When to use "throw" keyword

[abalfazl hossein]

 


Class myclass
{
  public static void main(String arg[])
   {
    int a, b, c;
      try
         {
          a=20;
          b=0;
          c=a/b;
         }
     catch(ArithmeticException e)
         {
          System.out.println("zero divide error");
         }
   }
}

   

There is a zero divide , and a message shows and an exception caught.

 try {
        readFromFile("foo");
        ...
    }
    catch ( FileNotFoundException e ) {
        // Handle file not found
        ...
    }
    catch ( IOException e ) {
        // Handle read error
        ...
    }
    catch ( Exception e ) {
        // Handle all other errors
        ...
    }


   

There are exceptions fro open a file, and these are caught.

There is no need to use throw keyword. But if you have your own exception that is you write it yourself.Is it right?

When to use throw keyword?

[Bear Bibeault]

Whenever you want to initiate the throwing of an exception -- it does not have to be one that you wrote.

[Seetharaman Venkatasamy]

abalfazl hossein wrote: if you have your own exception that is you write it yourself.Is it right?

No; As Bear said, you can throw an Exception which is already in API . example

    
if(true)
throw new NullPointerException();

[Alex Parvan]

Let's say you have a Servlet or a Web Service (a java application running on a server) and a client that wants to use a method on that server. If something goes wrong on the server, you might want to throw an exception, your code will look like this:

Server:

public class ServerApplication throws IOException {
    public File getFile() {
        File file = new File("/read.txt");
        if (file == null) throw new IOException("There was an error while reading the file");
        else return file;
    }
}

Client:

try {
    server.getFile();
} catch (IOException e) {
    System.out.println(e.getMessage());
}

It's not such a good example, but it's the only one i can come up with I hope this puts you on the right track.

[abalfazl hossein]

http://www.faqs.org/docs/javap/c9/s3.html

There are times when it makes sense for a program to deliberately throw an exception. This is the case when the program discovers some sort of exceptional or error condition, but there is no reasonable way to handle the error at the point where the problem is discovered. The program can throw an exception in the hope that some other part of the program will catch and handle the exception.

http://www.ccs.neu.edu/course/com3118/EXCEPTION.html

throws

If a method is capable of causing an exception that it does not handle, it must specify this behaviour so that callers of the method can guard themselves against that exception.

type method-name(paramlist) throws exception-list

{

//…..

}

throws clause lists the types of exceptions that a method might throw.

Necessary for all exceptions, except those of type Error or RuntimeException, or any of their subclasses.

Why isn't Necessary for Error or RuntimeException?

[Jesper de Jong]

abalfazl hossein wrote:Why isn't Necessary for Error or RuntimeException?

Because RuntimeException and any other exceptions that subclass RuntimeException are unchecked exceptions. See Lesson: Exceptions in Sun's Java Tutorials to learn more about the difference between checked and unchecked exceptions.