Question

Suppose I have a class Point and a function to process Point instances

class Point { private final int x, y; ... } 
...
void handlePoints(Iterable<Point> points) { for (Point p: points) {...} }

Now I would like to read points from a file. Each line of the file contains two numbers, so I have a function (“factory method”) to create a point from a line.

Point makePoint(String line) { ... }

What should I do now? I can write a function to read the file to a list of points and call the handlePoints function.

List<Point> readPoints(BufferedReader reader) {...} // use makePoint here

void handlePoints(BufferedReader reader) {
   List<Point> points = readPoints(reader); 
   handlePoints(points);
}

Unfortunately this function does not seem particularly elegant since it creates an unnecessary list of points in memory.

Wouldn’t it be better to use iterators ?

void handlePoints(Iterator<Point> points) {...}

Iterator<Point> readPoints(BufferedReader reader) {...} // use makePoint here

void handlePoints(BufferedReader reader) {
   Iterator<Point> points = readPoints(reader); 
   handlePoints(points);
}

Does it make sense? Won’t be this code too “noisy” in Java?

Answer 1

If you don’t need to have all points in memory, think of something more along these lines:

while (reader.ready())
{
  String line = reader.readLine();
  Point point = makePoint(line);
  handlePoint(point);
}

How to do this with an iterator and handlePoints: (code for handling exceptions to be added)

class PointIterator implements Iterator<Point>
{
  BufferedReader reader;
  PointIterator(BufferedReader myReader) { reader = myReader; };
  @Override
  public boolean hasNext() { return myReader.ready(); };
  @Override
  public Point next() { return makePoint(myReader.readLine()); };
  @Override
  public void remove()
  { throw new UnsupportedOperationException("Remove not supported!"); };
}

And because handlePoints takes an Iterable:

class PointIterable implements Iterable<Point>
{
  BufferedReader reader;
  public PointIterable(BufferedReader myReader) { reader = myReader; };
  @Override
  public Iterator<Point> iterator() { return new PointIterator(reader); }
}

To use:

handlePoints(new PointIterable(reader));