add Config.empty variants that take origin description; use singleton for default-named empty object

src/main/java/com/typesafe/config/Config.java

@@ -81,13 +81,22 @@ public final class Config {
     }
 
     public static ConfigRoot emptyRoot(String rootPath) {
-        return ConfigImpl.emptyRoot(rootPath);
+        return emptyRoot(rootPath, null);
     }
 
     public static ConfigObject empty() {
-        return ConfigImpl.empty();
+        return empty(null);
     }
 
+    public static ConfigRoot emptyRoot(String rootPath, String originDescription) {
+        return ConfigImpl.emptyRoot(rootPath, originDescription);
+    }
+
+    public static ConfigObject empty(String originDescription) {
+        return ConfigImpl.empty(originDescription);
+    }
+
+
     public static ConfigRoot systemPropertiesRoot(String rootPath) {
         return ConfigImpl.systemPropertiesRoot(rootPath);
     }
@@ -250,7 +259,7 @@ public final class Config {
      * ConfigObject interface. The units parsed are interpreted as powers of
      * two, that is, the convention for memory rather than the convention for
      * disk space.
-     * 
+     *
      * @param input
      *            the string to parse
      * @param originForException

src/main/java/com/typesafe/config/impl/ConfigImpl.java

@@ -111,14 +111,17 @@ public class ConfigImpl {
     }
 
     /** For use ONLY by library internals, DO NOT TOUCH not guaranteed ABI */
-    public static ConfigRoot emptyRoot(String rootPath) {
-        return SimpleConfigObject.empty(new SimpleConfigOrigin(rootPath))
+    public static ConfigRoot emptyRoot(String rootPath, String originDescription) {
+        String desc = originDescription != null ? originDescription : rootPath;
+        return SimpleConfigObject.empty(new SimpleConfigOrigin(desc))
                 .asRoot(
                 Parser.parsePath(rootPath));
     }
 
-    public static ConfigObject empty() {
-        return SimpleConfigObject.empty();
+    public static ConfigObject empty(String originDescription) {
+        return SimpleConfigObject
+                .empty(originDescription != null ? new SimpleConfigOrigin(
+                        originDescription) : null);
     }
 
     /** For use ONLY by library internals, DO NOT TOUCH not guaranteed ABI */

src/main/java/com/typesafe/config/impl/SimpleConfigObject.java

@@ -113,16 +113,20 @@ final class SimpleConfigObject extends AbstractConfigObject {
         return new HashSet<ConfigValue>(value.values());
     }
 
-    final static String EMPTY_NAME = "empty config";
+    final private static String EMPTY_NAME = "empty config";
+    final private static SimpleConfigObject emptyInstance = empty(new SimpleConfigOrigin(
+            EMPTY_NAME));
 
     final static SimpleConfigObject empty() {
-        return new SimpleConfigObject(new SimpleConfigOrigin(EMPTY_NAME),
-                Collections.<String, AbstractConfigValue> emptyMap());
+        return emptyInstance;
     }
 
     final static SimpleConfigObject empty(ConfigOrigin origin) {
-        return new SimpleConfigObject(origin,
-                Collections.<String, AbstractConfigValue> emptyMap());
+        if (origin == null)
+            return empty();
+        else
+            return new SimpleConfigObject(origin,
+                    Collections.<String, AbstractConfigValue> emptyMap());
     }
 
     final static SimpleConfigObject emptyMissing(ConfigOrigin baseOrigin) {

src/test/scala/com/typesafe/config/impl/PublicApiTest.scala

@@ -71,4 +71,13 @@ class PublicApiTest extends TestUtils {
             conf.getInt("fromProps.one")
         }
     }
+
+    @Test
+    def emptyObjects() {
+        assertEquals(0, Config.empty().size())
+        assertEquals(0, Config.empty("foo").size())
+        assertEquals("foo", Config.empty("foo").origin().description())
+        assertEquals(0, Config.emptyRoot("foo.bar").size())
+        assertEquals("foo.bar", Config.emptyRoot("foo.bar").origin().description())
+    }
 }