ArrayList to Array: Why is `toArray(new MyClass[0])` Faster Than `toArray(new MyClass[myList.size()])`?

The Array Conundrum: .toArray(new MyClass[0]) vs .toArray(new MyClass[myList.size()])

When dealing with ArrayLists, converting them to arrays often arises. To do so, one can employ either of these two techniques:

// Option 1: Specify Array Size
MyClass[] arr = myList.toArray(new MyClass[myList.size()]);

// Option 2: Blank Array
MyClass[] arr = myList.toArray(new MyClass[0]);

In terms of performance, an unexpected finding emerged: on Hotspot 8, using an empty array surprisingly outperforms the presized version:

Benchmark                      (n)  Mode  Samples    Score   Error  Units
c.a.p.SO29378922.preSize         1  avgt       30    0.025 ▒ 0.001  us/op
c.a.p.SO29378922.preSize       100  avgt       30    0.155 ▒ 0.004  us/op
c.a.p.SO29378922.preSize      1000  avgt       30    1.512 ▒ 0.031  us/op
c.a.p.SO29378922.preSize      5000  avgt       30    6.884 ▒ 0.130  us/op
c.a.p.SO29378922.preSize     10000  avgt       30   13.147 ▒ 0.199  us/op
c.a.p.SO29378922.preSize    100000  avgt       30  159.977 ▒ 5.292  us/op
c.a.p.SO29378922.resize          1  avgt       30    0.019 ▒ 0.000  us/op
c.a.p.SO29378922.resize        100  avgt       30    0.133 ▒ 0.003  us/op
c.a.p.SO29378922.resize       1000  avgt       30    1.075 ▒ 0.022  us/op
c.a.p.SO29378922.resize       5000  avgt       30    5.318 ▒ 0.121  us/op
c.a.p.SO29378922.resize      10000  avgt       30   10.652 ▒ 0.227  us/op
c.a.p.SO29378922.resize     100000  avgt       30  139.692 ▒ 8.957  us/op

The JVM and JIT compiler optimize creation and initialization of new, correctly sized arrays. However, these optimizations cannot be applied to user-created arrays. Therefore, using an empty array allows the JVM to perform these optimizations, resulting in improved performance.

The above is the detailed content of ArrayList to Array: Why is `toArray(new MyClass[0])` Faster Than `toArray(new MyClass[myList.size()])`?. For more information, please follow other related articles on the PHP Chinese website!