61. Throwing an exception

A method is able to throw an exception just as we saw in the Exceptions tutorial. In the code below I throw an exception if the caller to determineAverage() tries to pass a String through as a parameter:

def determineAverage(...values) throws IllegalArgumentException {
    for (item in values) {
        if (item in String) {
            throw new IllegalArgumentException()
        }
    }
    return values.sum() / values.size()
}

//This works:
assert determineAverage(12, 18) == 15

//This does not work - we get an exception
determineAverage(5, 5, 8, 'kitten')

None of that code is new to you except for the throws IllegalArgumentException that forms part of the method’s signature¹. The use of throws helps describe our method a little better by making callers aware of what to expect.

Multiple exceptions can be listed against throws, as seen in the example below:

def determineAverage(...values)
        throws IllegalArgumentException, NullPointerException {
    for (item in values) {
        if (item in String) {
            throw new IllegalArgumentException()
        }
    }
    return values.sum() / values.size()
}

Groovy doesn’t require that you explicitly provide a throws listing if your method throws an exception or passes up an exception that it doesn’t handle. However, if your method is to be used by others, I’d suggest that including throws is worth the effort.

You may note that, in that last example, I placed throws on a second line - this helps readability as it breaks up the display of the signature just slightly.