@Override
    public final void writeTo(Output output, Map<K, V> map) throws IOException
    {
        for (Entry<K, V> entry : map.entrySet())
        {
            // allow null keys and values.
            output.writeObject(1, entry, entrySchema, true);
        }
    }
Transfers the value from the input to the output.
protostuff-collectionschema/src/main/java/io/protostuff/MapSchema.java