Tuesday, February 21, 2012

Difference between throw and throws in Exception handling - Java Example

Difference between throw and throws keyword on Exception handling in Java is a popular core java interview question. Exception handling being an important part of Java programming language, complete knowledge of all keywords related to exception handling e.g. try, catch, finally, throw and throws is important. 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");

Difference between throw and throws in Exception handling in Javaif 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.

        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;
            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.

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Anonymous said...

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Praneeti said...

Can you please give an example of throw and throws, to understand difference between them better ? My understanding of throw is very limited but I know we can also use throw not just to throw exception but also to return from method or switch block.

Anonymous said...

Can we throw Errors in Java ? like throw java.lang.OutOfMemroyError etc

raj said...

'throws' is always used with a method and specifies this method may raise given exception(exception name followed by keyword throws)..
throw is used to raise an exception manually..

nitin rawat said...

how is message is print in the custom exception??
as we didnt use System.out.println??

Anonymous said...

Throw is used create custom exception

Anonymous said...

Can anyone please explain the diff....i still dont understand

vemalababu ravuri said...

can anyone please briefly explain the difference between throw and thrown..... still i can't understand...

Anonymous said...

I know that, error and exceptions are the problems in java coding, then why we generate Exceptions manually using this 'throw' keyword?

Pema said...

import java.io.IOException;
class Testthrows1{
void m()throws IOException{
throw new IOException("device error");//checked exception
void n()throws IOException{
void p(){
}catch(Exception e){System.out.println("exception handled");}
public static void main(String args[]){
Testthrows1 obj=new Testthrows1();
System.out.println("normal flow...");

Pema said...

In exceptional handling throw keyword is best used only for checked exceptions
ie compile time exceptions

throw keyword doesnot handle runtime exceptions like ArithmeticException etc.

throw keyword will throw the said exception and stops where as try catch methods will handle runtime exceptions and continues
the regular
flow of the program

throws keyword is used declare exceptions so that if there any exceptions ocuurring during the coding the compile will
prompt us saying please handle the exception
simply speaking throws keyword is like an alarm!!

Anonymous said...

number 3 does NOT make sense...are you missing some words in that sentence. Sounds "run-off"ish

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