twalthr commented on a change in pull request #8360: 
[FLINK-12393][table-common] Add the user-facing classes of the new type system
URL: https://github.com/apache/flink/pull/8360#discussion_r282347087
 
 

 ##########
 File path: 
flink-table/flink-table-common/src/main/java/org/apache/flink/table/types/DataType.java
 ##########
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+/*
+ * 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.table.types;
+
+import org.apache.flink.annotation.PublicEvolving;
+import org.apache.flink.table.api.DataTypes;
+import org.apache.flink.table.api.ValidationException;
+import org.apache.flink.table.types.logical.LogicalType;
+import org.apache.flink.util.Preconditions;
+
+import javax.annotation.Nullable;
+
+import java.io.Serializable;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Objects;
+
+/**
+ * Describes the data type of a value in the table ecosystem. Instances of 
this class can be used to
+ * declare input and/or output types of operations.
+ *
+ * <p>The {@link DataType} class has two responsibilities: declaring a logical 
type and giving hints
+ * about the physical representation of data to the optimizer. While the 
logical type is mandatory,
+ * hints are optional but useful at the edges to other APIs.
+ *
+ * <p>The logical type is independent of any physical representation and is 
close to the "data type"
+ * terminology of the SQL standard. See {@link 
org.apache.flink.table.types.logical.LogicalType} and
+ * its subclasses for more information about available logical types and their 
properties.
+ *
+ * <p>Physical hints are required at the edges of the table ecosystem. Hints 
indicate the data format
+ * that an implementation expects. For example, a data source could express 
that it produces values for
+ * logical timestamps using a {@link java.sql.Timestamp} class instead of 
using {@link java.time.LocalDateTime}.
+ * With this information, the runtime is able to convert the produced class 
into its internal data
+ * format. In return, a data sink can declare the data format it consumes from 
the runtime.
+ *
+ * @see DataTypes for a list of supported data types and instances of this 
class.
+ */
+@PublicEvolving
+public abstract class DataType implements Serializable {
+
+       protected LogicalType logicalType;
+
+       protected @Nullable Class<?> conversionClass;
+
+       private DataType(LogicalType logicalType, @Nullable Class<?> 
conversionClass) {
+               this.logicalType = Preconditions.checkNotNull(logicalType, 
"Logical type must not be null.");
+               this.conversionClass = performEarlyClassValidation(logicalType, 
conversionClass);
+       }
+
+       /**
+        * Returns the corresponding logical type.
+        *
+        * @return a parameterized instance of {@link LogicalType}
+        */
+       public LogicalType getLogicalType() {
+               return logicalType;
+       }
+
+       /**
+        * Returns the corresponding conversion class for representing values. 
If no conversion class was
+        * defined manually, the default conversion defined by the logical type 
is used.
+        *
+        * @see LogicalType#getDefaultConversion()
+        *
+        * @return the expected conversion class
+        */
+       public Class<?> getConversionClass() {
+               if (conversionClass == null) {
+                       return logicalType.getDefaultConversion();
+               }
+               return conversionClass;
+       }
+
+       /**
+        * Adds a hint that null values are not expected in the data for this 
type.
+        *
+        * @return a new, reconfigured data type instance
+        */
+       public abstract DataType notNull();
+
+       /**
+        * Adds a hint that null values are expected in the data for this type 
(default behavior).
+        *
+        * <p>This method exists for explicit declaration of the default 
behavior or for invalidation of
+        * a previous call to {@link #notNull()}.
+        *
+        * @return a new, reconfigured data type instance
+        */
+       public abstract DataType andNull();
+
+       /**
+        * Adds a hint that data should be represented using the given class 
when entering or leaving
+        * the table ecosystem.
+        *
+        * @return a new, reconfigured data type instance
+        */
+       public abstract DataType bridgedTo(Class<?> newConversionClass);
+
+       @Override
+       public String toString() {
+               return logicalType.toString();
+       }
+
+       @Override
+       public boolean equals(Object o) {
+               if (this == o) {
+                       return true;
+               }
+               if (o == null || getClass() != o.getClass()) {
+                       return false;
+               }
+               DataType dataType = (DataType) o;
+               return logicalType.equals(dataType.logicalType) &&
+                       Objects.equals(conversionClass, 
dataType.conversionClass);
+       }
+
+       @Override
+       public int hashCode() {
+               return Objects.hash(logicalType, conversionClass);
+       }
+
+       /**
+        * This method should catch the most common errors. However, another 
validation is required in
+        * deeper layers as we don't know whether the data type is used for 
input or output declaration.
+        */
+       private static <C> Class<C> performEarlyClassValidation(
+                       LogicalType logicalType,
+                       Class<C> candidate) {
+
+               if (candidate != null &&
+                               !logicalType.supportsInputConversion(candidate) 
&&
+                               
!logicalType.supportsOutputConversion(candidate)) {
+                       throw new ValidationException(
+                               String.format(
+                                       "Logical type '%s' does not support a 
conversion from or to class '%s'.",
+                                       logicalType.asSummaryString(),
+                                       candidate.getName()));
+               }
+               return candidate;
+       }
+
+       // 
--------------------------------------------------------------------------------------------
+
+       /**
+        * A data type that does not contain further data types (e.g. {@code 
INT} or {@code BOOLEAN}).
+        *
+        * @see DataTypes for a list of supported data types
+        */
+       public static final class AtomicDataType extends DataType {
+
+               public AtomicDataType(LogicalType logicalType, @Nullable 
Class<?> conversionClass) {
+                       super(logicalType, conversionClass);
+               }
+
+               public AtomicDataType(LogicalType logicalType) {
+                       super(logicalType, null);
+               }
+
+               @Override
+               public DataType notNull() {
+                       return new AtomicDataType(
+                               logicalType.copy(false),
+                               conversionClass);
+               }
+
+               @Override
+               public DataType andNull() {
+                       return new AtomicDataType(
+                               logicalType.copy(true),
+                               conversionClass);
+               }
+
+               @Override
+               public DataType bridgedTo(Class<?> newConversionClass) {
+                       return new AtomicDataType(
+                               logicalType,
+                               Preconditions.checkNotNull(newConversionClass, 
"New conversion class must not be null."));
+               }
+       }
+
+       /**
+        * A data type that contains an element type (e.g. {@code ARRAY} or 
{@code MULTISET}).
+        *
+        * @see DataTypes for a list of supported data types
+        */
+       public static final class ElementDataType extends DataType {
 
 Review comment:
   The naming is on purpose as `MapType` is also a collection data type. The 
naming should not reflect the SQL type families but just serve as a container 
name.

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