Java code to generate random number of given(n) length

I have to generate a random number of length given as input (20 , 256 , 2000 , 3000). What should i use . Any best practices?

There are various methods telling range like below:

int rangeMin = 1,
    rangeMax = 100,
    generatedNumber;
generatedNumber = rangeMin + rand() % rangeMax;

but i need to work on length of interger