code: void removeExportedKeys(Table toDrop) {
  
          // toDrop.schema may be null because it is not registerd
          Schema schema = (Schema) schemaMap.get(toDrop.getSchemaName().name);
  
          for (int i = 0; i < schema.tableList.size(); i++) {
              Table table = (Table) schema.tableList.get(i);
  
              for (int j = table.constraintList.length - 1; j >= 0; j--) {
                  Table refTable = table.constraintList[j].getRef();
  
                  if (toDrop == refTable) {
                      table.removeConstraint(j);
                  }
              }
          }
      }
docs: Removes any foreign key Constraint objects (exported keys) held by any
  tables referenced by the specified table. <p>
  
  This method is called as the last step of a successful call to
  dropTable() in order to ensure that the dropped Table ceases to be
  referenced when enforcing referential integrity.
  
  @param  toDrop The table to which other tables may be holding keys.
  This is a table that is in the process of being dropped.