Learn Java for Web Development

416 APPENDIX B: Introduction to Groovy

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.