`myList.toArray(new MyClass[myList.size()]) vs. myList.toArray(new MyClass[0]): Which Offers Better Performance in Java?`

Performance Implications of toArray with Array Initialization

The question of whether myList.toArray(new MyClass[myList.size()]) or myList.toArray(new MyClass[0]) offers better performance has piqued curiosity among Java developers. While the second approach appears more succinct, the assumption that the empty array is omitted from creation might not always hold true.

Benchmark Results

To shed light on this matter, a microbenchmark was conducted using Java HotSpot 8. The results, presented below, reveal that the counterintuitive toArray(new MyClass[0]) method consistently outperforms its toArray(new MyClass[myList.size()]) counterpart:

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

Explanation

The Java Virtual Machine (JVM) and JIT compiler employ optimizations that allow them to efficiently create and initialize arrays of the appropriate size. These optimizations are not available when arrays are manually created.

Conclusion

Based on the benchmark results and further analysis presented in the blog post Arrays of Wisdom of the Ancients, it is evident that myList.toArray(new MyClass[0]) is the recommended approach for situations where performance is a concern. The empty array creation overhead is effectively eliminated by the JVM's optimizations, resulting in faster performance.

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