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.transforms;
  
  import java.lang.reflect.Method;
  import org.apache.beam.sdk.values.TypeDescriptor;
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * A {@link SerializableFunction} which is not a <i>functional interface</i>. Concrete subclasses
   * allow us to infer type information, which in turn aids {@link org.apache.beam.sdk.coders.Coder
   * Coder} inference.
   */
  @SuppressWarnings({
    "nullness" // TODO(https://github.com/apache/beam/issues/20497)
  })
  public abstract class SimpleFunction<InputT, OutputT> extends InferableFunction<InputT, OutputT>
      implements SerializableFunction<InputT, OutputT> {
  
    private final @Nullable SerializableFunction<InputT, OutputT> fn;
  
    protected SimpleFunction() {
      this.fn = null;
      // A subclass must override apply if using this constructor. Check that via
      // reflection.
      try {
        Method methodThatMustBeOverridden =
            SimpleFunction.class.getDeclaredMethod("apply", Object.class);
        Method methodOnSubclass = getClass().getMethod("apply", Object.class);
  
        if (methodOnSubclass.equals(methodThatMustBeOverridden)) {
          throw new IllegalStateException(
              "Subclass of SimpleFunction must override 'apply' method"
                  + " or pass a SerializableFunction to the constructor,"
                  + " usually via a lambda or method reference.");
        }
  
      } catch (NoSuchMethodException exc) {
        throw new RuntimeException("Impossible state: missing 'apply' method entirely", exc);
      }
    }
  
    protected SimpleFunction(SerializableFunction<InputT, OutputT> fn) {
      this.fn = fn;
    }
  
    @Override
    public OutputT apply(InputT input) {
      return fn.apply(input);
    }
  
    public static <InputT, OutputT>
        SimpleFunction<InputT, OutputT> fromSerializableFunctionWithOutputType(
            SerializableFunction<InputT, OutputT> fn, TypeDescriptor<OutputT> outputType) {
      return new SimpleFunctionWithOutputType<>(fn, outputType);
    }
  
    /**
     * A {@link SimpleFunction} built from a {@link SerializableFunction}, having a known output type
     * that is explicitly set.
     */
    private static class SimpleFunctionWithOutputType<InputT, OutputT>
        extends SimpleFunction<InputT, OutputT> {
  
      private final TypeDescriptor<OutputT> outputType;
  
      public SimpleFunctionWithOutputType(
          SerializableFunction<InputT, OutputT> fn, TypeDescriptor<OutputT> outputType) {
        super(fn);
        this.outputType = outputType;
      }
  
      @Override
      public TypeDescriptor<OutputT> getOutputTypeDescriptor() {
        return outputType;
      }
    }
  }
  