Getting a character at a given index in a String and returing as a String

Question

Hello,
How do I simply grab one character in a string and return it as a string? I saw the substring() method but it does not like when I invoke:
String test = "abcd";
test = test.substring(0,0);
**Above produces a runetime error. ***I need something that does this:
String test = "abcd"
test.needtodo(0) returns String "a".
test.needtodo(1) returns String "b".
ETC.
---- Thanks for the help.

Answer 1

Ok I used Kaj's method, but what I am trying to do still inst working properly. Here is what I am trying to do:
public String stringToBinaryStringWithLeadingZeroz(String input) {
     String binaryString="";
     int sizeOf = input.length();
     for(int i=0; i < sizeOf; i++)
          int intRepresentation = Integer.parseInt(String.valueOf(input.charAt(i)), 16);
          binaryString += Integer.toBinaryString(intRepresentation);
     return binaryString;
}I am trying to take in a String that is Hex and return a string that is binary with leading zeros.
So if the input is: stringToBinaryStringWithLeadingZeroz("3");
It should be returning 0011 but its still only returning 11. It shouldnt be returning 0011 though. I am passing each character indivudally into the method toBinaryString() which just returns the result and appends onto the result.
** SCRATCHING MY HEAD **