Fastest way to pad a number in Java to a certain number of digits?

I would stop doing optimizations based on micro-benchmarks and go for something that looks elegant codewise, such as String. Format("%0"+numDigits+"d", testNumber).

Inserting padding characters one by one is obviously slow. If performance is really that big a concern, you could use predefined string constants of lengts 1..n-1 instead (where n is the biggest expected length), stored in an ArrayList at the corresponding indexes.

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