### Question:
ColumnCombination implements Serializable, Comparable<Object> { public Set<ColumnIdentifier> getColumnIdentifiers() { return columnIdentifiers; } ColumnCombination();  ColumnCombination(ColumnIdentifier... columnIdentifier); Set<ColumnIdentifier> getColumnIdentifiers(); void setColumnIdentifiers(Set<ColumnIdentifier> identifiers); @Override String toString(); String toString(Map<String, String> tableMapping, Map<String, String> columnMapping); static ColumnCombination fromString(Map<String, String> tableMapping, Map<String, String> columnMapping, String str); @Override int hashCode(); @Override int compareTo(Object o); @Override boolean equals(Object obj); static final String COLUMN_CONNECTOR; }### Answer:
@Test public void testGetColumnIdentifiers() { ColumnIdentifier expectedColumn1 = new ColumnIdentifier("table1", "column1"); ColumnIdentifier expectedColumn2 = new ColumnIdentifier("table2", "column2"); ColumnIdentifier expectedColumn3 = new ColumnIdentifier("table3", "column3"); ColumnCombination columnCombination = new ColumnCombination(expectedColumn2, expectedColumn3, expectedColumn1); Set<ColumnIdentifier> actualColumnIdentifiers = columnCombination.getColumnIdentifiers(); assertTrue(3 == actualColumnIdentifiers.size()); assertTrue(actualColumnIdentifiers.contains(expectedColumn1)); assertTrue(actualColumnIdentifiers.contains(expectedColumn2)); assertTrue(actualColumnIdentifiers.contains(expectedColumn3)); }