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   /*
    * Copyright 2012 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.util;
  
  /**
   * Key which can be used to access {@link Attribute} out of the {@link AttributeMap}. Be aware that it is not be
   * possible to have multiple keys with the same name.
   *
   * @param <T>   the type of the {@link Attribute} which can be accessed via this {@link AttributeKey}.
   */
  @SuppressWarnings("UnusedDeclaration") // 'T' is used only at compile time
  public final class AttributeKey<T> extends AbstractConstant<AttributeKey<T>> {
  
      private static final ConstantPool<AttributeKey<Object>> pool = new ConstantPool<AttributeKey<Object>>() {
          @Override
          protected AttributeKey<Object> newConstant(int id, String name) {
              return new AttributeKey<Object>(id, name);
          }
      };
  
      /**
       * Returns the singleton instance of the {@link AttributeKey} which has the specified {@code name}.
       */
      @SuppressWarnings("unchecked")
      public static <T> AttributeKey<T> valueOf(String name) {
          return (AttributeKey<T>) pool.valueOf(name);
      }
  
      /**
       * Returns {@code true} if a {@link AttributeKey} exists for the given {@code name}.
       */
      public static boolean exists(String name) {
          return pool.exists(name);
      }
  
      /**
       * Creates a new {@link AttributeKey} for the given {@code name} or fail with an
       * {@link IllegalArgumentException} if a {@link AttributeKey} for the given {@code name} exists.
       */
      @SuppressWarnings("unchecked")
      public static <T> AttributeKey<T> newInstance(String name) {
          return (AttributeKey<T>) pool.newInstance(name);
      }
  
      @SuppressWarnings("unchecked")
      public static <T> AttributeKey<T> valueOf(Class<?> firstNameComponent, String secondNameComponent) {
          return (AttributeKey<T>) pool.valueOf(firstNameComponent, secondNameComponent);
      }
  
      private AttributeKey(int id, String name) {
          super(id, name);
      }
  }