Checking if a collection is null or empty in Groovy

Question

I need to perform a null or empty check on a collection; I think that !members?.empty is incorrect. Is there a groovier way to write the following?

if (members && !members.empty) {
    // Some Work
}

Avi

Answer 1

There is indeed a Groovier Way.

if (members) {
    //Some work
}

does everything if members is a collection. Null check as well as empty check (Empty collections are coerced to false). Hail Groovy Truth. :)

Kanal

A more "groovier" way is that for example if you are interested in the maximum age out of the members then you can write the following: members?.age.max() – Nuristan

Note: members?.age.max() blows up with "Cannot invoke method max() on null object" when members is null. You would need members?.age?.max() – Dickman

@Nuristan I think you mean members*.age.max() – Kile

no: GreenGiant's solution is the best: check List members = null; and List members = [ [age: 12], [age: 24], [age: null], null ] against both the solutions – Nuristan

This type of check is working for most of the cases but if your purpose is to check if a variable is null then you might end up to an edge case where the variable is not null but a boolean false – Galbanum

this solution is perfect for the case where multiple statements needs this check. it works fine for list also for checking not-null and non-empty. for single statement I prefer to use null-safe operator as GreenGiant suggests – Wallah

Can we test if a parameter is null in this way? if(!members){ // do something } – Tisatisane

@Tisatisane You will run into issues if you try !members if your intention is to do something only when members is null. See the examples here which showcases how Groovy Truth works. It will be better if you explicitly check for null members == null – Kanal

Answer 2

FYI this kind of code works (you can find it ugly, it is your right :) ) :

def list = null
list.each { println it }
soSomething()

In other words, this code has null/empty checks both useless:

if (members && !members.empty) {
    members.each { doAnotherThing it }
}

def doAnotherThing(def member) {
  // Some work
}

Garretgarreth

This code will fail on line 2 null.each will throw an NPE. The second block of code uses the original user's post. I think you may have misunderstood the question. – Ils

Answer 3

!members.find()

I think now the best way to solve this issue is code above. It works since Groovy 1.8.1 http://docs.groovy-lang.org/docs/next/html/groovy-jdk/java/util/Collection.html#find(). Examples:

def lst1 = []
assert !lst1.find()

def lst2 = [null]
assert !lst2.find()

def lst3 = [null,2,null]
assert lst3.find()

def lst4 = [null,null,null]
assert !lst4.find()

def lst5 = [null, 0, 0.0, false, '', [], 42, 43]
assert lst5.find() == 42

def lst6 = null; 
assert !lst6.find()

Event

the collection that has 1 null element is not empty, so your suggestion is wrong – Kronfeld

What if the collection is null? – Dorado

def lst6 = null; assert !lst6.find() it's correct - no error occurs – Event