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.udf;
  
  import java.io.Serializable;
  import java.lang.annotation.ElementType;
  import java.lang.annotation.Retention;
  import java.lang.annotation.RetentionPolicy;
  import java.lang.annotation.Target;
  
  /**
   * A scalar function that can be executed as part of a SQL query. Subclasses must contain exactly
   * one method annotated with {@link ApplyMethod}, which will be applied to the SQL function
   * arguments at runtime.
   *
   * <p>For example:
   *
   * <pre><code>
   * public class IncrementFn extends ScalarFn {
   *  {@literal @ApplyMethod}
   *   public Long increment(Long i) {
   *     return i + 1;
   *   }
   * }
   * </code></pre>
   */
  public abstract class ScalarFn implements Serializable {
    /**
     * Annotates the single method in a {@link ScalarFn} implementation that is to be applied to SQL
     * function arguments.
     */
    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.METHOD)
    public static @interface ApplyMethod {}
  }
  