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.transforms.display.DisplayData;
  import org.apache.beam.sdk.transforms.display.HasDisplayData;
  import org.apache.beam.sdk.values.TypeDescriptor;
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * A {@link ProcessFunction} 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.
   *
   * <p>See {@link SimpleFunction} for providing robust type information where a {@link
   * SerializableFunction} is required.
   */
  @SuppressWarnings({
    "nullness" // TODO(https://github.com/apache/beam/issues/20497)
  })
  public abstract class InferableFunction<InputT, OutputT>
      implements ProcessFunction<InputT, OutputT>, HasDisplayData {
  
    private final @Nullable ProcessFunction<InputT, OutputT> fn;
  
    protected InferableFunction() {
      this.fn = null;
      // A subclass must override apply if using this constructor. Check that via
      // reflection.
      try {
        Method methodThatMustBeOverridden =
            InferableFunction.class.getDeclaredMethod("apply", Object.class);
        Method methodOnSubclass = getClass().getMethod("apply", Object.class);
  
        if (methodOnSubclass.equals(methodThatMustBeOverridden)) {
          throw new IllegalStateException(
              "Subclass of InferableFunction must override 'apply' method"
                  + " or pass a ProcessFunction to the constructor,"
                  + " usually via a lambda or method reference.");
        }
  
      } catch (NoSuchMethodException exc) {
        throw new RuntimeException("Impossible state: missing 'apply' method entirely", exc);
      }
    }
  
    protected InferableFunction(ProcessFunction<InputT, OutputT> fn) {
      this.fn = fn;
    }
  
    @Override
    public OutputT apply(InputT input) throws Exception {
      return fn.apply(input);
    }
  
    public static <InputT, OutputT>
        InferableFunction<InputT, OutputT> fromProcessFunctionWithOutputType(
            ProcessFunction<InputT, OutputT> fn, TypeDescriptor<OutputT> outputType) {
      return new InferableFunctionWithOutputType<>(fn, outputType);
    }
  
    /**
     * Returns a {@link TypeDescriptor} capturing what is known statically about the input type of
     * this {@link InferableFunction} instance's most-derived class.
     *
     * <p>See {@link #getOutputTypeDescriptor} for more discussion.
     */
    public TypeDescriptor<InputT> getInputTypeDescriptor() {
      return new TypeDescriptor<InputT>(this) {};
    }
  
    /**
     * Returns a {@link TypeDescriptor} capturing what is known statically about the output type of
     * this {@link InferableFunction} instance's most-derived class.
     *
     * <p>In the normal case of a concrete {@link InferableFunction} subclass with no generic type
     * parameters of its own (including anonymous inner classes), this will be a complete non-generic
     * type, which is good for choosing a default output {@code Coder<OutputT>} for the output {@code
     * PCollection<OutputT>}.
     */
    public TypeDescriptor<OutputT> getOutputTypeDescriptor() {
      return new TypeDescriptor<OutputT>(this) {};
    }
  
    /**
     * {@inheritDoc}
     *
     * <p>By default, does not register any display data. Implementors may override this method to
     * provide their own display data.
     */
    @Override
    public void populateDisplayData(DisplayData.Builder builder) {}
  
    /**
     * A {@link InferableFunction} built from a {@link ProcessFunction}, having a known output type
     * that is explicitly set.
     */
    private static class InferableFunctionWithOutputType<InputT, OutputT>
        extends InferableFunction<InputT, OutputT> {
  
      private final TypeDescriptor<OutputT> outputType;
  
      public InferableFunctionWithOutputType(
          ProcessFunction<InputT, OutputT> fn, TypeDescriptor<OutputT> outputType) {
        super(fn);
        this.outputType = outputType;
      }
  
      @Override
      public TypeDescriptor<OutputT> getOutputTypeDescriptor() {
        return outputType;
      }
    }
  }
  