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
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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 checkif (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; }}
UI mixed in computation method
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 stringsbefore input was closed. "); System.exit(-1); }}
Decision to halt without full context
Moral: Separate error detection and handling
• In this example– Let echoLines() 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 contextIO exception not
caught
Handling in mainpublic 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 clausestatic 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• Analogous to specifying class methods in interface – a form of
comments checked by Java
• 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
Checked vs. Unchecked
• Unchecked– No rules
• Checked1. uncaught in method body => acknowledged in method header2. unacknowledged in method header => caught in method body
(from 1) 3. unacknowledged in interface method-header => unacknowledged
in class method-header4. unacknowledged in interface method-header => caught in method
body (from 2 and 3)5. Interface can be used to force method implementations to catch
exceptions
Interface/Class relationshippublic interface StringEnumeration {
...
public String nextElement();
}
public class AnInputStreamScanner implements StringEnumeration {
...
public String nextElement() {
try {
return inputStream.readLine();
} catch (IOException e) {
return “”;
}
}
}
unacknowledged in interface method-header => caught in method body
Interface/Class relationshippublic interface StringEnumeration {
...
public String nextElement();
}
public class AnInputStreamScanner implements StringEnumeration {
...
public String nextElement() throws IOException{
return inputStream.readLine();
}
}
unacknowledged in interface method-header => caught in method body
Interface/Class relationshippublic 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 interfacepackage 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 exceptionspublic 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
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 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 message
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 Duplicationtry {
echoLines(numberOfInputLines(args));
} catch (IOException e) {
System.out.println(e);
System.exit(-1);
}
AMissingArgumentException is subclass
Removing Code Duplicationtry {
echoLines(numberOfInputLines(args));
} catch (IOException e) {
System.out.println(e);
System.exit(-1);
} catch (AMissingArgumentException e) {
System.out.println(e);
} AMissingArgumentException processed hereList exception subclass
before superclass Unreachable block
Interface/Class relationshippublic 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 relationshippublic 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 relationshippublic 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 relationshippublic 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 relationshippublic 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 relationshippublic 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
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 (AMissingArgumentException e) {
}
}
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
• Should overstate rather than understate bad side effects– Dizziness, headache
• 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 loopBad style, exception handler processes expected
eventmore efficient: no extra if
check
Intra-method propagationwhile (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– Scanning: 5 ( 2 } 1
• Dependent errors cannot be: – Parsing: 5 + 2 4 / - 2
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