For the input -2147483648, your program outputs just minus .

In convert(), I personally wouldn't bother with int units = num;, and I'd just use num.

Instead of instantiating a StringBuilder in convertHundredsImpl(), I'd just pass the original StringBuilder to the helper function. For consistency then, I'd also pass a StringBuilder to the convertUnitsImpl() helper. In accordance with the design of the StringBuilder class, I'd have those helpers return the same StringBuilder as well, to allow for fluent chaining, like convertHundredsImpl(sb, thousands).append(" thousand ").

The while (true) loop in main() gives you no graceful way to exit. It should be while (scanner.hasNextInt()) instead.

For the input -2147483648, your program outputs just minus . Also, you never handle 0 anywhere.

In convert(), I personally wouldn't bother with int units = num;, and I'd just use num.

Instead of instantiating a StringBuilder in convertHundredsImpl(), I'd just pass the original StringBuilder to the helper function. For consistency then, I'd also pass a StringBuilder to the convertUnitsImpl() helper. In accordance with the design of the StringBuilder class, I'd have those helpers return the same StringBuilder as well, to allow for fluent chaining, like convertHundredsImpl(sb, thousands).append(" thousand ").

The while (true) loop in main() gives you no graceful way to exit. It should be while (scanner.hasNextInt()) instead.