Method reference clarification

Question about the task

Convert a list to an array

Java Syntax, Level 19, Lesson 2

Under discussion

What is the difference between doing this

strings.toArray(new String[]{});

and this

strings.toArray(String[]::new);

They both work but what is the benefit of choosing the latter over the former?

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UltraGameCoder Java Developer

12 July 2022, 23:53

They aren't much different from one another, the second one (String[]::new) just takes a few extra steps. But lets get to know how much more. When you look back at the decompiled bytecode, this is what you'll get:

strings.<String>toArray(new String[0]); //Lets call this array1

(String[])strings.toArray(x$0 -> new String[x$0]); //Lets call this array2

What does this tell us? Well, to begin with, they use different toArray methods. But what do they do? array1 will use ArrayLists's toArray(T[] a) method. And array2 uses the Collection's default interface method toArray(IntFunction<T[]> generator). But now you may wonder, how are these different? Well, when array2 is passed through the toArray(IntFunction<T[]> generator) method, it calls its refering IntFunction with the length 0

generator.apply(0)

when that's done, it results in the creation of a

new String[0]

array object which then is passed over to the Collection's interface method

toArray(T[] a)

which, if you look closely, will just call the same method that was called for array1. So in the end, the collection interface catches array2 as another function to supply himself with an array object. And then passes it along to the same method array1 would go through.