def?persist(newLevel: StorageLevel): this.type = {
　　if (storageLevel != StorageLevel.NONE && newLevel != storageLevel) {
　　　　throw new UnsupportedOperationException(?"Cannot change storage level of an RDD after it was already assigned a level")
　　}
　　sc.persistRDD(this)

　　sc.cleaner.foreach(_.registerRDDForCleanup(this))
　　storageLevel = newLevel
　　this
}

/** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
def?persist(): this.type = persist(StorageLevel.MEMORY_ONLY)

?

/** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
def?cache(): this.type = persist()