Index: lucene/core/src/java/org/apache/lucene/util/Constants.java
===================================================================
--- lucene/core/src/java/org/apache/lucene/util/Constants.java	(revision 1300016)
+++ lucene/core/src/java/org/apache/lucene/util/Constants.java	(working copy)
@@ -17,6 +17,8 @@
  * limitations under the License.
  */
 
+import java.lang.management.ManagementFactory;
+
 import org.apache.lucene.LucenePackage;
 
 /**
@@ -51,6 +53,7 @@
 
   public static final boolean JRE_IS_64BIT;  
   public static final boolean JRE_IS_MINIMUM_JAVA7;
+  public static final boolean JRE_USES_COMPRESSED_OOPS;
   static {
     // NOTE: this logic may not be correct; if you know of a
     // more reliable approach please raise it on java-dev!
@@ -73,6 +76,26 @@
       v7 = false;
     }
     JRE_IS_MINIMUM_JAVA7 = v7;
+    
+    boolean compressedOops = false;
+    if (JRE_IS_64BIT) try {
+      final Class<?> beanClazz = Class.forName("com.sun.management.HotSpotDiagnosticMXBean");
+      final Object hotSpotBean = ManagementFactory.newPlatformMXBeanProxy(
+        ManagementFactory.getPlatformMBeanServer(),
+        "com.sun.management:type=HotSpotDiagnostic",
+        beanClazz
+      );
+      final Object vmOption = hotSpotBean.getClass().getMethod("getVMOption", String.class)
+        .invoke(hotSpotBean, "UseCompressedOops");
+      compressedOops = Boolean.valueOf(
+        vmOption.getClass().getMethod("getValue")
+        .invoke(vmOption).toString()
+      ).booleanValue();
+    } catch (Exception e) {
+      // if anything fails before, we assume that oops are not compressed
+      compressedOops = false;
+    }
+    JRE_USES_COMPRESSED_OOPS = compressedOops; 
   }
 
   // this method prevents inlining the final version constant in compiled classes,
Index: lucene/core/src/java/org/apache/lucene/util/RamUsageEstimator.java
===================================================================
--- lucene/core/src/java/org/apache/lucene/util/RamUsageEstimator.java	(revision 1300016)
+++ lucene/core/src/java/org/apache/lucene/util/RamUsageEstimator.java	(working copy)
@@ -45,7 +45,7 @@
   public final static int NUM_BYTES_DOUBLE = 8;
   public final static int NUM_BYTES_CHAR = 2;
   public final static int NUM_BYTES_OBJECT_HEADER = 8;
-  public final static int NUM_BYTES_OBJECT_REF = Constants.JRE_IS_64BIT ? 8 : 4;
+  public final static int NUM_BYTES_OBJECT_REF = (Constants.JRE_IS_64BIT && !Constants.JRE_USES_COMPRESSED_OOPS) ? 8 : 4;
   public final static int NUM_BYTES_ARRAY_HEADER = NUM_BYTES_OBJECT_HEADER + NUM_BYTES_INT + NUM_BYTES_OBJECT_REF;
 
   private MemoryModel memoryModel;
Index: lucene/core/src/test/org/apache/lucene/util/TestRamUsageEstimator.java
===================================================================
--- lucene/core/src/test/org/apache/lucene/util/TestRamUsageEstimator.java	(revision 1300016)
+++ lucene/core/src/test/org/apache/lucene/util/TestRamUsageEstimator.java	(working copy)
@@ -35,6 +35,21 @@
     rue.estimateRamUsage(strings);
   }
   
+  public void testCompressedOops() {
+    System.out.println("NOTE: This JVM is 64bit: " + Constants.JRE_IS_64BIT);    
+    System.out.println("NOTE: This JVM uses CompressedOops: " + Constants.JRE_USES_COMPRESSED_OOPS);
+    if (Constants.JRE_IS_64BIT) {
+      if (Constants.JRE_USES_COMPRESSED_OOPS) {
+        assertEquals(4, RamUsageEstimator.NUM_BYTES_OBJECT_REF);
+      } else {
+        assertEquals(8, RamUsageEstimator.NUM_BYTES_OBJECT_REF);
+      }
+    } else {
+      assertFalse(Constants.JRE_USES_COMPRESSED_OOPS);
+      assertEquals(4, RamUsageEstimator.NUM_BYTES_OBJECT_REF);
+    }
+  }
+  
   private static final class Holder {
     long field1 = 5000L;
     String name = "name";