get group builder

compiler/src/main/cpp/capnpc-java.c++

@@ -613,7 +613,7 @@ private:
             kj::strTree(
               kj::mv(unionDiscrim.builderIsDecl),
               spaces(indent), "  public final ", titleCase, ".Builder get", titleCase, "() {\n",
-              spaces(indent), "    throw new Error();\n",
+              spaces(indent), "    return new ", scope, titleCase, ".Builder(_builder);\n",
               spaces(indent), "  }\n",
               spaces(indent), "  public final ", titleCase, ".Builder init", titleCase, "() {\n",
               spaces(indent), "    throw new Error();\n",

runtime/src/main/java/org/capnproto/BuilderArena.java

@@ -33,4 +33,8 @@ public final class BuilderArena implements Arena {
     public AllocateResult allocate(int amount) {
         throw new Error("unimplemented");
     }
+
+    public final Vector<ByteBuffer> getSegmentsForOutput() {
+        throw new Error();
+    }
 }

runtime/src/main/java/org/capnproto/MessageBuilder.java

@@ -22,4 +22,6 @@ public final class MessageBuilder {
         AnyPointer.Builder ptr = new AnyPointer.Builder(PointerBuilder.getRoot(rootSegment, location));
         return ptr.initAsStruct(factory);
     }
+
+    //public final getSegmentsForOutput()
 }