Differences between Exception and Error

Question

Errors should not be caught or handled (except in the rarest of cases). Exceptions are the bread and butter of exception handling. The Javadoc explains it well:

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.

Look at a few of the subclasses of Error, taking some of their JavaDoc comments:

• AnnotationFormatError - Thrown when the annotation parser attempts to read an annotation from a class file and determines that the annotation is malformed.
• AssertionError - Thrown to indicate that an assertion has failed.
• LinkageError - Subclasses of LinkageError indicate that a class has some dependency on another class; however, the latter class has incompatibly changed after the compilation of the former class.
• VirtualMachineError - Thrown to indicate that the Java Virtual Machine is broken or has run out of resources necessary for it to continue operating.

There are really three important subcategories of Throwable:

• Error - Something severe enough has gone wrong the most applications should crash rather than try to handle the problem,
• Unchecked Exception (aka RuntimeException) - Very often a programming error such as a NullPointerException or an illegal argument. Applications can sometimes handle or recover from this Throwable category -- or at least catch it at the Thread's run() method, log the complaint, and continue running.
• Checked Exception (aka Everything else) - Applications are expected to be able to catch and meaningfully do something with the rest, such as FileNotFoundException and TimeoutException...

This slide showing Java's exception hierarchy by @georgios-gousios concisely explains the differences between Errors and Exceptions in Java.

Errors tend to signal the end of your application as you know it. It typically cannot be recovered from and should cause your VM to exit. Catching them should not be done except to possibly log or display and appropriate message before exiting.

Example: OutOfMemoryError - Not much you can do as your program can no longer run.

Exceptions are often recoverable and even when not, they generally just mean an attempted operation failed, but your program can still carry on.

Example: IllegalArgumentException - Passed invalid data to a method so that method call failed, but it does not affect future operations.

These are simplistic examples, and there is another wealth of information on just Exceptions alone.

Errors -

1. Errors in java are of type java.lang.Error.
2. All errors in java are unchecked type.
3. Errors happen at run time. They will not be known to compiler.
4. It is impossible to recover from errors.
5. Errors are mostly caused by the environment in which application is running.
6. Examples : java.lang.StackOverflowError, java.lang.OutOfMemoryError

Exceptions -

1. Exceptions in java are of type java.lang.Exception.
2. Exceptions include both checked as well as unchecked type.
3. Checked exceptions are known to compiler where as unchecked exceptions are not known to compiler because they occur at run time.
4. You can recover from exceptions by handling them through try-catch blocks.
5. Exceptions are mainly caused by the application itself.
6. Examples : Checked Exceptions : SQLException, IOException
   Unchecked Exceptions : ArrayIndexOutOfBoundException, ClassCastException, NullPointerException

further reading : http://javaconceptoftheday.com/difference-between-error-vs-exception-in-java/

Sun puts it best:

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch.

The description of the Error class is quite clear:

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions. The ThreadDeath error, though a "normal" condition, is also a subclass of Error because most applications should not try to catch it.

A method is not required to declare in its throws clause any subclasses of Error that might be thrown during the execution of the method but not caught, since these errors are abnormal conditions that should never occur.

Cited from Java's own documentation of the class Error.

In short, you should not catch Errors, except you have a good reason to do so. (For example to prevent your implementation of web server to crash if a servlet runs out of memory or something like that.)

An Exception, on the other hand, is just a normal exception as in any other modern language. You will find a detailed description in the Java API documentation or any online or offline resource.