### Question:
XYSeriesCollection extends AbstractIntervalXYDataset implements IntervalXYDataset, DomainInfo, RangeInfo, 
        VetoableChangeListener, PublicCloneable, Serializable { public int indexOf(XYSeries series) { ParamChecks.nullNotPermitted(series, "series"); return this.data.indexOf(series); } XYSeriesCollection();  XYSeriesCollection(XYSeries series); @Override DomainOrder getDomainOrder(); void addSeries(XYSeries series); void removeSeries(int series); void removeSeries(XYSeries series); void removeAllSeries(); @Override int getSeriesCount(); List<XYSeries> getSeries(); int indexOf(XYSeries series); XYSeries getSeries(int series); XYSeries getSeries(Comparable key); @Override Comparable getSeriesKey(int series); int getSeriesIndex(Comparable key); @Override int getItemCount(int series); @Override Number getX(int series, int item); @Override Number getStartX(int series, int item); @Override Number getEndX(int series, int item); @Override Number getY(int series, int index); @Override Number getStartY(int series, int item); @Override Number getEndY(int series, int item); @Override boolean equals(Object obj); @Override Object clone(); @Override int hashCode(); @Override double getDomainLowerBound(boolean includeInterval); @Override double getDomainUpperBound(boolean includeInterval); @Override Range getDomainBounds(boolean includeInterval); double getIntervalWidth(); void setIntervalWidth(double width); double getIntervalPositionFactor(); void setIntervalPositionFactor(double factor); boolean isAutoWidth(); void setAutoWidth(boolean b); @Override Range getRangeBounds(boolean includeInterval); @Override double getRangeLowerBound(boolean includeInterval); @Override double getRangeUpperBound(boolean includeInterval); @Override void vetoableChange(PropertyChangeEvent e);  }### Answer:
@Test public void testIndexOf() { XYSeries s1 = new XYSeries("S1"); XYSeries s2 = new XYSeries("S2"); XYSeriesCollection dataset = new XYSeriesCollection(); assertEquals(-1, dataset.indexOf(s1)); assertEquals(-1, dataset.indexOf(s2)); dataset.addSeries(s1); assertEquals(0, dataset.indexOf(s1)); assertEquals(-1, dataset.indexOf(s2)); dataset.addSeries(s2); assertEquals(0, dataset.indexOf(s1)); assertEquals(1, dataset.indexOf(s2)); dataset.removeSeries(s1); assertEquals(-1, dataset.indexOf(s1)); assertEquals(0, dataset.indexOf(s2)); XYSeries s2b = new XYSeries("S2"); assertEquals(0, dataset.indexOf(s2b)); }