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.values;
  
  import static org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Preconditions.checkArgument;
  
  import java.lang.reflect.ParameterizedType;
  import java.lang.reflect.Type;
  import java.lang.reflect.TypeVariable;
  import org.apache.beam.sdk.util.Preconditions;
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * Captures a free type variable that can be used in {@link TypeDescriptor#where}. For example:
   *
   * <pre>{@code
   * static <T> TypeDescriptor<List<T>> listOf(Class<T> elementType) {
   *   return new TypeDescriptor<List<T>>() {}
   *       .where(new TypeParameter<T>() {}, elementType);
   * }
   * }</pre>
   */
  public abstract class TypeParameter<T> {
    final TypeVariable<?> typeVariable;
  
    public TypeParameter() {
      Type superclass = getClass().getGenericSuperclass();
      checkArgument(superclass instanceof ParameterizedType, "%s isn't parameterized", superclass);
      Preconditions.checkArgumentNotNull(superclass);
      typeVariable = (TypeVariable<?>) ((ParameterizedType) superclass).getActualTypeArguments()[0];
    }
  
    @Override
    public int hashCode() {
      return typeVariable.hashCode();
    }
  
    @Override
    public boolean equals(@Nullable Object obj) {
      if (!(obj instanceof TypeParameter)) {
        return false;
      }
      TypeParameter<?> that = (TypeParameter<?>) obj;
      return typeVariable.equals(that.typeVariable);
    }
  
    @Override
    public String toString() {
      return typeVariable.toString();
    }
  }
  