Java Exceptions: Throw, Try and Catch

Exceptions are used when writing code for "exceptional situations" such as:

• The user tried to open a file that doesn't exist
• There's an error in a configuration file
• A server could not be reached due to a network problem

When such event occur, it's appropriate to throw an exception. The statements below a throw statement are skipped:

Execution will continue as normal when the exception is caught. You catch an exception using try and catch in the following way:

If the exception isn't caught within the method, it will propagate to the previous method:

Checked vs Unchecked Exceptions

If the exception being thrown is checked, the method needs to include a throws declaration to allow it to propagate. See Java Exception Types and Java Keyword: throws.

Uncaught Exceptions

If the exception is never caught a stack trace will be printed and the program (or at least the thread) will crash. For an example of how this might look, see Java: Stack trace.

Other Ways of Throwing

Apart from an explicit throw statement, an exception can be thrown due to programming errors, or calling an API method that throws an exception. A few examples:

String text = null;
text.length();

String text = "abc";
Integer.parseInt(text);