Exceptions Problems with error reporting so far –Either ignored exceptions or terminated program on first error. –Error handling and regular code mixed –User interface and computation code mixed

Argument Printer package main; public class AnArgPrinter{ public static void main(String args[]) { System.out.println(args[0]); } Exception will be reported to the user

Error check if (args.length == 0 ) { System.out.println("Did not specify the argument to be printed. Terminating program."); System.exit(-1); } else { System.out.println(args[0]); } Regular and error code mixed together

Exception handler try { System.out.println(args[0]); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Did not specify the argument to be printed. Terminating program."); System.exit(-1); } Regular and error code separate

Printing multiple arguments: main public static void main (String args[]) { echoLines(numberOfInputLines(args)); }

numberOfInputLines static int numberOfInputLines(String[] args) { try { return Integer.parseInt(args[0]); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Did not enter an argument.") return 0; } Computation and UI mixed Arbitrary legal value returned, program not halted

echoLines static void echoLines (int numberOfInputLines) { try { for (int inputNum = 0; inputNum < numberOfInputLines; inputNum++) System.out.println(inputStream.readLine()); } catch (IOException e) { System.out.println("Did not input " + numberOfInputLines + " input strings before input was closed. "); System.exit(-1); } Decision to halt without full context

Moral: Separate error detection and handling In this example –Let echo lines and numberOfInputLines not do the error handling. –All they do is error reporting –Main does error handling and associated UI

Error code solution Pass back error codes to main –Works for procedures as we can make it return value instead of void –Does not work for functions as error code may be legal return value Integer function returning all possible integer values

Global variable solution Store error codes in common variables –Does not work when there are multiple calls to the same method A call may overwrite value written by another call –Variable may accessed by other methods sharing its scope

Exception propagation Java lets exceptions be “returned instead of regular values. These propagate through call chain until some method handles them

Propagating echoLines static void echoLines (int numberOfInputLines) throws IOException { for (int inputNum = 0; inputNum < numberOfInputLines; inputNum++) System.out.println(inputStream.readLine( )); } Tells caller that passing it the exception

Propagating numberOfInputLines static int numberOfInputLines(String[] args) throws ArrayIndexOutOfBoundsException { return Integer.parseInt(args[0]); }

Handling in main public static void main (String args[]) { try { echoLines(numberOfInputLines(args)); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Did not enter an argument. Assuming a single input line.”); echoLines(1); } catch (IOException e) { System.out.println("Did not input the correct number of input strings before input was closed. "); } Has context IO exception not caught

Handling in main public static void main (String args[]) { try { echoLines(numberOfInputLines(args)); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Did not enter an argument. Assuming a single input line.”); try { echoLines(1); } catch (IOException ioe) { System.out.println("Did not input the one input string, which is the default in case of missing argument, before input was closed. "); } } catch (IOException e) { System.out.println("Did not input the correct number of input strings before input was closed. "); } Must be different names

Passing the buck in main public static void main (String args[]) throws IOException, ArrayIndexOutOfBoundsException { echoLines(numberOfInputLines(args)); } Bad idea as interpreter’s messages may be meaningless to the user

Omitting IOException in throws clause static void echoLines (int numberOfInputLines) { for (int inputNum = 0; inputNum < numberOfInputLines; inputNum++) System.out.println(inputStream.readLine( )); } Java complains IOException neither handled nor declared Caller does not know what it must handle

Omitting ArrayIndexOutOfBoundsException in throws clause static int numberOfInputLines(String[] args) { return Integer.parseInt(args[0]); } No complaints from Java

Java has two kinds of exceptions Unchecked exceptions –Called “runtime”, but all exceptions are runtime! –Subclasses of RunTimeException E.g. ArrayIndexOutofBoundsException –Uncaught exceptions need not be declared Checked exceptions –Uncaught exceptions must be declared Rationale for division?

Misleading header static void safeArrayIndexer throws ArrayIndexOutOfBoundsException () { String args[] = {“hello”, “goodbye”}; System.out.println(args[1]); } Array index does not imply exception Java cannot tell the difference Array index out of bounds guaranteed to not happen

Reasons for exceptions User error –Programmer cannot prevent it –Should be acked Internal error –Programmer can prevent –A method that can be erroneous probably is not really erroneous –Acking is probably misleading

Justification of Java Rules Java rules justified if: Checked (Non-runtime) exceptions = user errors Unchecked (runtime) exceptions = internal errors

Problems with Java rules Unchecked exceptions can be caused by user error static int numberOfInputLines(String[] args) { return Integer.parseInt(args[0]); }

Approach 1: Voluntarily list exception static int numberOfInputLines(String[] args) throws ArrayIndexOutOfBoundsException { return Integer.parseInt(args[0]); } No way to force every caller that does not handle it to ack it.

Approach 2: Convert to checked exception static int numberOfInputLines(String[] args) throws IOException { try { return Integer.parseInt(args[0]); } catch (ArrayIndexOutOfBoundsException e) { throw new IOException (“First argument missing”); } Exception object thrown explicitly message

Checked vs. Unchecked Unchecked –No rules Checked –uncaught in method body => acknowledged in method header –unacknowledged in method header => caught in method body –unacknowledged in interface method-header => unacknowledged in class method-header –Interface can be used to force method implementations to catch exceptions

Interface/Class relationship public interface StringEnumeration {... public String nextElement(); } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() { try { return inputStream.readLine(); } catch (IOException e) { return “”; }

Interface/Class relationship public interface StringEnumeration {... public String nextElement(); } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() throws IOException{ return inputStream.readLine(); }

Interface/Class relationship public interface StringEnumeration {... public String nextElement(); } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() throws java.util.NoSuchElementException { if (!hasMoreElements()) throw new java.util.NoSuchElementException(); return inputStream.readLine(); } Unchecked, as most users will call hasMoreElements before nextElement()

Standard Enumeration interface package java.util; public interface Enumeration {... public Object nextElement(); } public class AnInputStreamScanner implements java.util.Enumeration{... public Object nextElement() throws java.util.NoSuchElementException { if (!hasMoreElements()) throw new java.util.NoSuchElementException(); return inputStream.readLine(); } Good idea to throw this exception when no more elements

Throwing multiple exceptions public interface StringEnumeration {... public String nextElement() throws java.io.IOException; } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() throws java.util.NoSuchElementException, java.io.IOException { if (!hasMoreElements()) throw new java.util.NoSuchElementException(); return inputStream.readLine(); } When no more elements When next element erroneous because of user error (e.g. scanning rules violated)

Printing debugging information catch (ArrayIndexOutOfBoundsException e) { System.out.println(e); } catch (ArrayIndexOutOfBoundsException e) { e.printStackTrace(); } e.getMessage() Stack when exception is thrown

Printing stack Stack that existed when exception was thrown

Exceptions in initialization int numberOfInputLines = numberOfInputLines() –Checked exception not being handled or acknowledged Do initialization in method –public static void main (String args) { try { numberOfInputLines = numberOfInputLines() } catch (IOException e) { } }

Approach 2: Convert to existing checked exception static int numberOfInputLines(String[] args) throws IOException { try { return Integer.parseInt(args[0]); } catch (ArrayIndexOutOfBoundsException e) { throw new IOException(“First argument missing”); } Exception object thrown explicitly

Approach 3: Convert to new checked exception static int numberOfInputLines(String[] args) throws IOException { try { return Integer.parseInt(args[0]); } catch (ArrayIndexOutOfBoundsException e) { throw new AMissingArgumentException(“First argument missing”); } Our own exception

Creating Exception Class public class AMissingArgumentException extends java.io.IOException { public AMissingArgumentException(String message) { super(message); } No interface! Not adding any methods

Checked vs. Unchecked Programmer-defined Exceptions An exception class must be subclass of existing exception classes Subclass of RunTimeException is unchecked All other exceptions are checked Java does not define exception interfaces Neither can we as a result

Handling programmer-defined exceptions try { echoLines(numberOfInputLines(args)); } catch (AMissingArgumentException e) { System.out.println(e); System.exit(-1); } catch (IOException e) { System.out.println(e); System.exit(-1); }

Removing Code Duplication try { echoLines(numberOfInputLines(args)); } catch (IOException e) { System.out.println(e); System.exit(-1); } AMissingArgumentException is subclass

Removing Code Duplication try { echoLines(numberOfInputLines(args)); } catch (IOException e) { System.out.println(e); System.exit(-1); } catch (AMissingArgumentException e) { System.out.println(e); } AMissingArgumentException processed here List exception subclass before superclass Unreachable block

Interface/Class relationship public interface StringEnumeration { public boolean hasMoreElements(); public String nextElement() throws Exception; } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() throws IOException{ return inputStream.readLine(); } Stronger advertisement allowed void print (StringEnumeration stringEnumeration) { try { while (stringEnumeration.hasMoreElements()) System.out.println(stringEnumeration.nextElement()); } catch (Exception e) {…} } Can handle IOException

Interface/Class relationship public interface StringEnumeration {... public String nextElement() throws IOException; } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() { try { return inputStream.readLine(); } catch (IOException e) { return “”; } Stronger advertisement allowed void print (StringEnumeration stringEnumeration) { try { while (stringEnumeration.hasMoreElements()) System.out.println(stringEnumeration.nextElement()); } catch (IOException e) {…} }

Interface/Class relationship public interface StringEnumeration { public boolean hasMoreElements(); public String nextElement() throws IOException; } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() throws Exception { return inputStream.readLine() + … ; } Weaker advertisement not allowed void print (StringEnumeration stringEnumeration) { try { while (stringEnumeration.hasMoreElements()) System.out.println(stringEnumeration.nextElement()); } catch (IOException e) {…} } Cannot handle Exception

Interface/Class relationship public interface StringEnumeration { public boolean hasMoreElements(); public String nextElement(); } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() throws IOException{ return inputStream.readLine(); } Weaker advertisement not allowed void print (StringEnumeration stringEnumeration) { while (stringEnumeration.hasMoreElements()) System.out.println(stringEnumeration.nextElement()); } Not handling IOException

Implementation/body relationship public interface StringEnumeration { public boolean hasMoreElements(); public String nextElement() throws Exception; } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() throws Exception { return inputStream.readLine(); } void print (StringEnumeration stringEnumeration) { try { while (stringEnumeration.hasMoreElements()) System.out.println(stringEnumeration.nextElement()); } catch (Exception e) {…} } Can handle IOException Stronger advertisement allowed

Implementation/body relationship public interface StringEnumeration { public boolean hasMoreElements(); public String nextElement() throws AMissingArgumentException; } public class AnInputStreamScanner implements StringEnumeration {... public String nextElement() throws AMissingArgumentException { return inputStream.readLine(); } void print (StringEnumeration stringEnumeration) { try { while (stringEnumeration.hasMoreElements()) System.out.println(stringEnumeration.nextElement()); } catch (AMissingArgumentException e) {…} } Cannot handle IOException Weaker advertisement not allowed

IS-A Rule for Exceptions Exception of type T1 uncaught in method body => exception of type T2, where T1 IS-A T2, acknowledged in method header Exception of type T1 acknowledged in interface method-header => exception of type T2, where T2 IS-A T1, acknowledged in class method-header If you are bad, you should not say you are good. –People will be disappointed If you are good, you can say you are bad –Don’t let people down

Catching expected events try { for (;;) { String s = inputStream.readLine(); process(s); } } catch (IOException e) { }; Using EOF to terminate the loop Bad style, exception handler processes expected event more efficient: no extra if check

Intra-method propagation while (enumeration.hasMoreElements()) try { System.out.println((String) enumeration.nextElement()); } catch (ClassCastException e) { e.printStackTrace());} try { while (enumeration.hasMoreElements()) System.out.println((String) enumeration.nextElement()); } catch (ClassCastException e) {e.printStackTrace());} println terminated and exception propagated to enclosing loop, which is also terminated, and catch executed Println terminated, catch executed, and loop continues

Terminating loop vs. statement Independent errors can be collected –5 % 2 ^ 1 Dependent errors cannot be: – / - 2