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Difference between throw and throws in Exception handling

3

May 19, 2014 by admin

Main difference between throw and throws is in there usage and functionality. where throws is used in method signature to declare Exception possibly thrown by any method, throw is actually used to throw Exception in Java code, here is an example of both throw and throws keyword which makes it easy to understand difference between them.

 public void shutdown() throws IOException{
        throw new IOException(“Unable to shutdown”);
 }
if you see the code above, throws keyword is used to declare Exception thrown by this method shutdwon(), by looking at method signature we know that its throwing IOException. while inside method code throw keyword is actually used to throw instance of IOException. here are couple of more differences between throw and throws keyword in java:

Difference between throw and throws in Exception handling – Java Example

1) You can declare multiple exception thrown by method in throws keyword by separating them in common e.g. throws IOException, ArrayIndexBoundException etc, while you can only throw one instance of exception using throw keyword e.g. throw new IOException(“not able to open connection”).
2) throws keyword gives a method flexibility of throwing an Exception rather than handling it. with throws keyword in method
signature a method suggesting its caller to prepare for Exception declared in throws clause, specially in case of checked Exception and provide sufficient handling of them. On the other hand throw keyword transfer control of execution to caller by throwing an instance of Exception. throw keyword can also be used in place of return as shown in below example:
 private static boolean shutdown() {
        throw new UnsupportedOperationException(“Not yet implemented”);
    }
as in below method shutdown should return boolean but having throw in place compiler understand that this method will always throw exception .
3) throws keyword cannot be used anywhere exception method signature while throw keyword can be used inside method or static initializer block provided sufficient exception handling as shown in example.
static{
        try {
            throw new Exception(“Not able to initialized”);
        } catch (Exception ex) {
            Logger.getLogger(ExceptionTest.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
worth remembering is that static initializer block should complete normally.
4) throw keyword can also be used to break a switch statement without using break keyword as shown in below example:
int number = 5;
switch(number){
            case 1:
                throw new RuntimeException(“Exception number 1″);
            case 2:
                throw new RuntimeException(“Exception number 2″);
        }
We have seen differences between throw and throws in Java and they should be used accordingly. Let me know if you come across any other difference on throw vs throws in java.

 


3 comments »

  1. Hem Chandra Bhatt says:

    Hi,

    This is great post .Thank you !

  2. Muthu says:

    Good description about throw and throws.

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