Download exceptions

Programming errors (faults) occur in two basic forms:
Syntax errors
A syntax error is a violation of the notational rules of
the programming language. (synonym: compile-time errors)
Logic errors
A logic error results in potentially faulty behavior.
Some logic errors are detected by the Java VM.
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
A detected error is generally called a run-time error.
Java run-time errors produce exceptions.
An exception is an abnormal condition that occurs during
software execution.
Three Example Exceptions
String str = null;
str = str.toLowerCase();
int j = 0;
int k = 25/j;
double[ ] arr = new double[3];
arr[3] = 29.4;
When an exception occurs, it is said that “the exception is ”___________”.
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
Exception
reports
driley> java Driver
Exception in thread "main" java.lang.NullPointerException
at Driver.main(Driver.java:5)
driley>
String str = null;
str = str.toLowerCase();
driley> java Driver
int j = 0;
int k = 25 / j;
Exception in thread "main" java.lang.ArithmeticException: / by zero
at Driver.main(Driver.java:8)
driley>
double[ ] arr = new double[3];
arr[3] = 29.4;
driley> java Driver
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException
at Driver.main(Driver.java:11)
driley>
The Exception message includes the following information:
• name of the method executing when the exception is thrown.
• type of exception that occurred.
• traceback of all active methods (and class(es), line number(s)).
Exception traceback
driley> java Driver
Exception occurs in
line
of method
in class
Exception in thread "main" java.lang.NullPointerException
at Thing.doSomething(Thing.java:9)
at Thing.<init>(Thing.java:5)
at Driver2.main(Driver2.java:3)
doSomething called from
driley>
line
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
of
constructor <init>
Thing constructor called from
line
of
method of
class.
It is also possible to simulate an exception by executing a throw instruction.
throw ExceptionObject;
where ExceptionObject is an object reference conforming to Exception
(Exception is a class from java.lang.)
Example
public class SimpleFraction {
private int numerator, denominator;
public class runFractTest {
public SimpleFraction(int n, int d) {
IllegalArgumentException error;
if (d != 0) {
numerator = n;
denominator = d;
} else {
error = new IllegalArgumentException(
“Fraction denominator of 0”);
throw error;
}
}
public double realValue() {
return numerator / denominator;
}
...
The Object of Data Abstraction
public static void main(String[ ] args) {
SimpleFraction myFrac;
myFrac = new SimpleFraction(1, 2);
System.out.println(myFrac.realValue());
myFrac = new SimpleFraction(1, 0);
System.out.println(myFrac.realValue());
}
}
}
and Structure, David D. Riley
© Addison Wesley pub.
program
executing
normally
exception
is thrown
program
is interrupted
exception
handler
executes
program
terminates
???
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
try Syntax
try {
tryInstructionBody
} catchClauses
finallyClause
catchClauses Syntax (zero or more repetitions of the following)
catch (exceptionClassName parmName ) {
exceptionHandlerBody
}
finallyClause Syntax (optional)
finally {
finallyBody
}
...Body denotes a sequence of instructions.
exceptionClassName - class conforming to Throwable.
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
parmName - serves as a parameter passed into
try Semantics
 Execution of the try begins by
executing the tryInstructionBody.
Trace the following:
System.out.println( “La Crosse” );
try {
 During the execution of tryInstructionBody
System.out.println( “River Falls” );
the catchClauses serve as exception handlers. String str;
System.out.println( str.trim() );
The appropriate catchClause is selected by
System.out.println( “Eau Claire” );
conformance to a catchClauseName
}
catch ( NullPointerException e) {
System.out.println( “Platteville” );
 If a finallyClause is included, then
}
it always executes after the try.
catch ( ArithmeticException e) {
System.out.println( “Oshkosh” );
}
finally {
System.out.println( “Whitewater” );
}
What if this line is deleted?
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
Details of Exception Handling Behavior
When an exception is thrown execution proceeds as follows:
1)Normal instruction execution is suspended.
2)If the immediately enclosing try contains a matching catch, then the
conforming catch clause serves as the exception handler.
Otherwise, the current try body is aborted and the thrown exception
is forwarded to the next instruction after the try. (If this is the last
instruction in a method, then the exception is forwarded to the
location of the call.)
3)Repeat Step 2 until an exception handler is located or all try instructions
are exhausted.
4)The exception handler is executed, followed by finallyClause (if present).
The remainder of the try instruction containing the catchClause is
aborted
and execution proceeds with the next instruction after the try.
5)If no matching catchClause is found, then this is considered to be an
uncaught exception and the program terminates with a traceback.
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
Example
Start trace by executing cud.
public void cud() {
try {
System.out.println( "cud: before horn" );
horn();
System.out.println( "cud: after horn" );
udder();
System.out.println( "cud: after udder" );
}
catch ( Exception e ) {
System.out.println( "cud handler" );
}
}
public void horn() {
try {
System.out.println( "horn: before udder" );
udder();
System.out.println( "horn: after udder" );
}
catch ( Exception e ) {
System.out.println( "horn handler" );
}
}
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
public void udder() {
System.out.println( ”udder: before exception" );
throw( new ArithmeticException() );
System.out.println( ”udder: after exception" );
}
All Java exceptions are represented by objects. (These objects are
automatically passed as parameters to the exception handler.)
The superclass of all exception
objects is _________________.
Exceptions partitioned into two categories.
Unchecked Exceptions
Throwable
«constructor»
+ Throwable()
...
«other»
+ String getMessage()
+ void printStackTrace()
+ String toString()
...
 are often severe, unpredictable.
 may (or may not) be handled by a try instruction.
Checked Exceptions
 should be used for user-created exceptions.
 must either be handled by a catch or included in a
throws declaration. (throws explained later.)
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
Classes from java.lang.
Error subclasses (abridged)
Object
OutOfMemoryError
StackOverflowError
Throwable
Exception subclasses (abridged)
Error
IOException
Exception
RuntimeException (abridged)
RuntimeException
Unchecked
Checked
Unchecked
ArithmeticException
ClassCastException
IllegalArgumentException
IndexOutOfBoundsException
NegativeArraySizeException
NullPointerException
SecurityException
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
Example
try {
...
}
catch ( NullPointerException e ) {
System.out.println( ”null handler" );
}
catch ( IndexOutOfBoundsException e ) {
System.out.println( ”index handler" );
}
catch ( OutOfMemoryError e ) {
System.out.println( ”memory handler" );
}
catch (Exception e ) {
System.out.println( e );
}
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
When a method is called that can potentially throw a checked exception,
then the code must do one of two things:
1) Be enclosed within a try instruction with a matching catch clause.
OR
2) The surrounding method must include a throws suffix.
Example
public void cud() throws IOException {
...
}
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.
Exceptions provide the opportunity to capture runtime errors.
Such errors are best reported in method preconditions.
Example
/* pre: arr.length != 0 (throws ArithmeticException)
* post: result = (summation of arr[0] through arr[arr.length-1]) / arr.length */
private double arrayAve(int[ ] arr) {
int total == 0;
for( int j = 0; j != arr.length; j++) {
total = total + arr[j];
}
return total / arr.length;
}
The Object of Data Abstraction
and Structure, David D. Riley
© Addison Wesley pub.