Interpolation in Java: Mapping a random number from one range to another

I have generated a number z from a range (x,y) and want to map z to another range (p,q)

This question appears to be answered here using matlab:

https://www.mathworks.com/matlabcentral/answers/379380-mapping-of-a-random-number-in-one-range-to-another-range

I need to do this in java but also need to understand what this 'interp' is doing.