Optimize emptySet/Map/List() in Collections class

2012-10-16  Ivan Maidanski  <ivmai@mail.ru>

	* java/util/Collections.java:
	(emptySet(), EmptySet.iterator(), emptyList(), emptyMap(),
	EmptyMap.entrySet(), EmptyMap.keySet(), EmptyMap.values()): Suppress
	"unchecked" warnings.
	(emptySet(), emptyList(), emptyMap()): Don't create new instance (use
	the corresponding immutable container instance); remove FIXME.
	(EmptySet.equals(Object), EmptyList.equals(Object),
	EmptyMap.entrySet(), EmptyMap.equals(Object), EmptyMap.keySet(),
	EmptyMap.values()): Add generic typing.
	(SingletonList.subList(int, int)): Use emptyList() instead of
	EMPTY_LIST (to eliminate "unchecked" warning).
	(SynchronizedCollection.toArray(T[])): Rename T type to E (to
	suppress compiler warning about type hiding).

2 files changed, 34 insertions, 14 deletions

diff --git a/ChangeLog b/ChangeLog
index f78f39328..580546334 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,19 @@
+2012-10-16  Ivan Maidanski  <ivmai@mail.ru>
+
+	* java/util/Collections.java:
+	(emptySet(), EmptySet.iterator(), emptyList(), emptyMap(),
+	EmptyMap.entrySet(), EmptyMap.keySet(), EmptyMap.values()): Suppress
+	"unchecked" warnings.
+	(emptySet(), emptyList(), emptyMap()): Don't create new instance (use
+	the corresponding immutable container instance); remove FIXME.
+	(EmptySet.equals(Object), EmptyList.equals(Object),
+	EmptyMap.entrySet(), EmptyMap.equals(Object), EmptyMap.keySet(),
+	EmptyMap.values()): Add generic typing.
+	(SingletonList.subList(int, int)): Use emptyList() instead of
+	EMPTY_LIST (to eliminate "unchecked" warning).
+	(SynchronizedCollection.toArray(T[])): Rename T type to E (to
+	suppress compiler warning about type hiding).
+
 2012-10-15  Andrew John Hughes  <gnu_andrew@member.fsf.org>
 
 	* configure.ac: Set to 0.99.1pre, as
diff --git a/java/util/Collections.java b/java/util/Collections.java
index 828c6ecea..b970dd8f6 100644
--- a/java/util/Collections.java
+++ b/java/util/Collections.java
@@ -120,10 +120,10 @@ public class Collections
    * @return an empty parameterized set.
    * @since 1.5
    */
+  @SuppressWarnings("unchecked")
   public static final <T> Set<T> emptySet()
   {
-    /* FIXME: Could this be optimized? */
-    return new EmptySet<T>();
+    return (Set<T>) EMPTY_SET;
   }
 
   /**
@@ -161,6 +161,7 @@ public class Collections
      * @return A non-iterating iterator.
      */
     // This is really cheating! I think it's perfectly valid, though.
+    @SuppressWarnings("unchecked")
     public Iterator<T> iterator()
     {
       return (Iterator<T>) EMPTY_LIST.iterator();
@@ -196,7 +197,7 @@ public class Collections
      */
     public boolean equals(Object o)
     {
-      return o instanceof Set && ((Set) o).isEmpty();
+      return o instanceof Set<?> && ((Set<?>) o).isEmpty();
     }
 
     /**
@@ -288,10 +289,10 @@ public class Collections
    * @return an empty parameterized list.
    * @since 1.5
    */
+  @SuppressWarnings("unchecked")
   public static final <T> List<T> emptyList()
   {
-    /* FIXME: Could this be optimized? */
-    return new EmptyList<T>();
+    return (List<T>) EMPTY_LIST;
   }
 
   /**
@@ -369,7 +370,7 @@ public class Collections
      */
     public boolean equals(Object o)
     {
-      return o instanceof List && ((List) o).isEmpty();
+      return o instanceof List<?> && ((List<?>) o).isEmpty();
     }
 
     /**
@@ -480,10 +481,10 @@ public class Collections
    * @return an empty parameterized map.
    * @since 1.5
    */
+  @SuppressWarnings("unchecked")
   public static final <K,V> Map<K,V> emptyMap()
   {
-    /* FIXME: Could this be optimized? */
-    return new EmptyMap<K,V>();
+    return (Map<K,V>) EMPTY_MAP;
   }
 
   /**
@@ -511,9 +512,10 @@ public class Collections
      * There are no entries.
      * @return The empty set.
      */
+    @SuppressWarnings("unchecked")
     public Set<Map.Entry<K, V>> entrySet()
     {
-      return EMPTY_SET;
+      return (Set<Map.Entry<K, V>>) EMPTY_SET;
     }
 
     // The remaining methods are optional, but provide a performance
@@ -546,7 +548,7 @@ public class Collections
      */
     public boolean equals(Object o)
     {
-      return o instanceof Map && ((Map) o).isEmpty();
+      return o instanceof Map<?,?> && ((Map<?,?>) o).isEmpty();
     }
 
     /**
@@ -572,9 +574,10 @@ public class Collections
      * No entries.
      * @return The empty set.
      */
+    @SuppressWarnings("unchecked")
     public Set<K> keySet()
     {
-      return EMPTY_SET;
+      return (Set<K>) EMPTY_SET;
     }
 
     /**
@@ -601,9 +604,10 @@ public class Collections
      * Collection, will work. Besides, that's what the JDK uses!
      * @return The empty set.
      */
+    @SuppressWarnings("unchecked")
     public Collection<V> values()
     {
-      return EMPTY_SET;
+      return (Collection<V>) EMPTY_SET;
     }
 
     /**
@@ -1854,7 +1858,7 @@ public class Collections
     public List<T> subList(int from, int to)
     {
       if (from == to && (to == 0 || to == 1))
-        return EMPTY_LIST;
+        return emptyList();
       if (from == 0 && to == 1)
         return this;
       if (from > to)
@@ -2480,7 +2484,7 @@ public class Collections
      * @throws ArrayStoreException if the type of any element of the
      *         collection is not a subtype of the element type of a.
      */
-    public <T> T[] toArray(T[] a)
+    public <E> E[] toArray(E[] a)
     {
       synchronized (mutex)
         {