public int getOutputIndexCount() {
    return outputIndex_.size();
  }
<pre>
Output only. The index that locates the explained prediction output.

If the prediction output is a scalar value, output_index is not populated.
If the prediction output has multiple dimensions, the length of the
output_index list is the same as the number of dimensions of the output.
The i-th element in output_index is the element index of the i-th dimension
of the output vector. Indices start from 0.
</pre>

<code>repeated int32 output_index = 4 [(.google.api.field_behavior) = OUTPUT_ONLY];</code>

@return The count of outputIndex.
java-vertexai/proto-google-cloud-vertexai-v1/src/main/java/com/google/cloud/vertexai/api/Attribution.java