Exception handling throw and throws

[Sonali Sehgal]

Can someone please explain the difference between Exception handling throw and throws clause?

[N.Senthil Kumar]

http://java.sun.com/docs/books/tutorial/essential/exceptions

throw is used to throw an exception manually where as throws is used in the case of checked exceptions to reintimate the compiler that we have handled the exception. so throws is to be used at the time of defining a method and also at the time of calling that function which rises an checked exception.

class MyException extends Exception //to create our own exception
{
      public String toString() //overriding the method toString() to print the desired msg.
     {
         return Can not divide a no. with one: + MyException ;
     }
     public static void main(String args[]) throws MyException //use of throws
     {
       int a = Integer.parseInt(args[0]);
       int b = Integer.parseInt(args[1]);
      
       if(b == 1)
          throw new MyException(); // rises an MyException if we try to divide a no. with 1
      else
         System.out.println((float)a/b);
      }  
}

if we want to rise our own exception we have to use either throws or to handle the exception by try-catch. if not it gives the compile time error.

and throw is to rise the exception manually In the above prog. I rised an exception when you try to divide a no with 1.(own Exception)

[Sonali Sehgal]

I am now clear on throw clause can you please explain more on throws clause....???

[Sagar Rohankar]

'throws' clause is appended to function and tells the compiler, "Ok if this function throws any (checked/unchecked) exception, then just pass it on to the calling function, and let him handle this exception."

Example:

public void foo() throws SQLException //checked exception
{
    // Some operation
     throw new SQLException(); // No need to 'handle' it, we had defined throws 
                                             // clause

}

public void callFoo()
{
         try
         {
                        foo();
         }
          catch(Exception ex){}

HTH

[Ankit Garg]

Remember that if your method can throw any checked exception at any point and doesn't handle it, then it must declare that exception in its throws clause. This is not required for unchecked exceptions. Declaring any unchecked exceptions in throws clause is useless but allowed...

[Sonali Sehgal]

Thanks a lot Sagar and Ankit ...but I still have a question

    
class MyException extends Exception{
	void do someMethod(){
	doStuff();
	}

void doStuff() throws MyException{
	
	try{
	throw new MyException();
	}

	catch(MyException me) {
	throw me;
	}
	
}

Output:-
This is a Kathy S book example it says that it will not compile.....
MyException.java3:reported exception MyException; must be caught or declared to be thrown
doStuff();

So in order to execute this program how can we actually fix it or implement throws clause....

[Sonali Sehgal]

Hi Sagar,
I read your program and Kathy S book....The book says that in the program code of yours if it throws a checked exception but you are not declaring it...
It goes followed by the following example:-

   
void doStuff() {
doMore();
}
void doMore() {
throw new IOException();
}

 

[Ankit Garg]

Sonali Sehgal wrote:Thanks a lot Sagar and Ankit ...but I still have a question

    
class MyException extends Exception{
	void do someMethod(){
	doStuff();
	}

void doStuff() throws MyException{
	
	try{
	throw new MyException();
	}

	catch(MyException me) {
	throw me;
	}
	
}

Output:-
This is a Kathy S book example it says that it will not compile.....
MyException.java3:reported exception MyException; must be caught or declared to be thrown
doStuff();

So in order to execute this program how can we actually fix it or implement throws clause....

throws clause is used properly in this program. doStuff method throws MyException and has declared it in its throws clause. doStuff is called in someMethod(). But some method neither declared MyException in its throws clause nor catches it. That's why you are getting an error. The two possible solutions for this are

	void do someMethod() throws MyException{
		doStuff();
	}

	void do someMethod(){
		try {
			doStuff();
		} catch(MyException me) {
			//some code
		}
	}

[Sonali Sehgal]

Hi Ankit,

Thanks a lot now I understand the declaration and rules of throws clause..........I am now able to understand all the above programs........

[Sagar Rohankar]

Sonali Sehgal wrote:Hi Sagar,
I read your program and Kathy S book....The book says that in the program code of yours if it throws a checked exception but you are not declaring it...
It goes followed by the following example:-

   
void doStuff() {
     doMore(); // U've to handle it
}
void doMore() {
      throw new IOException();
} 

Re read my first post, particularly this line:
...then just pass it on to the calling function, and let him handle this exception
That's mean, calling function (doStuff()) must handle the exception which are declared in called functions (doMore()) 'throws' clause.

HTH