What happens if a method throws an exception that was not specified in the method declaration with "throws"

Question

I've never used the "throws" clause, and today a mate told me that I had to specify in the method declaration which exceptions the method may throw. However, I've been using exceptions without problems without doing it, so, why is it needed if, in fact, it's needed?

Answer 1

Java has two different types of exceptions: checked Exceptions and unchecked Exceptions.

Unchecked exceptions are subclasses of RuntimeException and you don't have to add a throws declaration. All other exceptions have to be handled in the method body, either with a try/catch statement or with a throws declaration.

Example for unchecked exceptions: IllegalArgumentException that is used sometimes to notify, that a method has been called with illegal arguments. No throws needed.

Example for checked exceptions: IOException that some methods from the java.io package might throw. Either use a try/catch or add throws IOException to the method declaration and delegate exception handling to the method caller.

Answer 2

If a method is declared with the throws keyword then any other method that wishes to call that method must either be prepared to catch it or declare that itself will throw an exception.

For instance if you want to pause the application you must call Thread.sleep(milliseconds);

But the declaration for this method says that it will throw an InterruptedException

Declaration:

public static void sleep(long millis) throws InterruptedException 

So if you wish to call it for instance in your main method you must either catch it:

public static void main(String args[]) {     try {         Thread.sleep(1000);     } catch(InterruptedException ie) {         System.out.println("Opps!");     } } 

Or make the method also declare that it is throwing an exception:

public static void main(String args[]) throws InterruptedException {     Thread.sleep(1000); }