Just a quick java iterator example.

IteratorExample.java

// assume some stuff like this exists
ArrayList<String> lines = ...
BufferedWriter writer = ...

// get your iterator instance.
Iterator<String> iterator = lines.iterator();
String write = null;

// iterate + check inside of the while loop condition test.
while (iterator.hasNext() && (write = iterator.next()) != null) {
  writer.append(write);
  writer.flush();
  writer.newLine();
}