@Override
  public String toString() {
    StringBuilder sb = new StringBuilder();
    sb.append("Not Null Constraints for " + databaseName + "." + tableName + ":");
    sb.append("[");
    if (notNullConstraints != null && notNullConstraints.size() > 0) {
      for (Map.Entry<String, String> me : notNullConstraints.entrySet()) {
        sb.append(" {Constraint Name: " + me.getKey());
        sb.append(", Column Name: " + me.getValue());
        sb.append("},");
      }
      sb.setLength(sb.length()-1);
    }
    sb.append("]");
    return sb.toString();
  }
NotNullConstraintInfo is a metadata structure containing the not null constraints
associated with a table.
ql/src/java/org/apache/hadoop/hive/ql/metadata/NotNullConstraint.java