Concating 2 numbers without using java libraries

public static long concatenateNumbers(long x, long y) {
        long temp = y;
        do {
            temp = temp / 10;
            x = x * 10;
        } while (temp > 0);
        return (x + y);
    }

Example: 230, 400 => 230400


How does this work?
x = 230; y =400
temp  = y; => 400
case 1: temp = 400/10 = 40 ; x = 230*10 = 2300

case 2: temp =40> 0; 40/10=4; x = 2300*10 = 23000

case 3: temp =4> 0; 4/10=0; x = 23000*10 = 230000

case 2: temp =0> 0 ; //false.
x + y =>  230000+400= 230400



Why do.. while ? why can't while..do ??
Example: 230, 0= > 2300
If i use while(temp> 0) // here itself it will fail, since temp = 0;

How do with java libraries ?

public static long concatenateNumbers(long x, long y) {
            retrun Long.parseLong( Long.toString(x)+Long.toString(y));
    }