code: public java.util.List<String> getEc2SubnetIds() {
          if (ec2SubnetIds == null) {
              ec2SubnetIds = new com.amazonaws.internal.SdkInternalList<String>();
          }
          return ec2SubnetIds;
      }
docs: <p>
  Applies to clusters that use the instance fleet configuration. When multiple EC2 subnet IDs are specified, Amazon
  EMR evaluates them and launches instances in the optimal subnet.
  </p>
  <note>
  <p>
  The instance fleet configuration is available only in Amazon EMR versions 4.8.0 and later, excluding 5.0.x
  versions.
  </p>
  </note>
  
  @return Applies to clusters that use the instance fleet configuration. When multiple EC2 subnet IDs are
  specified, Amazon EMR evaluates them and launches instances in the optimal subnet.</p> <note>
  <p>
  The instance fleet configuration is available only in Amazon EMR versions 4.8.0 and later, excluding
  5.0.x versions.
  </p>