### Question:
DefaultCategoryDataset extends AbstractDataset implements CategoryDataset, PublicCloneable, Serializable { public void addValue(Number value, Comparable rowKey, Comparable columnKey) { this.data.addValue(value, rowKey, columnKey); fireDatasetChanged(); } DefaultCategoryDataset(); @Override int getRowCount(); @Override int getColumnCount(); @Override Number getValue(int row, int column); @Override Comparable getRowKey(int row); @Override int getRowIndex(Comparable key); @Override List<Comparable> getRowKeys(); @Override Comparable getColumnKey(int column); @Override int getColumnIndex(Comparable key); @Override List<Comparable> getColumnKeys(); @Override Number getValue(Comparable rowKey, Comparable columnKey); void addValue(Number value, Comparable rowKey,
                         Comparable columnKey); void addValue(double value, Comparable rowKey,
                         Comparable columnKey); void setValue(Number value, Comparable rowKey,
                         Comparable columnKey); void setValue(double value, Comparable rowKey,
                         Comparable columnKey); void incrementValue(double value,
                               Comparable rowKey,
                               Comparable columnKey); void removeValue(Comparable rowKey, Comparable columnKey); void removeRow(int rowIndex); void removeRow(Comparable rowKey); void removeColumn(int columnIndex); void removeColumn(Comparable columnKey); void clear(); @Override boolean equals(Object obj); @Override int hashCode(); @Override Object clone();  }### Answer:
@Test public void testAddValue() { DefaultCategoryDataset d1 = new DefaultCategoryDataset(); d1.addValue(null, "R1", "C1"); assertNull(d1.getValue("R1", "C1")); d1.addValue(new Double(1.0), "R2", "C1"); assertEquals(1.0, d1.getValue("R2", "C1")); try { d1.addValue(new Double(1.1), null, "C2"); fail("UnknownKeyException should have been thrown on unknown row"); } catch (IllegalArgumentException e) { assertEquals("Null 'key' argument.", e.getMessage()); } }