Groovy Closure with optional arguments

问题:

I want to define a closure which takes one argument (which i refer to with it ) sometimes i want to pass another additional argument to the closure. how can i do this?

回答1:

You could set the second argument to a default value (such as null):

def cl = { a, b=null ->
  if( b != null ) {
    print "Passed $b then "
  }
  println "Called with $a"
}

cl( 'Tim' )          // prints 'Called with Tim'
cl( 'Tim', 'Yates' ) // prints 'Passed Yates then Called with Tim

Another option would be to make b a vararg List like so:

def cl = { a, ...b ->
  if( b ) {
    print "Passed $b then "
  }
  println "Called with $a"
}

cl( 'Tim' )                    // prints 'Called with Tim'
cl( 'Tim', 'Yates' )           // prints 'Passed [Yates] then Called with Tim
cl( 'Tim', 'Yates', 'Groovy' ) // prints 'Passed [Yates, Groovy] then Called with Tim

回答2:

hopefully this helps

​def clr = {...a ->  
    print "Passed $a then "
    enter code here

}

​clr('Sagar')
clr('Sagar','Rahul')

回答3:

The variants from @tim_yates do not work with @TypeChecked (in a class context), at least with Groovy 2.4.11 where the default arg is ignored and does not compile :-(

So other (admittedly uglier) solutions that would work in this case are:

1. declaring the closure first seems to work fine (necessary for recursions anyway):

   def cl
   cl = { ... }
   

   • at least in Eclipse Neon / Groovy-Eclipse Plugin 2.9.2 the code completion/suggestions do not work either way using the closure later on in the same code block => so nothing lost as far as I can say

2. with @TypeChecked(value=TypeCheckingMode.SKIP) it would work for both, but then you would loose the type checking on the method (or class, depending on where you put it)

3. declare closure delegate cl2:

   @TypeChecked
   class Foo { 
   
     static main( String[] args ) {
   
       def cl = { a, b ->
         if( b != null )
           print "Passed $b then "
         println "Called with $a"
       }
       def cl2 = { a -> cl( a, null ) }
   
       cl2( 'Tim' )         // prints 'Called with Tim'
       cl( 'Tim', 'Yates' ) // prints 'Passed Yates then Called with Tim           
     }
   }
   

4. transform closure into class method, e.g.

   @TypeChecked
   class Foo { 
   
     cl( a, b=null ) {
       if( b != null )
         print "Passed $b then "
       println "Called with $a"
     }
   
     static main( String[] args ) {
       cl( 'Tim' )          // prints 'Called with Tim'
       cl( 'Tim', 'Yates' ) // prints 'Passed Yates then Called with Tim           
     }
   }