Calls to a collection’s toArray(E[]) method should specify a target array of zero size. This allows the JVM to optimize the memory allocation and copying as much as possible.

Previous versions of this rule (pre PMD 6.0.0) suggested the opposite, but current JVM implementations perform always better, when they have full control over the target array. And allocation an array via reflection is nowadays as fast as the direct allocation.

See also Arrays of Wisdom of the Ancients

Note: If you don’t need an array of the correct type, then the simple toArray() method without an array is faster, but returns only an array of type Object[].

Noncompliant Code Example

List<Foo> foos = getFoos();

// inefficient, the array needs to be zeroed out by the jvm before it is handed over to the toArray method
Foo[] fooArray = foos.toArray(new Foo[foos.size()]);

Compliant Solution

List<Foo> foos = getFoos();

// much better; this one allows the jvm to allocate an array of the correct size and effectively skip
// the zeroing, since each array element will be overridden anyways
Foo[] fooArray = foos.toArray(new Foo[0]);