We have modified your code:

class Output { 
    public static void main(String[] args) { 
         int i = 1;
         int j = 1;
         while (i < 5) {
             i++; 
             j = j * 2; 
             } 
             System.out.println(j);
    }
}   



In the code, value of i incremented 4 times, so at first, value of j=1, then 1*2=2
                                         value of j is changed to 2 so j=2, then 2*2=4
                                         value of j is changed to 4 so j=4, then 4*2=8
                                         value of j is chnaged to 8 so j=8, then 8*2=16
                                         So at last we get the value of j after last
    counter is 16, so output is 16.