Checked vs unchecked exceptions and the Throwable hierarchy

An exception is an object that signals something unexpected happened, and throwing it interrupts the normal flow of the program until some code catches it. All exceptions and errors in Java descend from java.lang.Throwable. It has two direct subclasses: Error, for serious problems in the JVM or environment that applications normally should not try to handle, such as OutOfMemoryError and StackOverflowError; and Exception, for conditions a program might reasonably handle. RuntimeException is a subclass of Exception.