`toArray(new MyClass[0]) or toArray(new MyClass[myList.size()]): Which is Faster for Java ArrayLists?`

toArray(new MyClass[0]) or toArray(new MyClass[myList.size()])?

In the context of Java ArrayLists, the toArray method can be used to convert an ArrayList to an array of a specified type. However, choosing between using an empty array or an array of the exact size of the list can impact performance.

Performance Considerations

Intuitively, it might seem that presizing the array, as in toArray(new MyClass[myList.size()]), would be more efficient. However, micro benchmarking reveals that using an empty array, toArray(new MyClass[0]), consistently outperforms the presizing approach, especially for larger lists.

Why Empty Array Is Faster

This counterintuitively faster performance is due to JVM optimizations. The JVM and JIT compiler have built-in optimizations for creating and initializing arrays, which are not utilized when creating the array manually.

Real-World Implications

While the difference in performance may not be significant for small arrays, it can become noticeable for larger ones. Therefore, for performance-critical situations or optimized inner loops, using toArray(new MyClass[0]) is recommended.

Summary

Contrary to initial expectations, using an empty array (toArray(new MyClass[0])) for ArrayLists exhibits superior performance compared to presizing the array. This is attributed to JVM optimizations that handle array creation and initialization efficiently, which cannot be leveraged when manually creating the array.

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