@java.lang.Override
  public java.lang.String getKey() {
    java.lang.Object ref = key_;
    if (ref instanceof java.lang.String) {
      return (java.lang.String) ref;
    } else {
      com.google.protobuf.ByteString bs = (com.google.protobuf.ByteString) ref;
      java.lang.String s = bs.toStringUtf8();
      key_ = s;
      return s;
    }
  }
<pre>
Required. The key of this data schema. This key should be matching the key
of user specified annotation and unique inside corpus. This value can be up
to 63 characters, and valid characters are /[a-z][0-9]-/. The first
character must be a letter, the last could be a letter or a number.
</pre>

<code>string key = 2 [(.google.api.field_behavior) = REQUIRED];</code>

@return The key.
java-visionai/proto-google-cloud-visionai-v1/src/main/java/com/google/cloud/visionai/v1/DataSchema.java