public java.lang.String getFilter() {
      java.lang.Object ref = filter_;
      if (!(ref instanceof java.lang.String)) {
        com.google.protobuf.ByteString bs = (com.google.protobuf.ByteString) ref;
        java.lang.String s = bs.toStringUtf8();
        filter_ = s;
        return s;
      } else {
        return (java.lang.String) ref;
      }
    }
<pre>
Optional. The filter applied to the returned list.
We only support filtering for the `deployed_image_index.image_index` field.
However, to filter by a corpus instead of an image index, simply use
`deployed_image_index.corpus`, which will return all endpoints with
`deployed_image_index.image_index` inside of the given corpus.
A basic filter on image index would look like:
  deployed_image_index.image_index =
    "projects/123/locations/us-central1/corpora/my_corpus/imageIndexes/my_image_index"
A basic filter on corpus would look like:
  deployed_image_index.corpus =
    "projects/123/locations/us-central1/corpora/my_corpus"
</pre>

<code>string filter = 4 [(.google.api.field_behavior) = OPTIONAL];</code>

@return The filter.
java-visionai/proto-google-cloud-visionai-v1/src/main/java/com/google/cloud/visionai/v1/ListIndexEndpointsRequest.java