### Question:
ColumnCombination implements Serializable, Comparable<Object> { @Override public String toString() { return columnIdentifiers.toString(); } 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 testToString() { ColumnIdentifier expectedColumn1 = new ColumnIdentifier("table56", "column1"); ColumnIdentifier expectedColumn2 = new ColumnIdentifier("table2", "column2"); ColumnCombination columnCombination = new ColumnCombination(expectedColumn1, expectedColumn2); String expectedStringRepresentation = "[" + expectedColumn2.toString() + ", " + expectedColumn1.toString() + "]"; String actualStringRepresentation = columnCombination.toString(); assertEquals(expectedStringRepresentation, actualStringRepresentation); }