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   /*
    * Copyright 2015 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.handler.codec.http;
  
  import java.util.Collections;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map.Entry;
  import java.util.Set;
  
  public class EmptyHttpHeaders extends HttpHeaders {
      static final Iterator<Entry<CharSequence, CharSequence>> EMPTY_CHARS_ITERATOR =
              Collections.<Entry<CharSequence, CharSequence>>emptyList().iterator();
  
      public static final EmptyHttpHeaders INSTANCE = instance();
  
      /**
       * @see InstanceInitializer#EMPTY_HEADERS
       * @deprecated Use {@link EmptyHttpHeaders#INSTANCE}
       * <p>
       * This is needed to break a cyclic static initialization loop between {@link HttpHeaders} and {@link
       * EmptyHttpHeaders}.
       */
      @Deprecated
      static EmptyHttpHeaders instance() {
          return InstanceInitializer.EMPTY_HEADERS;
      }
  
      protected EmptyHttpHeaders() {
      }
  
      @Override
      public String get(String name) {
          return null;
      }
  
      @Override
      public Integer getInt(CharSequence name) {
          return null;
      }
  
      @Override
      public int getInt(CharSequence name, int defaultValue) {
          return defaultValue;
      }
  
      @Override
      public Short getShort(CharSequence name) {
          return null;
      }
  
      @Override
      public short getShort(CharSequence name, short defaultValue) {
          return defaultValue;
      }
  
      @Override
      public Long getTimeMillis(CharSequence name) {
          return null;
      }
  
      @Override
      public long getTimeMillis(CharSequence name, long defaultValue) {
          return defaultValue;
      }
  
      @Override
      public List<String> getAll(String name) {
          return Collections.emptyList();
      }
  
      @Override
      public List<Entry<String, String>> entries() {
          return Collections.emptyList();
      }
  
      @Override
      public boolean contains(String name) {
          return false;
      }
  
      @Override
      public boolean isEmpty() {
          return true;
      }
 
     @Override
     public int size() {
         return 0;
     }
 
     @Override
     public Set<String> names() {
         return Collections.emptySet();
     }
 
     @Override
     public HttpHeaders add(String name, Object value) {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public HttpHeaders add(String name, Iterable<?> values) {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public HttpHeaders addInt(CharSequence name, int value) {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public HttpHeaders addShort(CharSequence name, short value) {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public HttpHeaders set(String name, Object value) {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public HttpHeaders set(String name, Iterable<?> values) {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public HttpHeaders setInt(CharSequence name, int value) {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public HttpHeaders setShort(CharSequence name, short value) {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public HttpHeaders remove(String name) {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public HttpHeaders clear() {
         throw new UnsupportedOperationException("read only");
     }
 
     @Override
     public Iterator<Entry<String, String>> iterator() {
         return entries().iterator();
     }
 
     @Override
     public Iterator<Entry<CharSequence, CharSequence>> iteratorCharSequence() {
         return EMPTY_CHARS_ITERATOR;
     }
 
     /**
      * This class is needed to break a cyclic static initialization loop between {@link HttpHeaders} and
      * {@link EmptyHttpHeaders}.
      */
     @Deprecated
     private static final class InstanceInitializer {
         /**
          * The instance is instantiated here to break the cyclic static initialization between {@link EmptyHttpHeaders}
          * and {@link HttpHeaders}. The issue is that if someone accesses {@link EmptyHttpHeaders#INSTANCE} before
          * {@link HttpHeaders#EMPTY_HEADERS} then {@link HttpHeaders#EMPTY_HEADERS} will be {@code null}.
          */
         @Deprecated
         private static final EmptyHttpHeaders EMPTY_HEADERS = new EmptyHttpHeaders();
 
         private InstanceInitializer() {
         }
     }
 }