How do you create a method to throw an exception in java?

Question

My homework says we must follow these instructions to create the throwException method.

throwException method

• Should take in an Exception object as its only parameter and return nothing
• Take that exception and throw it

How do you create this method?

What I have so far is:

public void throwException (Exception ex) throws Exception {
}

is this correct?

Answer 1

Your current method covers the first point - to take an exception as parameter.
The throws Exception is unnecessary if you dont throw it from a method, but it is needed to cover the second point.

As commenters pointed, you just need to use the throw keyword to throw an exception, so the method could look like:

public void throwException (Exception ex) throws Exception {
    //some other code maybe?
    throw ex;
}

This implementation has a little flaw. When the null is passed as a parameter to it, the method will throw a NullPointerException, because the throw keyword accepts objects of the Throwable class or its subclasses (Exception is a subclass of Throwable).
To avoid NullPointerException (which is an unchecked-exception), simple if statement can be used:

public void throwException (Exception ex) throws Exception {
    if (ex != null) {
        throw ex;
    }
    //just for presentation,below it throws new Exception
    throw new Exception("ex parameter was null");
}

Edit:
As @Slaw suggested, in that very small case adding the null-check and throwing new Exception just disguises the NullPointerException. Without the null-check and throw new... the NPE will be thrown from that method and its stacktrace will show exact line of throw ex when the null is passed to that method.
The NPE is a subtype of RuntimeException class and the subtypes of RuntimeException doesn't need to be explicitly declared in method signature when they are thrown from that method. Like here:

public static void throwNPE(Exception e) {
    throw new NullPointerException();
}

The RuntimeException and its subclasses are called an unchecked-exceptions. Other classes extending one of Exception or Throwable classes are call checked-exceptions, because if a method throws them, it must declare that exception (or superclass) in the signature or explicitly try-catch it.

The proper use of null-check would be when the method would throw a more specific kind of exception (like IOException or a new subclass of Exception/Throwable) and the when the other method using the one which throws that new type of Exception would try-catch that specific type the NPE wouldn't be caught.

Just for good practices, when dealing with try-catch it's much better to catch exact types of thrown Exceptions instead of general Exception/Throwable. It helps to understand the real cause of exception, debug and fix code.