Convert a postfix expression to an array of numbers and operators. For example: "31 4 5 7/* - " should convert to an array with each digit and operator as an individual item. There can or cannot be space between numbers and operators just like there is no space between 7 and /, but there is space between * and -. Also 31 should be one item and not two items like 3 and 1 in the array.

The code should be in Java.