How to have optional named arguments in Groovy method

Question

Using the documented mixture of named and positional arguments (or args with default values for that matter), how to invoke a method without specifying any named param?

I'm trying to extend existing shared method without breaking everybody else's code, and this approach looked promising, but the following example fails:

def test(Map args, some, thing='default value'){
    "$some $thing";
}

//good - adding any named parameter works
//test('yet', 'another good', anything:'notneeded');

//but not specifying named parameter fails
test('this', 'fails');

I was not able to find documentation about this behavior, and it looks strange.

first param is "map". I guess calling will be test(args:null, "this", "pass") works fine — Jayan, Jan 08 '19 at 17:01
For me method call like test("key":"value", "this", "passed" worked fine. You may want to edit question with old method definition and its callers and new expected callers. — Jayan, Jan 09 '19 at 03:12

blackbuild · Answer 1 · 2019-11-22T14:21:14.137

The groovy parser needs some information to work with in order to determine which method to execute.

So if you write:

test('yet', 'another good', anything:'notneeded')

This is translated into:

test([anything:'notneeded'], 'yet', 'another good')

i.e. all named parameter styled arguments (with the colon) are put into a map and placed at the start of the argument list. All remaining parameters are placed after that.

So Groovy now looks for a signature test(Map, String, String) and correctly finds your method.

If there is no named parameter, this conversion does not happen and the signature would be test(String, String), which does not have a matching method.

So the solution is to create an additional method that matches the call without named parameters:

def test(some, thing='default value'){
    test([:], some, thing)
}

That way, both named and not named calls are supported.

score 0 · Answer 2 · answered Jan 12 '19 at 21:08

It depends on what the existing method signature looks like. Let's say you have an existing method with 1 parameter, like this:

def test(String input) { ... }

If you want to add the ability to pass one additional positional parameter you can simply add an overload:

def test(String input, String output) { ... }

If you want to add optional named parameters, you could add this overload:

def test(Map namedArgs, String input) { ... }

If you call "test('')" it will run the original method. "test('one', 'two')" will run the secobd version. "test('one', two: 'two')" will run the third.

You may want to add a 4th overload if you want to support the 2 positional parameters and one or more named parameters.

How to have optional named arguments in Groovy method

2 Answers2