code: public ExpressionSpecBuilder addUpdate(UpdateAction updateAction) {
          final String operator = updateAction.getOperator();
          List<UpdateAction> list = updates.get(operator);
          if (list == null) {
              list = new LinkedList<UpdateAction>();
              updates.put(operator, list);
          }
          list.add(updateAction);
          return this;
      }
docs: Fluent API to add the given Update expression for a request.
  <p>
  For example:
  <pre class="brush: java">
  import static com.amazonaws.services.dynamodbv2.xspec.ExpressionSpecBuilder.*;
  ...
  builder
  // SET num1 = num1 + 20
  .addUpdate(
  N("num1").set(N("num1").plus(20)))
  // SET string-attr = "string-value"
  .addUpdate(
  S("string-attr").set("string-value")
  )