use this
if (choice == 1)
output = Integer.toString(input, 2);
else if (choice == 2)
output = Integer.toString(input, 8);
// if the user chooses choice #2, it will convert from base 10 to base of 8
else if (choice == 3)
output = Integer.toString(input, 16);
else
System.out.println("invalid entry");