11.1 The Causes of Exceptions

If we do not succeed, then we run the risk of failure.
—J. Danforth Quayle (1990)

An exception is thrown for one of three reasons:

• An abnormal execution condition was synchronously detected by a Java Virtual Machine. Such conditions arise because:
  • evaluation of an expression violates the normal semantics of the Java language, such as an integer divide by zero, as summarized in §15.5
  • an error occurs in loading or linking part of the Java program (§12.2, §12.3)
  • some limitation a resource is exceeded, such as using too much memory

These exceptions are not thrown at an arbitrary point in the program, but rather at a point where they are specified as a possible result of an expression evaluation or statement execution.

• A throw statement (§14.16) was executed in Java code.
• An asynchronous exception occurred either because:
  • the method stop of class Thread (§20.20.16) was invoked
  • an internal error has occurred in the virtual machine (§11.5.2.2)

Exceptions are represented by instances of the class Throwable and instances of its subclasses. These classes are, collectively, the exception classes.