1. Program Errors Syntax errors Logic errors
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.

2. Run-time Errors Three Example Exceptions
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.

3. 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();
int j = 0;
int k = 25 / j;
double[ ] arr = new double[3];
arr[3] = 29.4;
driley> java Driver
Exception in thread "main" java.lang.ArithmeticException: / by zero
at Driver.main(Driver.java:8)
driley>
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)).

4. Exception traceback Exception occurs in line of method in class
driley> java Driver
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)
driley>
doSomething called from
line of
constructor <init>
Thing constructor called from
line of
method of
class.
The Object of Data Abstraction and Structure, David D. Riley
© Addison Wesley pub.

5. throw Instruction Example
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 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 (double)numerator / denominator;
. . .
public class runFractTest {
public static void main(String[ ] args) {
SimpleFraction myFrac;
myFrac = new SimpleFraction(1, 2);
System.out.println(myFrac.realValue());
myFrac = new SimpleFraction(1, 0); }
The Object of Data Abstraction and Structure, David D. Riley
© Addison Wesley pub.

6. program executing normally
Exception Flow
program executing normally
exception
is thrown
program
is interrupted
program
terminates
exception
handler
executes
???
The Object of Data Abstraction and Structure, David D. Riley
© Addison Wesley pub.

7. try Instruction try Syntax
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 catch.

8. try Semantics Trace the following:  Execution of the try begins by
executing the tryInstructionBody.
Trace the following:
System.out.println( “La Crosse” );
try {
System.out.println( “River Falls” );
String str;
System.out.println( str.trim() );
System.out.println( “Eau Claire” ); }
catch ( NullPointerException e) {
System.out.println( “Platteville” );
catch ( ArithmeticException e) {
System.out.println( “Oshkosh” );
finally {
System.out.println( “Whitewater” );
 During the execution of tryInstructionBody
the catchClauses serve as exception handlers.
The appropriate catchClause is selected by
conformance to a catchClauseName
What if this line is deleted?
 If a finallyClause is included, then
it always executes after the try.
The Object of Data Abstraction and Structure, David D. Riley
© Addison Wesley pub.

9. 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.

10. 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() {
System.out.println( "horn: before udder" );
System.out.println( "horn: after udder" );
System.out.println( "horn handler" );
public void udder() {
System.out.println( ”udder: before exception" );
throw( new ArithmeticException() );
System.out.println( ”udder: after exception" );
Example
Start trace by executing cud.
The Object of Data Abstraction and Structure, David D. Riley
© Addison Wesley pub.

11. Exception Types Unchecked Exceptions Checked Exceptions
All Java exceptions are represented by objects. (These objects are automatically passed as parameters to the exception handler.)
Throwable
«constructor»
+ Throwable()
. . .
«other»
+ String getMessage()
+ void printStackTrace()
+ String toString()
The superclass of all exception objects is _________________.
Exceptions partitioned into two categories.
Unchecked Exceptions
 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.

12. Exception Types Classes from java.lang. Error subclasses (abridged)
OutOfMemoryError
StackOverflowError
Unchecked
Object
Throwable
Error
Exception
RuntimeException
Checked
Exception subclasses (abridged)
IOException
RuntimeException (abridged)
ArithmeticException
ClassCastException
IllegalArgumentException
IndexOutOfBoundsException
NegativeArraySizeException
NullPointerException
SecurityException
The Object of Data Abstraction and Structure, David D. Riley
© Addison Wesley pub.

13. Example . . . } catch ( NullPointerException e ) {
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.

14. throws
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.
2) The surrounding method must include a throws suffix. OR
Example
public void cud() throws IOException {
. . . }
The Object of Data Abstraction and Structure, David D. Riley
© Addison Wesley pub.

15. Exceptions in Specifications
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.