Best way of generating a random number

Question

In order to get fake IDs for testing needs I wrote something that works and is reusable, but I'd like to know if there is better ways because my code seems heavy.

n is the total caracters of the string I want.

First, I think about the conversion between number and strings I could do better.

Got some ideas ?

final integer STRING_LENGTH = 14;
String aleaNumberN=String.valueOf((integer)(Math.ceil(Math.random()*9)));
for(integer j = 1; j<n; j++) {
    aleaNumberN += String.valueOf((integer)(Math.ceil(Math.random()*9)));
}
System.debug('aleaNumberN ? --> ' + aleaNumberN);
System.assert(aleaNumberN.length()==STRING_LENGTH);

Answer 1

final integer STRING_LENGTH = 3;
final integer MIN = Math.pow(10,STRING_LENGTH -1); //Returns 100
final integer MAX = Math.pow(10,STRING_LENGTH) - 1; // Returns 999
String aleaNumberN = String.valueOf(Math.Round(Math.Random() * (MAX-MIN) + MIN));

If you replace some vars:

final integer STRING_LENGTH = 3;
String aleaNumberN = String.valueOf(Math.Round(Math.Random() * ((Math.pow(10,STRING_LENGTH) - 1)-Math.pow(10,STRING_LENGTH -1)) + Math.pow(10,STRING_LENGTH -1)));

Edit: Be carefull with string length, maybe using long is required

Update (Having 0's in the left):

final integer STRING_LENGTH = 3;
final integer MAX = Math.pow(10,STRING_LENGTH) - 1; // Returns 999
String aleaNumberN = String.valueOf(Math.Round(Math.Random() * MAX)).leftPad(STRING_LENGTH,'0');