Question

I need to create fixed length records which consist mostly of space but which are sparsely populated at certain known positons. Essentially what I need to produce is a legacy file format in which the records consist of a large number of fixed length fields. I only need to populate a handful of these fields so I want to create the empty buffer full of spaces first, and write formatted fields of specifc length at specific positions

Answer 1

Try this:

StringBuilder b = new StringBuilder();// Or use StringBufer if you need synchronization
b.append("----------"); //use dash instead of space for visibility
int pos = 4; 

String replacement = "foo";
b.replace(pos, pos + replacement.length(), replacement); //Attention: if the length of the replacement is greater than the length of the original content, the exceeding chars will be appended
System.out.println(b); //----foo---