code: public DescribeSpotFleetInstancesResult withActiveInstances(ActiveInstance... activeInstances) {
          if (this.activeInstances == null) {
              setActiveInstances(new com.amazonaws.internal.SdkInternalList<ActiveInstance>(activeInstances.length));
          }
          for (ActiveInstance ele : activeInstances) {
              this.activeInstances.add(ele);
          }
          return this;
      }
docs: <p>
  The running instances. This list is refreshed periodically and might be out of date.
  </p>
  <p>
  <b>NOTE:</b> This method appends the values to the existing list (if any). Use
  {@link #setActiveInstances(java.util.Collection)} or {@link #withActiveInstances(java.util.Collection)} if you
  want to override the existing values.
  </p>
  
  @param activeInstances
  The running instances. This list is refreshed periodically and might be out of date.
  @return Returns a reference to this object so that method calls can be chained together.