# FileItemHeadersImpl.java

    /*
     * 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.commons.fileupload.util;

    import java.io.Serializable;
    import java.util.ArrayList;
    import java.util.Collections;
    import java.util.Iterator;
    import java.util.LinkedHashMap;
    import java.util.List;
    import java.util.Locale;
    import java.util.Map;

    import org.apache.commons.fileupload.FileItemHeaders;

    /**
     * Default implementation of the {@link FileItemHeaders} interface.
     *
     * @since 1.2.1
     */
    public class FileItemHeadersImpl implements FileItemHeaders, Serializable {

        /**
         * Serial version UID, being used, if serialized.
         */
        private static final long serialVersionUID = -4455695752627032559L;

        /**
         * Map of <code>String</code> keys to a <code>List</code> of
         * <code>String</code> instances.
         */
        private final Map<String, List<String>> headerNameToValueListMap = new LinkedHashMap<String, List<String>>();

        /**
         * {@inheritDoc}
         */
        @Override
        public String getHeader(String name) {
            String nameLower = name.toLowerCase(Locale.ENGLISH);
            List<String> headerValueList = headerNameToValueListMap.get(nameLower);
            if (null == headerValueList) {
                return null;
            }
            return headerValueList.get(0);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public Iterator<String> getHeaderNames() {
            return headerNameToValueListMap.keySet().iterator();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public Iterator<String> getHeaders(String name) {
            String nameLower = name.toLowerCase(Locale.ENGLISH);
            List<String> headerValueList = headerNameToValueListMap.get(nameLower);
            if (null == headerValueList) {
                headerValueList = Collections.emptyList();
            }
            return headerValueList.iterator();
        }

        /**
         * Method to add header values to this instance.
         *
         * @param name name of this header
         * @param value value of this header
         */
        public synchronized void addHeader(String name, String value) {
            String nameLower = name.toLowerCase(Locale.ENGLISH);
            List<String> headerValueList = headerNameToValueListMap.get(nameLower);
            if (null == headerValueList) {
                headerValueList = new ArrayList<String>();
                headerNameToValueListMap.put(nameLower, headerValueList);
            }
            headerValueList.add(value);
        }

    }