Introduction to Groovy - Learn Java for Web Development - page 412

Listing B-25. Calling a Closure

1. def closureVar = {println 'Hello world'}
2. println "closure is not called yet"
3. println " "
4. closureVar.call()

 Line 1: This line has the closure with no parameters and consists of a single

println statement. Because there are no parameters, the parameter List
and the -> separator are omitted. The closure is referenced by the identifier
closureVar .
Line 4: This line uses the explicit mechanism via the

call() method to invoke
the closure. You may also use the implicit nameless invocation approach:
closureVar() . As shown in the output, the closure prints “Hello world” when it is
called in line 4, not when it is defined in line 1.



Here is the output:

closure is not called yet
Hello world

Listing B-26 illustrates the same closure as in Listing B-25 but with the parameter.

Listing B-26. Closure with Parameter

1. def closureVar = {param -> println "Hello ${param}"}
2. closureVar.call('world')
3. closureVar ('implicit world')

 Line 2 : This is an explicit call with the actual argument 'world' .
 Line 3 : This is an implicit call with the actual argument 'implicit world' .

Hello world
Hello implicit world

As Listing B-27 illustrates, the formal parameters to a closure may be assigned default values.

Listing B-27. Parameters with Default Values

1. def sayHello= {str1, str2= " default world" -> println "${str1} ${str2}" }
2. sayHello("Hello", "world")
3. sayHello("Hello")

 Line 1 : The sayHello closure takes two parameters, of which one parameter,
str2 , has a default value.
 Line 3 : Only one actual parameter is provided to the closure, and the default
value of the second parameter is used.