src/main/java/com/googlecode/objectify/impl/KeyMetadata.java
@SuppressWarnings("unchecked")
	public <K extends IncompleteKey> void setKey(final FullEntity.Builder<K> container, final P pojo) {
		final IncompleteKey rawKey = getIncompleteKey(pojo);

		if (!(rawKey instanceof com.google.cloud.datastore.Key)) {
			// it's incomplete, make sure we can save it
			Preconditions.checkState(isIdNumeric(), "Cannot save an entity with a null String @Id: %s", pojo);
		}

		container.setKey((K)rawKey);
	}
@SuppressWarnings("unchecked")
	public <K extends IncompleteKey> void setKey(final FullEntity.Builder<K> container, final P pojo) {
		final IncompleteKey rawKey = getIncompleteKey(pojo);

		if (!(rawKey instanceof com.google.cloud.datastore.Key)) {
			// it's incomplete, make sure we can save it
			Preconditions.checkState(isIdNumeric(), "Cannot save an entity with a null String @Id: %s", pojo);
		}

		container.setKey((K)rawKey);
	}
Sets the key on a container from the POJO.