What you are trying to do can not be done with Strings because their data physically can not be changed in Java. If you want to change the data at specific subscripts, you must use class StringBuffer.
You can however do the following with Strings:
String myString = null;
myString = myString + 'A';
myString = myString + 'B';
myString = myString + 'C';
Understand though that each statement creates another myString reference to the String data each time it increases. myString references, let say 'A' after first line executes. Then the second line will have myString reference "AB". There is no reference to 'A' anymore so eventually the garbage collector will release its resource.
Hope this make sense.
Brian