I want to draw a Bollinger Band on my own (that is, NOT via adding BB to a chart).

What is the best practice for that? Currently I'm doing this:

  IPolyLineChartObject upper;
  IPolyLineChartObject middle;
  IPolyLineChartObject lower;

  public void onStart(IContext ctx) throws JFException
  {
    this.ctx = ctx;
    engine = ctx.getEngine();
    this.console = ctx.getConsole();
    indicators = ctx.getIndicators();
    chart = ctx.getChart(Instrument.EURUSD);
    upper = chart.getChartObjectFactory().createPolyLine();
    upper.setColor(Color.ORANGE);
    middle = chart.getChartObjectFactory().createPolyLine();
    middle.setColor(Color.RED);
    lower = chart.getChartObjectFactory().createPolyLine();
    lower.setColor(Color.ORANGE);
    chart.addToMainChart(upper);
    chart.addToMainChart(middle);
    chart.addToMainChart(lower);
    console.getOut().println("Started");
  }

  public void onTick(Instrument instrument, ITick tick) throws JFException
  {
  }


  public void onBar(Instrument instrument, Period period, IBar askBar, IBar bidBar) throws JFException
  {
    if (!instrument.equals(Instrument.EURUSD) || !period.equals(Period.FIFTEEN_MINS))
      return;
    double[] currentValues = indicators.bbands(instrument, period, OfferSide.ASK, IIndicators.AppliedPrice.CLOSE, timePeriod, nbDevUp, nbDevDn, maType, 1);
    long time = askBar.getTime();
    System.out.println(time+"/0: "+currentValues[0]+", 1: "+currentValues[1]+", 2: "+currentValues[2]);
    upper.addNewPoint(time, currentValues[0]);
    middle.addNewPoint(time, currentValues[1]);
    lower.addNewPoint(time, currentValues[2]);
  }

But the PolyLines grow and will certainly lead to an out of memory at some time, I don't find a way to remove points (e.g. the oldest). So what is the best practice to do that?

Thanks!

Currently it is not possible to remove points from a PolyLine. What you can consider is doing something similar to this:

        int pointCount = polyLine.getPointsCount();
        if (pointCount > 1000){
           IPolyLineChartObject polyLineNew = chart.getChartObjectFactory().createPolyLine();
           //copy last 500 points from polyline the old polyline
           for (int i = 500; i< pointCount; i++){
              polyLineNew.addNewPoint(polyLine.getTime(i), polyLine.getPrice(i));
           }
           chart.remove(polyLine);
           chart.addToMainChart(polyLineNew);
        }

Thanks. There seems to be an error with polylines. I'm getting this from time to time. Seems the last point will be connected to a point far in the past. This continues until I stop the app so every new point is connect to the past one.

Screenshot attached 3 polylines).

Just found this is a bug in your API:

https://www.dukascopy.com/swiss/english/forex/jforex/forum/viewtopic.php?f=16&t=42255

Is it already fixed or, if not, when will it be fixed?

This is a consequence of a fact, that currently a poly line is limited to 100 points, so as a temporary workaround please consider making a new poly line after each 100 points.
This will be fixed with the next release, follow the release information here:
viewtopic.php?f=71&t=28934