when when the the value value of of 00 is is passed passed as as an an argument: argument: public public void voidsetdenominator (int (intd) d) if if (d (d = = 0) 0) System.out.println( Fatal Error ); Error ); System.exit(1); System.exit(1); denominator denominator = d; d; public public void voidsetdenominator (int (intd) d) throws throwsilleglargumentexception if if (d (d = = 0) 0) throw throw new newilleglargumentexception ( Fatal ( Fatal Error ); Error ); denominator denominator = d; d;

38 Programmer-Defined Exception Classes A throw statement can throw an exception object of any exception class Instead of using a predefined class, exception classes can be programmerdefined These can be tailored to carry the precise kinds of information needed in the catch block A different type of exception can be defined to identify each different exceptional situation Every exception class to be defined must be a sub-class of some already defined exception class It can be a sub-class of any exception class in the standard Java libraries, or of any programmer defined exception class Constructors are the most important members to define in an exception class They must behave appropriately with respect to the variables and methods inherited from the base class Often, there are no other members, except those inherited from the base class The following exception class performs these basic tasks only

39 A Programmer-Defined Exception Class

40 Programmer-Defined Exception Class Guidelines Exception classes may be programmer-defined, but every such class must be a derived class of an already existing exception class The class Exception can be used as the base class, unless another class would be more suitable At least two constructors should be defined, sometimes more The exception class should allow for the fact that the method getmessage is inherited

41 Programmer-Defined Exceptions: AgeInputException class AgeInputException extends Exception private static final String DEFAULT_MESSAGE = "Input out of bounds"; private int lowerboun, upperbound, value; public AgeInputException(int low, int high, int input) this(default_message, low, high, input); public AgeInputException(String msg, int low, int high, int input) super(msg); if (low > high) throw new IllegalArgumentException(); lowerbound = low; upperbound = high; value = input; public int lowerbound() return lowerbound; public int upperbound() return upperbound; public int value() return value;

42 Class AgeInputVer5 Uses AgeInputException import java.util.scanner; class AgeInputVer5 private static final String DEFAULT_MESSAGE = "Your age:"; private static final int DEFAULT_LOWER_BOUND = 0; private static final int DEFAULT_UPPER_BOUND = 99; private int lowerbound, upperbound; public AgeInputVer5( ) throws IllegalArgumentException setbounds(default_lower_bound, DEFAULT_UPPER_BOUND); public AgeInputVer5(int low, int high) throws IllegalArgumentException if (low > high) throw new IllegalArgumentException( "Low (" + low + ") was " + "larger than high(" + high + ")"); else setbounds(low, high); public int getage() throws AgeInputException return getage(default_message);

43 public int getage(string prompt) throws AgeInputException Scanner T = new Scanner(System.in); String inputstr; int age; while (true) inputstr = prompt; try age = Integer.parseInt(inputStr); if (age < lowerbound age > upperbound) throw new AgeInputException("Input out of bound ", lowerbound, upperbound, age); return age; //input okay so return the value & exit catch (NumberFormatException e) System.out.println("\n"+ inputstr + " is invalid age."); System.out.print("Please enter age as an integer value : "); prompt = T.next()+T.nextLine(); private void setbounds(int low, int high) lowerbound = low; upperbound = high;

44 Main Using throws public class TestAgeInputUsingThrows public static void main( String[] args ) throws AgeInputException int entrantage=0; AgeInputVer5 input = new AgeInputVer5(25, 50); entrantage = input.getage("thirty"); System.out.println("Input Okay "); Thirty is invalid age. Please enter age as an integer value : fourty fourty is invalid age. Please enter age as an integer value : 40 Input Okay Thirty is invalid age. Please enter age as an integer value : fourty fourty is invalid age. Please enter age as an integer value : 55 Exception in thread "main" AgeInputException: Input out of bound at AgeInputVer5.getAge(AgeInputVersion5.java:42) at TestAgeInputVer5.main(TestAgeInputVer5.java:7)

45 public class Test2AgeInput Main Using try-catch public static void main( String[] args ) int entrantage; try AgeInputVer5 input = new AgeInputVer5(25, 50); entrantage = input.getage("thirty"); System.out.println("Input Okay "); catch (AgeInputException e) System.out.println("Error: " + e.value() + " is entered. It is " + "outside the valid range of [" + e.lowerbound() +", " + e.upperbound() + "]"); Thirty is invalid age. Please enter age as an integer value : fourty fourty is invalid age. Please enter age as an integer value : 40 Input Okay Thirty is invalid age. Please enter age as an integer value : fourty fourty is invalid age. Please enter age as an integer value : 55 Error: 55 is entered. It is outside the valid range of [25, 50]