code: public TableKeysAndAttributes addPrimaryKey(PrimaryKey primaryKey) {
          if (primaryKey != null) {
              if (primaryKeys == null)
                  primaryKeys = new ArrayList<PrimaryKey>();
              checkConsistency(primaryKey);
              this.primaryKeys.add(primaryKey);
          }
          return this;
      }
docs: Adds a primary key to be included in the batch get-item operation. A
  primary key could consist of either a hash-key or both a
  hash-key and a range-key depending on the schema of the table.