lindong28 commented on a change in pull request #37:
URL: https://github.com/apache/flink-ml/pull/37#discussion_r765496493



##########
File path: 
flink-ml-core/src/main/java/org/apache/flink/ml/param/ParamValidators.java
##########
@@ -95,4 +95,9 @@ public boolean validate(T value) {
             }
         };
     }
+
+    // Check if the parameter value array is not empty array.
+    public static <T> ParamValidator<T> nonEmptyArray() {

Review comment:
       Could you test this validator in `StageTest::testValidators(...)`?

##########
File path: 
flink-ml-lib/src/main/java/org/apache/flink/ml/common/param/HasHandleInvalid.java
##########
@@ -0,0 +1,43 @@
+/*
+ * 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.flink.ml.common.param;
+
+import org.apache.flink.ml.param.Param;
+import org.apache.flink.ml.param.ParamValidators;
+import org.apache.flink.ml.param.StringParam;
+import org.apache.flink.ml.param.WithParams;
+
+/** Interface for the shared handleInvalid param. */
+public interface HasHandleInvalid<T> extends WithParams<T> {
+    Param<String> HANDLE_INVALID =
+            new StringParam(
+                    "handleInvalid",
+                    "Strategy to handle invalid entries.",
+                    "ERROR",

Review comment:
       Could we use lower-case letters (i.e. `error`) as the value here for 
consistency with `HasDistanceMeasure`?

##########
File path: 
flink-ml-core/src/main/java/org/apache/flink/ml/linalg/SparseVector.java
##########
@@ -0,0 +1,177 @@
+/*
+ * 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.flink.ml.linalg;
+
+import org.apache.flink.api.common.typeinfo.TypeInfo;
+import org.apache.flink.ml.linalg.typeinfo.SparseVectorTypeInfoFactory;
+import org.apache.flink.util.Preconditions;
+
+import java.util.Arrays;
+import java.util.Objects;
+
+/** A sparse vector of double values. */
+@TypeInfo(SparseVectorTypeInfoFactory.class)
+public class SparseVector implements Vector {
+    public final int n;
+    public final int[] indices;
+    public final double[] values;
+
+    public SparseVector(int n, int[] indices, double[] values) {
+        this.n = n;
+        this.indices = indices;
+        this.values = values;
+        checkSizeAndIndicesRange();

Review comment:
       Instead of making three passes over the vector (i.e. 
`checkSizeAndIndicesRange`, `isIndicesSorted` and `checkDuplicatedIndices`). 
Could we make just one pass to improve efficiency?

##########
File path: 
flink-ml-lib/src/main/java/org/apache/flink/ml/feature/onehotencoder/OneHotEncoder.java
##########
@@ -0,0 +1,145 @@
+/*
+ * 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.flink.ml.feature.onehotencoder;
+
+import org.apache.flink.api.common.functions.FlatMapFunction;
+import org.apache.flink.api.common.functions.MapPartitionFunction;
+import org.apache.flink.api.common.typeinfo.Types;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.ml.api.Estimator;
+import org.apache.flink.ml.common.datastream.MapPartitionFunctionWrapper;
+import org.apache.flink.ml.param.Param;
+import org.apache.flink.ml.util.ParamUtils;
+import org.apache.flink.ml.util.ReadWriteUtils;
+import org.apache.flink.streaming.api.datastream.DataStream;
+import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
+import org.apache.flink.table.api.Table;
+import org.apache.flink.table.api.bridge.java.StreamTableEnvironment;
+import org.apache.flink.table.api.internal.TableImpl;
+import org.apache.flink.types.Row;
+import org.apache.flink.util.Collector;
+import org.apache.flink.util.Preconditions;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * An Estimator which implements the one-hot encoding algorithm.
+ *
+ * <p>See https://en.wikipedia.org/wiki/One-hot.
+ */
+public class OneHotEncoder
+        implements Estimator<OneHotEncoder, OneHotEncoderModel>,
+                OneHotEncoderParams<OneHotEncoder> {
+    private final Map<Param<?>, Object> paramMap = new HashMap<>();
+
+    public OneHotEncoder() {
+        ParamUtils.initializeMapWithDefaultValues(paramMap, this);
+    }
+
+    @Override
+    public OneHotEncoderModel fit(Table... inputs) {
+        Preconditions.checkArgument(inputs.length == 1);
+        Preconditions.checkArgument(getHandleInvalid().equals("ERROR"));

Review comment:
       Instead of asking individual algorithms to specify the raw string, could 
we add a `public static final String ERROR_INVALID = "error"`, similar to 
`Bucketizer::ERROR_INVALID` in Spark and `EuclideanDistanceMeasure::NAME` in 
Flink ML?

##########
File path: 
flink-ml-core/src/test/java/org/apache/flink/ml/linalg/VectorsTest.java
##########
@@ -0,0 +1,69 @@
+/*
+ * 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.flink.ml.linalg;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+
+/** Tests the behavior of Vectors. */
+public class VectorsTest {

Review comment:
       If this test is supposed to cover only `SparseVector`, could we name it 
as `SparseVectorTest`? Otherwise, could we specify in the test method name 
whether the test is for `SparseVector` or `DenseVector`?

##########
File path: 
flink-ml-lib/src/main/java/org/apache/flink/ml/common/param/HasHandleInvalid.java
##########
@@ -0,0 +1,43 @@
+/*
+ * 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.flink.ml.common.param;
+
+import org.apache.flink.ml.param.Param;
+import org.apache.flink.ml.param.ParamValidators;
+import org.apache.flink.ml.param.StringParam;
+import org.apache.flink.ml.param.WithParams;
+
+/** Interface for the shared handleInvalid param. */
+public interface HasHandleInvalid<T> extends WithParams<T> {

Review comment:
       Could we add Java doc explaining what happens when the strategy is 
`ERROR`?

##########
File path: 
flink-ml-lib/src/test/java/org/apache/flink/ml/feature/OneHotEncoderTest.java
##########
@@ -0,0 +1,282 @@
+/*
+ * 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.flink.ml.feature;
+
+import org.apache.flink.api.common.restartstrategy.RestartStrategies;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.configuration.Configuration;
+import org.apache.flink.ml.feature.onehotencoder.OneHotEncoder;
+import org.apache.flink.ml.feature.onehotencoder.OneHotEncoderModel;
+import org.apache.flink.ml.feature.onehotencoder.OneHotEncoderModelData;
+import org.apache.flink.ml.linalg.Vector;
+import org.apache.flink.ml.linalg.Vectors;
+import org.apache.flink.ml.util.ReadWriteUtils;
+import org.apache.flink.ml.util.StageTestUtils;
+import 
org.apache.flink.streaming.api.environment.ExecutionCheckpointingOptions;
+import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
+import org.apache.flink.table.api.Table;
+import org.apache.flink.table.api.bridge.java.StreamTableEnvironment;
+import org.apache.flink.types.Row;
+import org.apache.flink.util.CloseableIterator;
+
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+/** Tests OneHotEncoder and OneHotEncoderModel. */
+public class OneHotEncoderTest {
+    @Rule public final TemporaryFolder tempFolder = new TemporaryFolder();
+
+    private StreamExecutionEnvironment env;
+    private StreamTableEnvironment tEnv;
+    private Table trainTable;
+    private Table predictTable;
+    private Map<Double, Vector>[] expectedOutput;
+    private OneHotEncoder estimator;
+
+    @Before
+    public void before() {
+        Configuration config = new Configuration();
+        
config.set(ExecutionCheckpointingOptions.ENABLE_CHECKPOINTS_AFTER_TASKS_FINISH, 
true);
+        env = StreamExecutionEnvironment.getExecutionEnvironment(config);
+        env.setParallelism(4);
+        env.enableCheckpointing(100);
+        env.setRestartStrategy(RestartStrategies.noRestart());
+        tEnv = StreamTableEnvironment.create(env);
+
+        List<Row> trainData = Arrays.asList(Row.of(0.0), Row.of(1.0), 
Row.of(2.0), Row.of(0.0));
+
+        trainTable = 
tEnv.fromDataStream(env.fromCollection(trainData)).as("input");
+
+        List<Row> predictData = Arrays.asList(Row.of(0.0), Row.of(1.0), 
Row.of(2.0));
+
+        predictTable = 
tEnv.fromDataStream(env.fromCollection(predictData)).as("input");
+
+        expectedOutput =
+                new HashMap[] {
+                    new HashMap<Double, Vector>() {
+                        {
+                            put(0.0, Vectors.sparse(2, new int[] {0}, new 
double[] {1.0}));
+                            put(1.0, Vectors.sparse(2, new int[] {1}, new 
double[] {1.0}));
+                            put(2.0, Vectors.sparse(2, new int[0], new 
double[0]));
+                        }
+                    }
+                };
+
+        estimator = new 
OneHotEncoder().setInputCols("input").setOutputCols("output");
+    }
+
+    /**
+     * Executes a given table and collect its results. Results are returned as 
a map array. Each
+     * element in the array is a map corresponding to a input column whose key 
is the original value
+     * in the input column, value is the one-hot encoding result of that value.
+     *
+     * @param table A table to be executed and to have its result collected
+     * @param inputCols Name of the input columns
+     * @param outputCols Name of the output columns containing one-hot 
encoding result
+     * @return An array of map containing the collected results for each input 
column
+     */
+    private static Map<Double, Vector>[] executeAndCollect(
+            Table table, String[] inputCols, String[] outputCols) {
+        Map<Double, Vector>[] maps = new HashMap[inputCols.length];
+        for (int i = 0; i < inputCols.length; i++) {
+            maps[i] = new HashMap<>();
+        }
+        for (CloseableIterator<Row> it = table.execute().collect(); 
it.hasNext(); ) {
+            Row row = it.next();
+            for (int i = 0; i < inputCols.length; i++) {
+                maps[i].put(
+                        ((Number) row.getField(inputCols[i])).doubleValue(),
+                        (Vector) row.getField(outputCols[i]));
+            }
+        }
+        return maps;
+    }
+
+    @Test
+    public void testParam() {
+        OneHotEncoder estimator = new OneHotEncoder();
+
+        assertTrue(estimator.getDropLast());
+
+        
estimator.setInputCols("test_input").setOutputCols("test_output").setDropLast(false);
+
+        assertArrayEquals(new String[] {"test_input"}, 
estimator.getInputCols());
+        assertArrayEquals(new String[] {"test_output"}, 
estimator.getOutputCols());
+        assertFalse(estimator.getDropLast());
+
+        OneHotEncoderModel model = new OneHotEncoderModel();
+
+        assertTrue(model.getDropLast());
+
+        
model.setInputCols("test_input").setOutputCols("test_output").setDropLast(false);
+
+        assertArrayEquals(new String[] {"test_input"}, model.getInputCols());
+        assertArrayEquals(new String[] {"test_output"}, model.getOutputCols());
+        assertFalse(model.getDropLast());
+    }
+
+    @Test
+    public void testFitAndPredict() {
+        OneHotEncoderModel model = estimator.fit(trainTable);
+        Table outputTable = model.transform(predictTable)[0];
+        Map<Double, Vector>[] actualOutput =
+                executeAndCollect(outputTable, model.getInputCols(), 
model.getOutputCols());
+        assertArrayEquals(expectedOutput, actualOutput);
+    }
+
+    @Test
+    public void testDropLast() {
+        estimator.setDropLast(false);
+
+        expectedOutput =
+                new HashMap[] {
+                    new HashMap<Double, Vector>() {
+                        {
+                            put(0.0, Vectors.sparse(3, new int[] {0}, new 
double[] {1.0}));
+                            put(1.0, Vectors.sparse(3, new int[] {1}, new 
double[] {1.0}));
+                            put(2.0, Vectors.sparse(3, new int[] {2}, new 
double[] {1.0}));
+                        }
+                    }
+                };
+
+        OneHotEncoderModel model = estimator.fit(trainTable);
+        Table outputTable = model.transform(predictTable)[0];
+        Map<Double, Vector>[] actualOutput =
+                executeAndCollect(outputTable, model.getInputCols(), 
model.getOutputCols());
+        assertArrayEquals(expectedOutput, actualOutput);
+    }
+
+    @Test
+    public void testInputDataType() {
+        List<Row> trainData = Arrays.asList(Row.of(0), Row.of(1), Row.of(2), 
Row.of(0));
+
+        trainTable = 
tEnv.fromDataStream(env.fromCollection(trainData)).as("input");
+
+        List<Row> predictData = Arrays.asList(Row.of(0), Row.of(1), Row.of(2));
+        predictTable = 
tEnv.fromDataStream(env.fromCollection(predictData)).as("input");
+
+        expectedOutput =
+                new HashMap[] {
+                    new HashMap<Double, Vector>() {
+                        {
+                            put(0.0, Vectors.sparse(2, new int[] {0}, new 
double[] {1.0}));
+                            put(1.0, Vectors.sparse(2, new int[] {1}, new 
double[] {1.0}));
+                            put(2.0, Vectors.sparse(2, new int[0], new 
double[0]));
+                        }
+                    }
+                };
+
+        OneHotEncoderModel model = estimator.fit(trainTable);
+        Table outputTable = model.transform(predictTable)[0];
+        Map<Double, Vector>[] actualOutput =
+                executeAndCollect(outputTable, model.getInputCols(), 
model.getOutputCols());
+        assertArrayEquals(expectedOutput, actualOutput);
+    }
+
+    @Test
+    public void testNonIntegerDouble() {
+        List<Row> trainData = Arrays.asList(Row.of(0.5), Row.of(1.0), 
Row.of(2.0), Row.of(0.0));
+
+        trainTable = 
tEnv.fromDataStream(env.fromCollection(trainData)).as("input");
+        OneHotEncoderModel model = estimator.fit(trainTable);
+        Table outputTable = model.transform(predictTable)[0];
+        try {
+            outputTable.execute().collect().next();
+            Assert.fail("Expected IllegalArgumentException");
+        } catch (Exception e) {
+            Throwable exception = e;
+            while (exception.getCause() != null) {
+                exception = exception.getCause();
+            }
+            assertEquals(IllegalArgumentException.class, exception.getClass());
+            assertEquals("Value 0.5 cannot be parsed as indexed integer.", 
exception.getMessage());
+        }
+    }
+
+    @Test
+    public void testNonIntegerDouble2() {

Review comment:
       Could we make the name more self-explanatory regarding its difference 
with `testNonIntegerDouble`?
   
   Maybe something like `testIllegalTrainData` and `testIllegalPredictData`.
    




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