How Does Array Sizing Impact ArrayList to Array Conversion Performance in Java?

Sizing the Array in toArray

When converting an ArrayList to an array using the toArray method, there are two options for specifying the size of the array: using a预先调整大小的数组或一个空的数组。 Let's explore their performance implications.

Assuming we have an ArrayList called myList, we can use the following syntax to create an array:

MyClass[] arr = myList.toArray(new MyClass[myList.size()]); // Presized array
MyClass[] arr = myList.toArray(new MyClass[0]); // Empty array

Intuitively, resizing the array might seem more efficient, but benchmark results using Java Microbenchmark Harness (JMH) reveal a surprising finding. Creating an array with an empty size parameter (new MyClass[0]) consistently outperforms the presized version (new MyClass[myList.size()]).

The JVM and JIT compiler have optimizations that can efficiently create and initialize arrays of the correct size. By providing an empty array, the compiler can take advantage of these optimizations, leading to better performance. This advantage is especially noticeable as the array size increases.

Here are the benchmark results:

Size	Presized Array (μs)	Empty Array (μs)
1	0.025	0.019
100	0.155	0.133
1000	1.512	1.075
5000	6.884	5.318
10000	13.147	10.652
100000	159.977	139.692

As you can see, the empty array method offers a slight but consistent performance advantage. However, it's important to note that these results may vary depending on the specific JVM and JIT compiler optimizations, as well as the size and characteristics of your data.

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