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## src/java.base/share/classes/java/util/Map.java

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    rev 48045 : 8177290: add copy factory methods for unmodifiable List, Set, Map
    8184690: add Collectors for collecting into unmodifiable List, Set, and Map
    Reviewed-by: alanb, briangoetz, dholmes, jrose, rriggs, scolebourne

    @@ -108,21 +108,22 @@
      * indirectly contains itself. This includes the {@code clone()},
      * {@code equals()}, {@code hashCode()} and {@code toString()} methods.
      * Implementations may optionally handle the self-referential scenario, however
      * most current implementations do not do so.
      *
    - * <h2><a id="immutable">Immutable Map Static Factory Methods</a></h2>
    - * <p>The {@link Map#of() Map.of()} and
    - * {@link Map#ofEntries(Map.Entry...) Map.ofEntries()}
    - * static factory methods provide a convenient way to create immutable maps.
    + * <h2><a id="unmodifiable">Unmodifiable Maps</a></h2>
    + * <p>The {@link Map#of() Map.of},
    + * {@link Map#ofEntries(Map.Entry...) Map.ofEntries}, and
    + * {@link Map#copyOf Map.copyOf}
    + * static factory methods provide a convenient way to create unmodifiable maps.
      * The {@code Map}
      * instances created by these methods have the following characteristics:
      *
      * <ul>
    - * <li>They are <em>structurally immutable</em>. Keys and values cannot be added,
    - * removed, or updated. Calling any mutator method will always cause
    - * {@code UnsupportedOperationException} to be thrown.
    + * <li>They are <a href="Collection.html#unmodifiable"><i>unmodifiable</i></a>. Keys and values
    + * cannot be added, removed, or updated. Calling any mutator method on the Map
    + * will always cause {@code UnsupportedOperationException} to be thrown.
      * However, if the contained keys or values are themselves mutable, this may cause the
      * Map to behave inconsistently or its contents to appear to change.
      * <li>They disallow {@code null} keys and values. Attempts to create them with
      * {@code null} keys or values result in {@code NullPointerException}.
      * <li>They are serializable if all keys and values are serializable.

    @@ -1274,12 +1275,12 @@
             }
             return newValue;
         }

         /**
    -     * Returns an immutable map containing zero mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing zero mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @return an empty {@code Map}
          *

    @@ -1288,12 +1289,12 @@
         static <K, V> Map<K, V> of() {
             return ImmutableCollections.Map0.instance();
         }

         /**
    -     * Returns an immutable map containing a single mapping.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing a single mapping.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the mapping's key
          * @param v1 the mapping's value

    @@ -1305,12 +1306,12 @@
         static <K, V> Map<K, V> of(K k1, V v1) {
             return new ImmutableCollections.Map1<>(k1, v1);
         }

         /**
    -     * Returns an immutable map containing two mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing two mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the first mapping's key
          * @param v1 the first mapping's value

    @@ -1325,12 +1326,12 @@
         static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2) {
             return new ImmutableCollections.MapN<>(k1, v1, k2, v2);
         }

         /**
    -     * Returns an immutable map containing three mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing three mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the first mapping's key
          * @param v1 the first mapping's value

    @@ -1347,12 +1348,12 @@
         static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) {
             return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3);
         }

         /**
    -     * Returns an immutable map containing four mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing four mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the first mapping's key
          * @param v1 the first mapping's value

    @@ -1371,12 +1372,12 @@
         static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) {
             return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4);
         }

         /**
    -     * Returns an immutable map containing five mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing five mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the first mapping's key
          * @param v1 the first mapping's value

    @@ -1397,12 +1398,12 @@
         static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) {
             return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5);
         }

         /**
    -     * Returns an immutable map containing six mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing six mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the first mapping's key
          * @param v1 the first mapping's value

    @@ -1427,12 +1428,12 @@
             return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                    k6, v6);
         }

         /**
    -     * Returns an immutable map containing seven mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing seven mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the first mapping's key
          * @param v1 the first mapping's value

    @@ -1459,12 +1460,12 @@
             return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                    k6, v6, k7, v7);
         }

         /**
    -     * Returns an immutable map containing eight mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing eight mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the first mapping's key
          * @param v1 the first mapping's value

    @@ -1493,12 +1494,12 @@
             return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                    k6, v6, k7, v7, k8, v8);
         }

         /**
    -     * Returns an immutable map containing nine mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing nine mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the first mapping's key
          * @param v1 the first mapping's value

    @@ -1529,12 +1530,12 @@
             return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                    k6, v6, k7, v7, k8, v8, k9, v9);
         }

         /**
    -     * Returns an immutable map containing ten mappings.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * Returns an unmodifiable map containing ten mappings.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @param <K> the {@code Map}'s key type
          * @param <V> the {@code Map}'s value type
          * @param k1 the first mapping's key
          * @param v1 the first mapping's value

    @@ -1567,13 +1568,13 @@
             return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
                                                    k6, v6, k7, v7, k8, v8, k9, v9, k10, v10);
         }

         /**
    -     * Returns an immutable map containing keys and values extracted from the given entries.
    +     * Returns an unmodifiable map containing keys and values extracted from the given entries.
          * The entries themselves are not stored in the map.
    -     * See <a href="#immutable">Immutable Map Static Factory Methods</a> for details.
    +     * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
          *
          * @apiNote
          * It is convenient to create the map entries using the {@link Map#entry Map.entry()} method.
          * For example,
          *

    @@ -1600,36 +1601,38 @@
          * @since 9
          */
         @SafeVarargs
         @SuppressWarnings("varargs")
         static <K, V> Map<K, V> ofEntries(Entry<? extends K, ? extends V>... entries) {
    -        if (entries.length == 0) { // implicit null check of entries
    +        if (entries.length == 0) { // implicit null check of entries array
                 return ImmutableCollections.Map0.instance();
             } else if (entries.length == 1) {
    +            // implicit null check of the array slot
                 return new ImmutableCollections.Map1<>(entries[0].getKey(),
                                                        entries[0].getValue());
             } else {
                 Object[] kva = new Object[entries.length << 1];
                 int a = 0;
                 for (Entry<? extends K, ? extends V> entry : entries) {
    +                // implicit null checks of each array slot
                     kva[a++] = entry.getKey();
                     kva[a++] = entry.getValue();
                 }
                 return new ImmutableCollections.MapN<>(kva);
             }
         }

         /**
    -     * Returns an immutable {@link Entry} containing the given key and value.
    +     * Returns an unmodifiable {@link Entry} containing the given key and value.
          * These entries are suitable for populating {@code Map} instances using the
          * {@link Map#ofEntries Map.ofEntries()} method.
          * The {@code Entry} instances created by this method have the following characteristics:
          *
          * <ul>
          * <li>They disallow {@code null} keys and values. Attempts to create them using a {@code null}
          * key or value result in {@code NullPointerException}.
    -     * <li>They are immutable. Calls to {@link Entry#setValue Entry.setValue()}
    +     * <li>They are unmodifiable. Calls to {@link Entry#setValue Entry.setValue()}
          * on a returned {@code Entry} result in {@code UnsupportedOperationException}.
          * <li>They are not serializable.
          * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
          * Callers should make no assumptions about the identity of the returned instances.
          * This method is free to create new instances or reuse existing ones. Therefore,

    @@ -1653,6 +1656,32 @@
          */
         static <K, V> Entry<K, V> entry(K k, V v) {
             // KeyValueHolder checks for nulls
             return new KeyValueHolder<>(k, v);
         }
    +
    +    /**
    +     * Returns an <a href="#unmodifiable">unmodifiable Map</a> containing the entries
    +     * of the given Map. The given Map must not be null, and it must not contain any
    +     * null keys or values. If the given Map is subsequently modified, the returned
    +     * Map will not reflect such modifications.
    +     *
    +     * @implNote
    +     * If the given Map is an <a href="#unmodifiable">unmodifiable Map</a>,
    +     * calling copyOf will generally not create a copy.
    +     *
    +     * @param <K> the {@code Map}'s key type
    +     * @param <V> the {@code Map}'s value type
    +     * @param map a {@code Map} from which entries are drawn, must be non-null
    +     * @return a {@code Map} containing the entries of the given {@code Map}
    +     * @throws NullPointerException if map is null, or if it contains any null keys or values
    +     * @since 10
    +     */
    +    @SuppressWarnings({"rawtypes","unchecked"})
    +    static <K, V> Map<K, V> copyOf(Map<? extends K, ? extends V> map) {
    +        if (map instanceof ImmutableCollections.AbstractImmutableMap) {
    +            return (Map<K,V>)map;
    +        } else {
    +            return (Map<K,V>)Map.ofEntries(map.entrySet().toArray(new Entry[0]));
    +        }
    +    }
     }

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