Class SingleFieldBuilder<MType,BType,IType> (3.19.4)

SingleFieldBuilder implements a structure that a protocol message uses to hold a single field of another protocol message. It supports the classical use case of setting an immutable Message as the value of the field and is highly optimized around this.

It also supports the additional use case of setting a Message.Builder as the field and deferring conversion of that Builder to an immutable Message. In this way, it's possible to maintain a tree of Builder's that acts as a fully read/write data structure.
Logically, one can think of a tree of builders as converting the entire tree to messages when build is called on the root or when any method is called that desires a Message instead of a Builder. In terms of the implementation, the SingleFieldBuilder and RepeatedFieldBuilder classes cache messages that were created so that messages only need to be created when some change occurred in its builder or a builder for one of its descendants.

Builds the message and returns it.

Clears the value of the field.

Gets a builder for the field. If no builder has been created yet, a builder is created on demand by calling Message#toBuilder.

Get the message for the field. If the message is currently stored as a Builder, it is converted to a Message by calling Message.Builder#buildPartial on it. If no message has been set, returns the default instance of the message.

Gets the base class interface for the field. This may either be a builder or a message. It will return whatever is more efficient.

A builder becomes dirty whenever a field is modified -- including fields in nested builders -- and becomes clean when build() is called. Thus, when a builder becomes dirty, all its parents become dirty as well, and when it becomes clean, all its children become clean. The dirtiness state is used to invalidate certain cached values.

To this end, a builder calls markDirty() on its parent whenever it transitions from clean to dirty. The parent must propagate this call to its own parent, unless it was already dirty, in which case the grandparent must necessarily already be dirty as well. The parent can only transition back to "clean" after calling build() on all children.

Merges the field from another field.

Sets a message for the field replacing any existing value.