How to concatenate a number

I am trying to figure out how to concatenate the same number. For instance, if a randomly generated the number is 621, how do you make it 621621? I can do it in the output but cannot figure out how to do it with a statement.

I've tried the following, none of which have worked:

int n = (int)Math.floor(Math.random()*1000+1);

String a = "n" + "n";

int n = (int)Math.floor(Math.random()*1000+1);

int a = "n" + "n";

int n = (int)Math.floor(Math.random()*1000+1);

int a = "n + n";

int n = (int)Math.floor(Math.random()*1000+1);

int a = "(n + n)";