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Panic in Logo
A RuntimeException typically means something went unexpectedly wrong. Mostly we use it to fail fast on errors that shouldn’t occur during normal operation, or that we aren’t prepared to handle gracefully.
import java.io.File;
import java.io.IOException;
public class ExceptionExample {
public static void main(String[] args) {
// We'll use RuntimeException throughout this site to check for
// unexpected errors. This is the only program on the
// site designed to throw an uncaught exception.
throw new RuntimeException("a problem");
// A common use of exceptions is to abort if a method
// throws an exception that we don't know how to
// (or want to) handle. Here's an example of
// throwing an exception if we get an unexpected error when creating a new file.
/*
try {
File file = new File("/tmp/file");
if (!file.createNewFile()) {
throw new IOException("Failed to create file");
}
} catch (IOException e) {
throw new RuntimeException(e);
}
*/
}
}Running this program will cause it to throw an exception, print an error message and stack trace, and exit with a non-zero status.
When the first RuntimeException in main is thrown, the program exits without reaching the rest of the code. If you’d like to see the program try to create a temp file, comment out the first exception and uncomment the file creation code.
$ java ExceptionExample
Exception in thread "main" java.lang.RuntimeException: a problem
at ExceptionExample.main(ExceptionExample.java:8)Note that unlike some languages which use exceptions for handling of many errors, in Java it is common to use both checked exceptions and return values for error handling, depending on the situation.
[Next example: Try-Catch-Finally]