LineParsingIterator

import java.io.BufferedReader;
import java.io.IOException;
import java.util.Iterator;
import java.util.NoSuchElementException;

/** Streams the lines of a {@link BufferedReader} while applying some parsing method. */
abstract class LineParsingIterator<T> implements Iterator<T> {
	private final BufferedReader input;

	private T buffered; // parsed but not yet returned to the client
	private int lineNumber;

	public LineParsingIterator(BufferedReader input) {
		this.input = input;
	}

	/** @return {@code null} to discard the current line (e.g. comments) */
	protected abstract T parse(String line, int lineNumber);

	@Override
	public boolean hasNext() {
		if (buffered == null) buffered = findAndParseNext();
		return (buffered != null);
	}

	@Override
	public T next() {
		if (buffered == null) buffered = findAndParseNext();
		if (buffered == null) throw new NoSuchElementException();
		T result = buffered;
		buffered = null;
		return result;
	}

	@Override
	public void remove() {
		throw new UnsupportedOperationException();
	}

	private T findAndParseNext() {
		while (true) {
			String line = readLine();
			if (line == null) return null; // EOF
			T result = parse(line, lineNumber);
			if (result == null) continue;
			return result;
		}
	}

	private String readLine() {
		try {
			String line = input.readLine();
			if (line != null) lineNumber += 1;
			return line;
		} catch (IOException e) {
			throw new ParsingException(e);
		}
	}
}