findResults和Groovy中的collect有什么区别？

Question

Here is the code using collect

​def lst = [1,2,3,4];      
def newlst = [];       
newlst = lst.collect {element -> return element * element}       
println(newlst);

Here is the code using findResults

def lst2 = [1,2,3,4];      
def newlst2 = [];       
newlst2 = lst2.findResults {element -> return element * element}       
println(newlst2);

​Both seem to return [1, 4, 9, 16] so what is the difference? Thanks!

Answer 1

Basically the difference is how they deal with null values

collect when sees null will collect it, while findResults won't pick it.

In other words, the size of resulting collection is the same as the size of input when using collect .

Of course you could filter out the results but its an additional step

Here is a link to the example I've found in the internet

Example:

​def list = [1, 2, 3, 4]
println list.coll​​​​​​​​​​​​​​ect { it % 2 ? it : null}
// [1, null, 3, null] 
println list.findResults { it % 2 ? it : null}​
// [1,3]

Answer 2

When we need to check if returned list is empty then findResults seem more useful. Thanks to Mark for the answer.

def list = [1, 2, 3, 4] 

def l1 = list.collect { it % 100 == 0 ? it : null}
def l2 = list.findResults { it % 100 == 0 ? it : null}

if(l1){
 println("not null/empty " + l1)
}

if(l2){
  println("not null/empty " + l2)
}

​