java_gen: create DEFAULT instance for objects that support it (for Match)

Objects which consist only of parts that have a default value
now get a default value.

Match, in particular, now defines a default instance that wildcards
everything and is available as factory.matchWildcardAll()
diff --git a/java_gen/templates/of_class.java b/java_gen/templates/of_class.java
index 8a8d09d..829d257 100644
--- a/java_gen/templates/of_class.java
+++ b/java_gen/templates/of_class.java
@@ -56,6 +56,13 @@
 //:: for prop in msg.data_members:
     private final ${prop.java_type.public_type} ${prop.name};
 //:: #endfor
+//
+//:: if all(prop.default_value for prop in msg.data_members):
+    // Immutable default instance
+    final static ${impl_class} DEFAULT = new ${impl_class}(
+        ${", ".join(prop.default_name for prop in msg.data_members)}
+    );
+//:: #endif
 
     //:: if msg.data_members:
     // package private constructor - used by readers, builders, and factory
diff --git a/java_gen/templates/of_factory_class.java b/java_gen/templates/of_factory_class.java
index 5263226..395b524 100644
--- a/java_gen/templates/of_factory_class.java
+++ b/java_gen/templates/of_factory_class.java
@@ -72,6 +72,12 @@
     public Match.Builder buildMatch() {
         return new ${i.versioned_class(factory.version).name}.Builder();
     }
+
+    final static Match MATCH_WILDCARD_ALL = ${i.versioned_class(factory.version).name}.DEFAULT;
+
+    public Match matchWildcardAll() {
+        return MATCH_WILDCARD_ALL;
+    }
     //::     general_get_match_func_written = True
     //:: #endif
     //:: if len(i.writeable_members) <= 2:
diff --git a/java_gen/templates/of_factory_interface.java b/java_gen/templates/of_factory_interface.java
index 48ea34a..6a92e96 100644
--- a/java_gen/templates/of_factory_interface.java
+++ b/java_gen/templates/of_factory_interface.java
@@ -55,6 +55,7 @@
 //:: #endfor
 //:: if factory.name == 'OFFactory':
     Match.Builder buildMatch();
+    Match matchWildcardAll();
 //:: #endif
 
     OFMessageReader<${factory.base_class}> getReader();