content: /*
   * 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.beam.sdk.extensions.sql;
  
  import java.io.Serializable;
  
  /**
   * Interface to create a UDF in Beam SQL.
   *
   * <p>A static method {@code eval} is required. Here is an example:
   *
   * <blockquote>
   *
   * <pre>
   * public static class MyLeftFunction {
   *   public String eval(
   *       &#64;Parameter(name = "s") String s,
   *       &#64;Parameter(name = "n", optional = true) Integer n) {
   *     return s.substring(0, n == null ? 1 : n);
   *   }
   * }</pre>
   *
   * </blockquote>
   *
   * <p>The first parameter is named "s" and is mandatory, and the second parameter is named "n" and
   * is optional(always NULL if not specified).
   */
  public interface BeamSqlUdf extends Serializable {
    String UDF_METHOD = "eval";
  }
  