Programming Exception Handling

This page examines the differences in how programming languages detect and intercept events that disrupt the normal flow of a program's execution.

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Language Differences

Using exceptions in C# does not in general adversely affect performance, unlike C++.

VB error handling is limited to On Error GoTo statements.

Java Exception Handling

public void doDaIO() throws IOException { // set boolean flag if process is successful. ... if( !success ) { throw new EOFException("File error 1234"); } }

Exceptions result in unconditional but recoverable transfer of control (not a return) from the try block.

To issue exceptions the code cannot intercept on its own (such as data in the wrong format). It must first create an instance of java.lang.Throwable, then declare the signature of a method with a throws clause with throw objects.

Subclasses can declare subclasses of the class declared. For example, EOFException (End of File Exception) is a subclass of IOException.

Among the 50+ subclasses generated by the Java runtime system's Java Exception Classes are:

• ArithmeticException from divide by zero
• FileNotFoundException from a file Open failure
• IOException from a Bad Network Connection
• ArrayIndexOutOfBoundsException

Java developers write throw objects under the Exception subclass derived from the java.lang.Thowable interface. This Throwable class is the superclass of all errors and exceptions in the Java language. Only objects that are instances of this class (or one of its subclasses) are (can be) thrown by the throw statement. Similarly, only this class or one of its subclasses can be the argument type in a catch clause.

java.lang.Error subclasses (such as the VirtualMachineError subclass thrown by the JVM) are not recoverable. Errors arise when Events external to a program code occur, such as the JVM running out of memory, cause program code to stop.

Checked?

An exception is "checked" if the compiler checks whether coding exists to anticipate/handle that exception.

All classes are checked by the compiler except for java.lang.RuntimeException and Error subclasses, which are unchecked by the compiler.

The Error subclass of the Throwable class is a sibling class of the Exception subclass. In other words, the compiler does not require coding to declare or catch Exception Errors.

try { // The code to protect: Inventory.StockRoom srRet = srm.getStockRoom((java.lang.String)); } catch( ThrowableObjectType paramName) { // to see the message associated with the Exception: e.getMessage(); // to determine the origin of the Exception: e.printStackTrace(); // to show the Exception name and message: e.toString(); } catch( Inventory.NotAStockItem e) { // Handle "Not A Stocked Item" exception ... System.out.println("usage Advert input"); System.exit(1); } finally {    // Clean-up code (close files, etc.)    // regardless of whether or not an exception occured:    System.out.println( "Out of catch block"); }

To detect recoverable exceptions, methods are called from within a try block. Each catch blocks determines whether a type of exception have been thrown, then responds to them by passing off to exception handler code.

For example, when trying the Thread.wait() method, the code should include a catch for InterruptException.

The JRE terminates with a NullPointerException if it can't find an appropriate catch somewhere up the call stack.

A variable referenced in try or catch blocks must be declared before entering the try block.

Dick Baldwin has a Tutorial on Exception Handling

David R. Nadeau's Java Exceptions Tutorial

Exception handling in German is Ausnahmebehandlung.

Java Exception Classes

The java.lang package defines several classes and exceptions are subclasses of the RuntimeException class for use in exception handling Some of these classes are not checked to see if they are included in a method's Throws List of exceptions (they do not have to be declared), while some other classes are checked.

Exception	Description	Checked
java.lang.NoClassDefFoundError	the default package cannot be found in the folder list within PATH or CLASSPATH environment variable.	--
ArithmeticException	Arithmetic errors such as a divide by zero	NO
ArrayIndexOutOfBoundsException	Arrays index is not within array.length	NO
ArrayStoreException	Assignment of incompatible datatype	NO
ClassNotFoundException	Class is not found	YES
CloneNotSupportedException	Cloning an object that does not implement Cloneable	YES
IllegalAccessException	No access to class	YES
IllegalArgumentException	Illegal argument when calling a method	NO
IllegalStateException	Illegal state for application or environment	NO
IllegalThreadStateException	Operation not compatible with current thread	NO
InstantiationException	Instantiating an abstract class	YES
InterruptedException	One thread has been interrupted by another thread	YES
NegativeArraySizeException	Creating array of negative size	NO
NoSuchFieldException	Nonexistent field	YES
NoSuchMethodException	Nonexistent method	YES
NullPointerException	Invalid use of null reference	NO
NumberFormatException	Invalid string for conversion to number	NO
SecurityException	Security violation	NO
StringIndexOutOfBoundsException	Index for string is out of bounds	NO