Collection constructors should not be used as java.util.function.Function

It is very common to pass a collection constructor reference as an argument, for example Collectors.toCollection(ArrayList::new) takes the ArrayList::new constructor. When the method expects a java.util.function.Supplier it is perfectly fine. However when the me argument type is java.util.function.Function it means that an argument will be passed to the constructor.

The first argument of Collections constructors is usually an integer representing its "initial capacity". This is generally not what the developer expects, but the memory allocation is not visible at first glance.

This rule raises an issue when a collection constructor is passed by reference as a java.util.function.Function argument.

Noncompliant Code Example

Arrays.asList(1, 2, 54000).stream().collect(Collectors.toMap(Function.identity(), ArrayList::new));

Compliant Solution

Arrays.asList(1, 2, 54000).stream().collect(Collectors.toMap(Function.identity(), id -> new ArrayList<>()));