Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/4925#discussion_r26062079
--- Diff: mllib/src/main/scala/org/apache/spark/ml/attribute/package.scala
---
@@ -0,0 +1,44 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.ml
+
+import org.apache.spark.sql.DataFrame
+import org.apache.spark.ml.attribute.{Attribute, AttributeGroup}
+
+/**
+ * ==ML attributes==
+ *
+ * The ML pipeline API uses [[DataFrame]]s as ML datasets.
+ * Each dataset consists of typed columns, e.g., string, double, vector,
etc.
+ * However, knowing only the column type may not be sufficient to handle
the data properly.
+ * For instance, a double column with values 0.0, 1.0, 2.0, ... may
represent some label indices,
+ * which cannot be treated as numeric values in ML algorithms, and, for
another instance, we may
+ * want to know the names and types of features stored in a vector column.
+ * ML attributes are used to provide additional information to describe
columns in a dataset.
+ *
+ * ===ML columns===
+ *
+ * A column with ML attributes attached is called an ML column.
+ * The data in ML columns are stored as double values, i.e., an ML column
is either a scalar column
+ * of double values or a vector column.
+ * Columns of other types must be encoded into ML columns using
transformers.
+ * We use [[Attribute]] to describe a scalar ML column, and
[[AttributeGroup]] to describe a vector
+ * ML column.
+ * ML attributes are stored in the metadata field of the column schema.
+ */
+package object attribute
--- End diff --
We need `package-info.java` for Java. Let me include it with the next
update.
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