Syntax errors A syntax error is a violation of the notational rules of the programming language. (synonym: compile-time errors) Programming errors (faults)

Syntax errors A syntax error is a violation of the notational rules ofthe programming language. (synonym: compile-

time errors)

Programming errors (faults) occur in two basic forms:

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.

Java run-time errors produce exceptions.

Three Example Exceptions

A detected error is generally called a run-time error.

An exception is an abnormal condition that occurs during software execution.

String str = null;str = str.toLowerCase();

When an exception occurs, it is said that “the exception is ”___________”.

int j = 0;int k = 25/j;

double[ ] arr = new double[3];arr[3] = 29.4;


Exception reports

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)).

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.NullPointerException at Driver.main(Driver.java:5)driley>

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>

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>

Exception traceback

Exception occurs in line of methodin class

doSomething called from line of constructor

<init>

Thing constructor called from

line of method of class.


throw ExceptionObject;

Example

It is also possible to simulate an exception by executing a throw instruction.

where ExceptionObject is an object reference conforming to Exception (Exception is a class from java.lang.)

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); System.out.println(myFrac.realValue()); }

}

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; } . . .}


???

exceptionhandler

executes

programterminates

program is interrupted

exceptionis thrown

program executing normally


try Syntaxtry {

tryInstructionBody

} catchClausesfinallyClause

...Body denotes a sequence of instructions.

catchClauses Syntax (zero or more repetitions of the following)catch (exceptionClassName parmName ) {

exceptionHandlerBody

}

finallyClause Syntax (optional) finally {

finallyBody

} exceptionClassName - class conforming to Throwable.parmName - serves as a parameter passed into catch.


try Semantics

Execution of the try begins by executing the tryInstructionBody.

During the execution of tryInstructionBody the catchClauses serve as exception handlers.

The appropriate catchClause is selected by conformance to a catchClauseName

If a finallyClause is included, then it always executes after the try.

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” );}

Trace the following:

What if this line is deleted?The Object of Data Abstraction and Structure, David D. Riley© Addison Wesley pub.

Details of Exception Handling BehaviorWhen 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.


Example

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" ); }}

public void udder() { System.out.println( ”udder: before exception" ); throw( new ArithmeticException() ); System.out.println( ”udder: after exception" );}

Start trace by executing cud.


All Java exceptions are represented by objects. (These objects are automatically passed as parameters to the exception handler.)

Unchecked Exceptions are often severe, unpredictable. may (or may not) be handled by a try

instruction.

Exceptions partitioned into two categories.

The superclass of all exception objects is _________________.

Throwable

«constructor» + Throwable() . . .

«other» + String getMessage() + void printStackTrace() + String toString() . . .

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.)


Unchecked

Object

Throwable

Error Exception

RuntimeExceptionChecked

Unchecked

Error subclasses (abridged) OutOfMemoryErrorStackOverflowError

Exception subclasses (abridged)

IOException

RuntimeException (abridged) ArithmeticExceptionClassCastExceptionIllegalArgumentExceptionIndexOutOfBoundsExceptionNegativeArraySizeExceptionNullPointerExceptionSecurityException

Classes from java.lang.


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 ); }


When a method is called that can potentially throw a checked exception,then the code must do one of two things:

Example

1) Be enclosed within a try instruction with a matching catch clause.

2) The surrounding method must include a throws suffix.

OR

public void cud() throws IOException { . . .}


Exceptions provide the opportunity to capture runtime errors.

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;}

Such errors are best reported in method preconditions.


Documents

Syntax errors A syntax error is a violation of the notational rules of the programming language. (synonym: compile-time errors) Programming errors (faults)