Definition: An exception is an event which occurs during execution of the program which disrupts the normal flow of the instructions. Different types of errors can cause exceptions: problems which range from serious hardware errors, such as hard disk crash, to the simple programming errors, like trying to access the out-of-bounds array element. When such error occurs within a Java method, the method will create an exception object and hands it to the runtime system. The exception object do contains the information about exception including its type and state of the program when error occurred. Runtime system is then responsible to find some code to handle error. In the Java terminology, creating exception object and handing it to runtime system is called "throwing an exception".

The example above defines and implements the class called "ListOfNumbers". Upon construction, the ListOfNumbers class creates a Vector which contains ten Integer elements with sequential values 0 through 9. ListOfNumbers class also defines the method "writeList()" that writes the list of numbers into the text file "OutFile.txt". The writeList() method calls up two more methods that can throw exceptions.

This line invokes the constructor for FileOutputStream which throws an IOException when the file cannot be opened for any of the reason.

This method will throw ArrayIndexOutOfBoundsException if you pass an index whose value is very small (negative number) or very large (larger than number of elements contained by the Vector currently).

If you try to compile ListOfNumbers class, the compiler prints the error message about an exception thrown by FileOutputStream constructor, but it will not display the error message about an exception thrown by elementAt(). This is because the exception thrown by the FileOutputStream constructor, IOException, is non-runtime exception and the exception thrown by elementAt() method, ArrayIndexOutOfBoundsException, is the runtime exception. Java requires only to catch or declare non-runtime exceptions.

The first step for constructing an exception handler is to enclose statements that might throw exception within the try block. In general, try block looks something like this:

The segment "Java statements" in the code above consists one or more legal Java statements which could potentially throw an exception. To construct the exception handler for writeList() method from ListOfNumbers class, we need to enclose the exception-throwing statements of writeList() method within the try block. There are more than one ways to accomplish this task: we can put each statement which may potentially throw the exception within its own try statement, and provide the separate exception handlers for each of the try. Or you can put all the writeList() statements within the single try statement and associate multiple handlers to it. The following example uses single try statement for the entire method as the code tends to be easier to read.

The try statement governs the statements enclosed within it and it defines the scope of any exceptions handlers (which are established by subsequent catch blocks) associated with it. In other words, if an exception does occurs within the try statement, then that exception is handled by some appropriate exception handler which is associated with this try statement.

As studied before, the try statement defines the scope of the associated exception handlers. We do associate exception handlers with a try statement by providing one or more of the sequential catch blocks directly after a try block.

There can be no intervening code between the end of try statement and the beginning of first catch statement. The general form of catch statement is:

Catch statement governs series of legal Java statements. These statements are executed when the exception handler is invoked. The runtime system invokes exception handler when a exception whose type matches with that of catch statement's argument is thrown within handler's try block.

The writeList() method from ListOfNumbers class uses two exception handlers for the try statement, one handler for each of the two types of exceptions which can be thrown within a try block, ArrayIndexOutOfBoundsException and IOException.