2 files changed, 84 insertions, 2 deletions

diff --git a/java/src/main/java/com/google/protobuf/nano/ExtendableMessageNano.java b/java/src/main/java/com/google/protobuf/nano/ExtendableMessageNano.java
new file mode 100644
index 0000000..066774d
--- /dev/null
+++ b/java/src/main/java/com/google/protobuf/nano/ExtendableMessageNano.java
@@ -0,0 +1,70 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2013 Google Inc.  All rights reserved.
+// http://code.google.com/p/protobuf/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+package com.google.protobuf.nano;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Base class of those Protocol Buffer messages that need to store unknown fields,
+ * such as extensions.
+ */
+public abstract class ExtendableMessageNano extends MessageNano {
+    /**
+     * A container for fields unknown to the message, including extensions. Extension fields can
+     * can be accessed through the {@link getExtension()} and {@link setExtension()} methods.
+     */
+    protected List<UnknownFieldData> unknownFieldData;
+
+    @Override
+    public int getSerializedSize() {
+        int size = WireFormatNano.computeWireSize(unknownFieldData);
+        cachedSize = size;
+        return size;
+    }
+
+    /**
+     * Gets the value stored in the specified extension of this message.
+     */
+    public <T> T getExtension(Extension<T> extension) {
+        return WireFormatNano.getExtension(extension, unknownFieldData);
+    }
+
+    /**
+     * Sets the value of the specified extension of this message.
+     */
+    public <T> void setExtension(Extension<T> extension, T value) {
+        if (unknownFieldData == null) {
+            unknownFieldData = new ArrayList<UnknownFieldData>();
+        }
+        WireFormatNano.setExtension(extension, value, unknownFieldData);
+    }
+}
\ No newline at end of file
diff --git a/java/src/main/java/com/google/protobuf/nano/MessageNano.java b/java/src/main/java/com/google/protobuf/nano/MessageNano.java
index 0885cdb..0cf8416 100644
--- a/java/src/main/java/com/google/protobuf/nano/MessageNano.java
+++ b/java/src/main/java/com/google/protobuf/nano/MessageNano.java
@@ -38,6 +38,8 @@ import java.io.IOException;
  * @author wink@google.com Wink Saville
  */
 public abstract class MessageNano {
+    protected int cachedSize = -1;
+
     /**
      * Get the number of bytes required to encode this message.
      * Returns the cached size or calls getSerializedSize which
@@ -45,14 +47,24 @@ public abstract class MessageNano {
      * so the size is only computed once. If a member is modified
      * then this could be stale call getSerializedSize if in doubt.
      */
-    abstract public int getCachedSize();
+    public int getCachedSize() {
+        if (cachedSize < 0) {
+            // getSerializedSize sets cachedSize
+            getSerializedSize();
+        }
+        return cachedSize;
+    }
 
     /**
      * Computes the number of bytes required to encode this message.
      * The size is cached and the cached result can be retrieved
      * using getCachedSize().
      */
-    abstract public int getSerializedSize();
+    public int getSerializedSize() {
+        // This is overridden if the generated message has serialized fields.
+        cachedSize = 0;
+        return 0;
+    }
 
     /**
      * Serializes the message and writes it to {@code output}.  This does not