--- old/src/java.net.http/share/classes/java/net/http/HttpHeaders.java	2018-05-25 15:52:01.597086117 +0100
+++ new/src/java.net.http/share/classes/java/net/http/HttpHeaders.java	2018-05-25 15:52:01.313092158 +0100
@@ -25,62 +25,68 @@
 
 package java.net.http;
 
+import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
 import java.util.OptionalLong;
-import static java.util.Collections.emptyList;
-import static java.util.Collections.unmodifiableList;
+import java.util.TreeMap;
+import java.util.TreeSet;
+import java.util.function.BiPredicate;
+import static java.lang.String.CASE_INSENSITIVE_ORDER;
+import static java.util.Collections.unmodifiableMap;
 import static java.util.Objects.requireNonNull;
 
 /**
  * A read-only view of a set of HTTP headers.
  *
- * <p> An {@code HttpHeaders} is not created directly, but rather returned from
- * an {@link HttpResponse HttpResponse}. Specific HTTP headers can be set for
- * {@linkplain HttpRequest requests} through the one of the request builder's
- * {@link HttpRequest.Builder#header(String, String) headers} methods.
+ * <p> An {@code HttpHeaders} is not typically created directly, but rather
+ * returned from an {@link HttpRequest#headers() HttpRequest} or an
+ * {@link HttpResponse#headers() HttpResponse}. Specific HTTP headers can be
+ * set for a {@linkplain HttpRequest request} through one of the request
+ * builder's {@link HttpRequest.Builder#header(String, String) headers} methods.
  *
  * <p> The methods of this class ( that accept a String header name ), and the
- * Map returned by the {@link #map() map} method, operate without regard to
- * case when retrieving the header value.
+ * {@code Map} returned by the {@link #map() map} method, operate without regard
+ * to case when retrieving the header value(s).
+ *
+ * <p> An HTTP header name may appear more than once in the HTTP protocol. As
+ * such, headers are represented as a name and a list of values. Each occurrence
+ * of a header value is added verbatim, to the appropriate header name list,
+ * without interpreting its value. In particular, {@code HttpHeaders} does not
+ * perform any splitting or joining of comma separated header value strings. The
+ * order of elements in a header value list is preserved when {@link
+ * HttpRequest.Builder#header(String, String) building} a request. For
+ * responses, the order of elements in a header value list is the order in which
+ * they were received. The {@code Map} returned by the {@code map} method,
+ * however, does not provide any guarantee with regard to the ordering of its
+ * entries.
  *
  * <p> {@code HttpHeaders} instances are immutable.
  *
  * @since 11
  */
-public abstract class HttpHeaders {
+public final class HttpHeaders {
 
     /**
-     * Creates an HttpHeaders.
-     */
-    protected HttpHeaders() {}
-
-    /**
-     * Returns an {@link Optional} containing the first value of the given named
-     * (and possibly multi-valued) header. If the header is not present, then
-     * the returned {@code Optional} is empty.
-     *
-     * @implSpec
-     * The default implementation invokes
-     * {@code allValues(name).stream().findFirst()}
+     * Returns an {@link Optional} containing the first header string value of
+     * the given named (and possibly multi-valued) header. If the header is not
+     * present, then the returned {@code Optional} is empty.
      *
      * @param name the header name
-     * @return an {@code Optional<String>} for the first named value
+     * @return an {@code Optional<String>} containing the first named header
+     *         string value, if present
      */
     public Optional<String> firstValue(String name) {
         return allValues(name).stream().findFirst();
     }
 
     /**
-     * Returns an {@link OptionalLong} containing the first value of the
-     * named header field. If the header is not present, then the Optional is
-     * empty. If the header is present but contains a value that does not parse
-     * as a {@code Long} value, then an exception is thrown.
-     *
-     * @implSpec
-     * The default implementation invokes
-     * {@code allValues(name).stream().mapToLong(Long::valueOf).findFirst()}
+     * Returns an {@link OptionalLong} containing the first header string value
+     * of the named header field. If the header is not present, then the
+     * Optional is empty. If the header is present but contains a value that
+     * does not parse as a {@code Long} value, then an exception is thrown.
      *
      * @param name the header name
      * @return  an {@code OptionalLong}
@@ -92,23 +98,19 @@
     }
 
     /**
-     * Returns an unmodifiable List of all of the values of the given named
-     * header. Always returns a List, which may be empty if the header is not
-     * present.
-     *
-     * @implSpec
-     * The default implementation invokes, among other things, the
-     * {@code map().get(name)} to retrieve the list of header values.
+     * Returns an unmodifiable List of all of the header string values of the
+     * given named header. Always returns a List, which may be empty if the
+     * header is not present.
      *
      * @param name the header name
-     * @return a List of String values
+     * @return a List of headers string values
      */
     public List<String> allValues(String name) {
         requireNonNull(name);
         List<String> values = map().get(name);
         // Making unmodifiable list out of empty in order to make a list which
         // throws UOE unconditionally
-        return values != null ? values : unmodifiableList(emptyList());
+        return values != null ? values : List.of();
     }
 
     /**
@@ -116,7 +118,9 @@
      *
      * @return the Map
      */
-    public abstract Map<String, List<String>> map();
+    public Map<String,List<String>> map() {
+        return headers;
+    }
 
     /**
      * Tests this HTTP headers instance for equality with the given object.
@@ -165,4 +169,84 @@
         sb.append(" }");
         return sb.toString();
     }
+
+    /**
+     * Returns an HTTP headers from the given map. The given map's key
+     * represents the header name, and its value the list of string header
+     * values for that header name.
+     *
+     * <p> An HTTP header name may appear more than once in the HTTP protocol.
+     * Such, <i>multi-valued</i>, headers must be represented by a single entry
+     * in the given map, whose entry value is a list that represents the
+     * multiple header string values. Leading and trailing whitespaces are
+     * removed from all string values retrieved from the given map and its lists
+     * before processing. Only headers that, after filtering, contain at least
+     * one, possibly empty string, value will be added to the HTTP headers.
+     *
+     * @apiNote The primary purpose of this method is for testing frameworks.
+     * Per-request headers can be set through one of the {@code HttpRequest}
+     * {@link HttpRequest.Builder#header(String, String) headers} methods.
+     *
+     * @param headerMap the map containing the header names and values
+     * @param filter a filter that can be used to inspect each
+     *               header-name-and-value pair in the given map to determine if
+     *               it should, or should not, be added to the to the HTTP
+     *               headers
+     * @return an HTTP headers instance containing the given headers
+     * @throws NullPointerException if any of: {@code headerMap}, a key or value
+     *        in the given map, or an entry in the map's value list, or
+     *        {@code filter}, is {@code null}
+     * @throws IllegalArgumentException if the given {@code headerMap} contains
+     *         any two keys that are equal ( without regard to case ); or if the
+     *         given map contains any key whose length, after trimming
+     *         whitespaces, is {@code 0}
+     */
+    public static HttpHeaders of(Map<String,List<String>> headerMap,
+                                 BiPredicate<String,String> filter) {
+        requireNonNull(headerMap);
+        requireNonNull(filter);
+        return headersOf(headerMap, filter);
+    }
+
+    // --
+
+    private static final HttpHeaders NO_HEADERS = new HttpHeaders(Map.of());
+
+    private final Map<String,List<String>> headers;
+
+    private HttpHeaders(Map<String,List<String>> headers) {
+        this.headers = headers;
+    }
+
+    // Returns a new HTTP headers after performing a structural copy and filtering.
+    private static HttpHeaders headersOf(Map<String,List<String>> map,
+                                         BiPredicate<String,String> filter) {
+        TreeMap<String,List<String>> other = new TreeMap<>(CASE_INSENSITIVE_ORDER);
+        TreeSet<String> notAdded = new TreeSet<>(CASE_INSENSITIVE_ORDER);
+        ArrayList<String> tempList = new ArrayList<>();
+        map.forEach((key, value) -> {
+            String headerName = requireNonNull(key).trim();
+            if (headerName.isEmpty()) {
+                throw new IllegalArgumentException("empty key");
+            }
+            List<String> headerValues = requireNonNull(value);
+            headerValues.forEach(headerValue -> {
+                headerValue = requireNonNull(headerValue).trim();
+                if (filter.test(headerName, headerValue)) {
+                    tempList.add(headerValue);
+                }
+            });
+
+            if (tempList.isEmpty()) {
+                if (other.containsKey(headerName)
+                        || notAdded.contains(headerName.toLowerCase(Locale.ROOT)))
+                    throw new IllegalArgumentException("duplicate key: " + headerName);
+                notAdded.add(headerName.toLowerCase(Locale.ROOT));
+            } else if (other.put(headerName, List.copyOf(tempList)) != null) {
+                throw new IllegalArgumentException("duplicate key: " + headerName);
+            }
+            tempList.clear();
+        });
+        return other.isEmpty() ? NO_HEADERS : new HttpHeaders(unmodifiableMap(other));
+    }
 }