throw & throws

[Chris Armstrong]

Hello
can some one tell me the diff between throw and throws with a an example
Thanks

[Amir Ghahrai]

throw and throws are two different things. the throw statement occurs inside a block e.g. a method body. To throw an exception, you would use
void aMethod() {
throw new Exception("new exception");
}
the throws clause, appears as a method header,
e.g.
void bMethod() throws FileNotFoundException {}
A method that can throw an exception, doesn't need to specify it in it's throws clause.
if you use a try catch block to call a method that throws an exception, that's fine. otherwise if you don't catch the exception, you must provide the throws clause in the header of the calling method.
example.......

public void aMethod() {
      throw new IndexOutOfBoundsException("outBounds");
}

in order to call this method you can use either

public void bMethod() {
    try {
        aMethod();
    }
    catch(IndexOutOfBoundsException e){}
}

or....

public void bMethod() throws IndexOutOfBoundsException {
      aMethod();
}

hope that clarifys it!

[Anthony Villanueva]

Just to add, a checked exception must be either declared in the throws clause of the method, or placed inside a try-catch block. A method does not have to declare a runtime exception in its throws clause, nor catch it.

[Veena Pointi]

Amir,
When you throw an exception using throw inside any method ,you should also declare it in methods header using throws clause.
So the following code doesn't compile.
void aMethod() {
throw new Exception("new exception");
}
Correct syntax would be
void aMethod() throws Exception {
throw new Exception("new exception");
}
Ususally you use throw statement with conditional statement.
For example

  
public void doSomeIO(String targetUr) throws MalformedURLException,EOFException{
URL url=new URL(targetUrl);
//Open the url & read from it
//set flag 'completed' when IO is completed satisfactorily
//so if we get here with completed==false,
//we got unexpected end of file
if(!completed){
throw new EOFException("Invalid file contents");
}
}

You are advertising every possible exception that your method might actually throw by declaring them in methods header using throws clause.
Hope that helps.
Veena