• An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.
• When an exceptional condition arsies, an object representing that exception is created and thrown in the method that caused the error.
• Exceptions can be generated by java run-time system, or they can be manually generated by your code.
  • Exceptions thrown by Java relate to fundamental errors that violate the rules of the java language or the constraints of the java execution
  • Manually generated exceptions are typically used to report some error condition to the caller of the method.

• Checked exceptions are exceptional conditions that a well-written application should anticipate and recover from.
• Checked exceptions are subject to the Catch or Specify Requirement.
• All exceptions are checked exceptions, except for those indicated by Error, RuntimeException, and their subclasses.

Errors and runtime exceptions are collectively known as unchecked exceptions.

• These are exceptional conditions that are external to the application, and that the application usually cannot anticipate or recover from
• Errors are not subject to the Catch or Specify Requirement
• Errors are those exceptions indicated by Error and its subclasses.

• These are exceptional conditions that are internal to the application, and that the application usually cannot anticipate or recover from
• These usually indicate programming bugs, such as logic errors or improper use of an API
• Runtime exceptions are not subject to the Catch or Specify Requirement
• Runtime exceptions are those indicated by RuntimeException and its subclasses.

• Java Exceptions are handled by 5 keywords:

  • try
  • catch
  • throw
  • throws
  • finally

• Program statements that want to be monitored for exceptions are contained within a try block. If an exception occurs in try block, it is thrown.

• If an exception occurs in the try block, it is thrown, we can catch this exception using catch and handle in some rationak manner.

• To manually throw exception, use the keyword throw.

• Any exception that is thrown out of a method must be specified as such by a throws clause.

• Any code that absolutely must be executed after a try block completes is put in a finally block.

try {
  // block of code to monitor for errors
}
catch (ExceptionType1 exOb) {
  // exception handler for ExceptionType1
}
catch (ExceptionType2 exOb) {
  // exception handler for ExceptionType2
}
  
// ...

finally {
  // block of code to be executed after try block ends
}

try {
  // block of code to monitor for errors
}
catch (ExceptionType exOb) {
  // exception handler for ExceptionType
}

It is possible for your program to throw an exception explicitly, using the throw statement

throw ThrowableInstance;