探究数组列表中的每个方法

Question

I met quite strange behaviour of each method on list of arrays in groovy. I have given piece of code.

def list = [
    [2, "foo"].toArray(),
    [4, "bar"].toArray()
]

list.each { def row ->
    println(row.length)
}

Which gives me pretty expecting result in console

2
2

Then I did small modification to this code

def list = [
    [2, "foo"].toArray(),
    [4, "bar"].toArray()
]

list.each { Object[] row ->
    println(row.length)
}

And result is

1
1

Because variable row is array with one element which is my original 2 elements array from list.

Is there some explanation for this? Im using groovy 1.8.8 or 2.1.2

Answer 1

I guess it's a feature rather than a bug :p

Object[] in a closure declaration has a special semantic for variable argument:

From http://groovy.codehaus.org/Closures+-+Formal+Definition :

Groovy has special support for excess arguments. A closure may be declared with its last argument of type Object[]. If the developer does this, any excess arguments at invocation time are placed in this array. This can be used as a form of support for variable numbers of arguments.

In your example, the argument passed to the closure will be wrapped again with a new Object array, containing list as the only element.

As an example:

def list = [
    [2, "foo"].toArray(),
    [4, "bar"].toArray()
]

def c = {Object[] args -> println args}
c(list)

Output:

[[[2, foo], [4, bar]]]