Дата-время:

Convert Instant to LocalDateTime:

LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneOffset.UTC);

Построчное чтение файлов:

Path file= Paths.get("file.txt");

try(BufferedReader reader = Files.newBufferedReader(file, Charset.forName("Windows-1251"))) {

    String currentLine;

    while((currentLine = reader.readLine()) != null) {
        //...
    }

}catch(IOException ex){
    ex.printStackTrace();
}

DOM

Сохранение в файл dom-документа

import org.w3c.dom.Document;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

Document document = ...;

try {
    TransformerFactory transformerFactory = TransformerFactory.newInstance();
    Transformer transformer = transformerFactory.newTransformer();
    DOMSource source = new DOMSource(document);
    StreamResult result = new StreamResult(new File("file.xml"));
    transformer.transform(source, result);

} catch (TransformerException | IOException e) {
   e.printStackTrace();
}