An exception object contains the information about the error condition. It includes information such as stacktrace and message. Perhaps, the most important information is the cause of the error. It is indicated by the name of the exception class used to create the exception object. Usually, you will use exception objects only to figure out the kind of error that occurred.
In order to understand exceptions fully, you need to understand the class hierarchy of exceptions first.
All exception classes are subclasses of the Throwable class. In other words,
the Throwable is the superclass of all exceptions in the exception class
hierarchy. The Throwable class, like any other class in Java, inherits the
Object class. You can throw and catch only the instances of the Throwable
class.
The exception class hierarchy is further divided into two main subclasses. * Exception * Error
The Exception class represents conditions that your program should catch and
handle. In order to create your own custom exception classes, you need to inherit
this class. FileNotFoundException is an example of such an exception. Further,
the class RuntimeException is a special subclass of the Exception class.
Exceptions which are intended to be unchecked should inherit this class.
NullPointerException is an example of such an exception.
You will learn more about checked and unchecked exceptions in the next section.
The second subclass of the Throwable class is the Error class. It defines
exceptions that your program should generally not catch. These exceptions are
usually thrown by the Java Virtual Machine to indicate errors related to the
environment. OutOfMemoryError is an example of such an error which indicates
memory exhaustion.
The following code segement shows the public methods found inside the Throwable
class. Please refere the Java documentation for more reference.
public class Throwable implements Serializable {
public String getMessage()
public String getLocalizedMessage()
public synchronized Throwable getCause()
public synchronized Throwable initCause(Throwable cause)
public String toString()
public void printStackTrace()
public void printStackTrace(PrintStream s)
public void printStackTrace(PrintWriter s)
public synchronized Throwable fillInStackTrace()
public StackTraceElement[] getStackTrace()
public void setStackTrace(StackTraceElement[] stackTrace)
public final synchronized void addSuppressed(Throwable exception)
public final synchronized Throwable[] getSuppressed()
}
Here is a list of some typical exceptions.
* IllegalArgumentException
This exception is thrown to indicate that you passed an incorrect argument
to a method.
* ArithmeticException
This exception is thrown to indicate that you tried to perform an illegal
arithmetic operation, such as dividing an integer by zero.
* IOException
This exception is thrown to indicate an error that the program encountered
when you tried to perform an I/O operation.
* ClassNotFoundException
This exception is thrown to indicate that class coud not be found.
Java includes hundreds of exceptions. You can learn more about them from the Java documentation.