Exception Handling Keywords

throw:- 

Exceptions can be thrown by either java run time environment or by the code itself. JRE throws 

exception when java’s rules are violated. An example is, when the code accesses an array 

location which is not available then ArrayIndexOutOfBoundsException is thrown. Pretty nice name right and obviously it explains the problem. All the java exception classes are not having any attributes and standard methods. It is a common design. Class name describes what exception it is and the hierarchy forms an organization chart kind of structure using which java exceptions are used.

Programmers can throw an exception to indicate a problem condition. Either java’s predefined exceptions can be used or custom exceptions can be created by extending the already available exceptions. Programmer can throw a custom exception or a predefined Java exception. Mostly custom exceptions are created based on business conditions. Programmer can use the Java keyword ‘throw‘ to generate an exception. It is done by instantiating the exception class of choice and then thrown.

try – catch:-    

 A thrown exception should be handled. If the program does not handles an exception, then it will be handled by the java run time environment. A block of code where an exception is expected should be surrounded by try – catch block. try indicates the start of the exception handling block and catch the end of the block. Following catch a block of code can be written which is the exception handling code block. This is the part the handles the exception. A catch will have an exception identified and it will catch only that type of exception. Type means the same exception and all its sub classes. There can be multiple catch blocks for a try block.

 

throws:-

 

When a Java method is going to throw an exception, to indicate that as part of the method signature ‘throws‘ keyword should be used followed by the exception. It means that the caller of this method should handle the exception given in the throws clause. There can be multiple exceptions declared to be thown by a method. If the caller of that method does not handles the exception, then it propagates to one level higher in the method call stack to the previous caller and similarly till it reaches base of the method call stack which will be the java’s run time system.

finally:- 

After catch block there can be one more block of code declared as ‘finally‘. Irrespective of whether an exception is thrown or not, the finally block of code will always be executed. Important piece of code that must be executed, even if a program fails belong to this finally block. Example would be closing a database connection, a file handle, etc.

General Exception Handling Structure:-  

try {

  // possible exception code block

} catch (ExceptionTypeA exception1) {

  // handle exception of type ExceptionTypeA and all it subclasses

} catch (ExceptionTypeB exception2) {

  // handle exception of type ExceptionTypeB and all it subclasses

} finally {

  // guarantee block: executes always irrespective of exception

}