Closures, another powerful Groovy feature, represent syntactical shorthands that can save you a few lines of code. You can read about them in detail in the official Groovy documentation.

Sorting

You can utilize the method sort on a collection or a map, passing the comparator in the form of a Closure.

def competitors = api.productExtension('Competition')

// a and b are named parameters and represent the arguments the Closure is called with. In this case it's the two elements being compared.
// you can rename a and b to whatever you want
def sortedByName = competitors.sort { a, b -> 
    // assuming attribute2 is a number, we need to convert it to BigDecimal in order to compare it correctly (as a number, not as a string)
	return a.attribute2?.toBigDecimal() <=> b.attribute2?.toBigDecimal() 
}

Iterating

Iterating through a collection or a map is far better when done with Closures.

def competitors = api.productExtension('Competition')
 
competitors.each { c -> api.trace('Competitor name', '', c.attribute1)}
 
// in case we don't name the closure parameter, we can reference it with variable called 'it'
competitors.each { api.trace('Competitor name', '', it.attribute1}
 
// iterating with an index
competitors.eachWithIndex { comp, i -> api.trace('Competitor ${i}', '', comp.attribute1) }
 
// you can also iterate through a map using an Entry
map.each { api.trace(it.key, '', it.value) }
 
// or using key and value
map.each { key, value -> api.trace(key, '', value) }

Filtering Collection

Create map from a collection

def competitors = api.productExtension('Competition')

// get competitors whose price (attribute2) is greater than 100
def filtered = competitors.findAll{ it.attribute2?.toBigDecimal() > 100 }

Transforming Collection

Transforming one collection of records into another on one line:

Create map from a collection

def competitors = api.productExtension('Competition')

// convert to a collection of competitors' names
def names = competitors.collect{ it.attribute1 }

Creating Map from Collection

Closures allow you to create a map out of a collection on one line:

Create map from a collection

def competitors = api.productExtension('Competition')

// return an object with the attribute1 as key and attribute2 as value
// notice the parentheses around the key
competitors.collectEntries{ [(it.attribute1) : it.attribute2] }

Strong Typing

Sometimes it is useful to create an object of a certain Class and not to rely on Groovy. You can do it like this:

// the list will be a HashSet
def list = [] as HashSet
 
// the map will be a TreeMap
def map = [:] as TreeMap