a 'char' is a primitive type, like int, float, and byte. an ArrayList can only hold objects. Character, Integer, Float, and Byte are the object versions of these classes. you can wrap a primitive type as an object like so:

Code:

ArrayList let = new ArrayList();
  String s_line = "This is a test";
  for(int i=0;i<s_line.length();i++)
  {
    char ch = s_line.charAt(i);
    Character co = new Character(ch);
    let.add(co);
} 

though storing chars in an ArrayList is a pretty inefficient way to work (which is why there's a distinction between primitive types and objects in Java), so i'd recommend using an array instead, e.g.:

char[] c = s_line.toCharArray();

then you can use the usual array method (expand(), .length,  append()) on the array.