What Is An Exception? (The Java™ Tutorials > Essential Java ...

Documentation The Java™ Tutorials Hide TOC Exceptions What Is an Exception? The Catch or Specify Requirement Catching and Handling Exceptions The try Block The catch Blocks The finally Block The try-with-resources Statement Putting It All Together Specifying the Exceptions Thrown by a Method How to Throw Exceptions Chained Exceptions Creating Exception Classes Unchecked Exceptions — The Controversy Advantages of Exceptions Summary Questions and Exercises Trail: Essential Java Classes Lesson: Exceptions Home Page > Essential Java Classes > Exceptions « Previous • Trail • Next »

The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.See Dev.java for updated tutorials taking advantage of the latest releases.See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.

What Is an Exception?

The term exception is shorthand for the phrase "exceptional event."

Definition: An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.

When an error occurs within a method, the method creates an object and hands it off to the runtime system. The object, called an exception object, contains information about the error, including its type and the state of the program when the error occurred. Creating an exception object and handing it to the runtime system is called throwing an exception.

After a method throws an exception, the runtime system attempts to find something to handle it. The set of possible "somethings" to handle the exception is the ordered list of methods that had been called to get to the method where the error occurred. The list of methods is known as the call stack (see the next figure).

The call stack.

The runtime system searches the call stack for a method that contains a block of code that can handle the exception. This block of code is called an exception handler. The search begins with the method in which the error occurred and proceeds through the call stack in the reverse order in which the methods were called. When an appropriate handler is found, the runtime system passes the exception to the handler. An exception handler is considered appropriate if the type of the exception object thrown matches the type that can be handled by the handler.

The exception handler chosen is said to catch the exception. If the runtime system exhaustively searches all the methods on the call stack without finding an appropriate exception handler, as shown in the next figure, the runtime system (and, consequently, the program) terminates.

Searching the call stack for the exception handler.

Using exceptions to manage errors has some advantages over traditional error-management techniques. You can learn more in the Advantages of Exceptions section.

« Previous • Trail • Next »

About Oracle | Contact Us | Legal Notices | Terms of Use | Your Privacy Rights

Copyright © 1995, 2024 Oracle and/or its affiliates. All rights reserved.

Previous page: Exceptions Next page: The Catch or Specify Requirement