I'm trying to draw a vertical line according to IChart wiki (https://www.dukascopy.com/wiki/index.php?title=IChart), but am getting this exception:
08:11:36 java.lang.NoSuchMethodException: com.dukascopy.charts.drawings.VLineChartObject.<init>(java.lang.String, long, double) @ jforex.ColorTest.onTick(ColorTest.java:37)

public void onTick(Instrument instrument, ITick tick) throws JFException {
        IChart chart = context.getChart(Instrument.EURUSD);
        double price1 = (tick.getAsk() + tick.getBid()) / 2;
        IVerticalLineChartObject vLine = chart.getChartObjectFactory().createVerticalLine("vLine", // line 37
            tick.getTime(), price1
        );
        chart.addToMainChart(vLine);         
}

The given examples in wiki are compatible to JForex API version 2.6.43, for earlier versions please use empty chart object constructors and thereafter set the price, time and other values to the newly created object. For VLINE use:

double price1 = (tick.getAsk() + tick.getBid()) / 2;
IVerticalLineChartObject vLine = chart.getChartObjectFactory().createVerticalLine();
vLine.setPrice(0, price1);
vLine.setTime(0, tick.getTime());
chart.addToMainChart(vLine);

Thank you. Is there a way to specify what the object's key should be?

Unfortunately, not, you can just retrieve the key of an already created IChartObject.