### Question:
ColumnIndexOperandNode extends OperandNode { @Override public String toString() { return String.format("_%d_", index); } ColumnIndexOperandNode(int index); int index(); @Override String toString();  }### Answer:
@Test public void testToString() { ColumnIndexOperandNode columnIndexOperandNode = new ColumnIndexOperandNode(10); assertEquals("_10_", columnIndexOperandNode.toString()); }