flink-libraries/flink-table/src/main/scala/org/apache/flink/table/sources/FieldComputer.scala

/*
 * 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,
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 * See the License for the specific language governing permissions and
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package org.apache.flink.table.sources

import org.apache.flink.table.api.ValidationException
import org.apache.flink.table.api.types.InternalType
import org.apache.flink.table.expressions.{Expression, ResolvedFieldReference}

/**
  * The [[FieldComputer]] interface returns an expression to compute the field of the table schema
  * of a [[TableSource]] from one or more fields of the [[TableSource]]'s return type.
  *
  * @tparam T The result type of the provided expression.
  */
abstract class FieldComputer[T] {

  /**
    * Returns the names of all fields that the expression of the field computer accesses.
    *
    * @return An array with the names of all accessed fields.
    */
  def getArgumentFields: Array[String]

  /**
    * Returns the result type of the expression.
    *
    * @return The result type of the expression.
    */
  def getReturnType: InternalType

  /**
    * Validates that the fields that the expression references have the correct types.
    *
    * @param argumentFieldTypes The types of the physical input fields.
    */
  @throws[ValidationException]
  def validateArgumentFields(argumentFieldTypes: Array[InternalType]): Unit

  /**
    * Returns the [[Expression]] that computes the value of the field.
    *
    * @param fieldAccesses Field access expressions for the argument fields.
    * @return The expression to extract the timestamp from the [[TableSource]] return type.
    */
  def getExpression(fieldAccesses: Array[ResolvedFieldReference]): Expression

}