code: public BType getBuilder() {
      if (builder == null) {
        // builder.mergeFrom() on a fresh builder
        // does not create any sub-objects with independent clean/dirty states,
        // therefore setting the builder itself to clean without actually calling
        // build() cannot break any invariants.
        builder = newBuilderForType(message.getDescriptorForType(), this);
        builder.mergeFrom(message); // no-op if message is the default message
        markClean(builder);
      }
      return builder;
    }
docs: Gets a builder for the field. If no builder has been created yet, a builder is created on
  demand by calling {@link Message#toBuilder}.
  
  @return The builder for the field