android-Near Field Communication（NFC）

》 Near Field Communication (NFC) is a set of short-range wireless technologies, typically requiring a distance of 4cm or less to initiate a connection. NFC allows you to share
small payloads of data between an NFC tag and an Android-powered device, or between two Android-powered devices.

》 Simple tags offer just read and write semantics, sometimes with one-time-programmable areas to make
the card read-only. More complex tags offer math operations, and have cryptographic hardware to authenticate access to a sector. The most sophisticated tags contain operating environments, allowing complex interactions with code executing on the tag. The data
stored in the tag can also be written in a variety of formats, but many of the Android framework APIs are based around a NFC
Forumstandard called NDEF (NFC Data Exchange Format).

Android-powered devices with NFC simultaneously support three main modes of operation:

Reader/writer mode, allowing the NFC device to read and/or write passive NFC tags and stickers.
P2P mode, allowing the NFC device to exchange data with other NFC peers; this operation mode is used by Android Beam.
Card emulation mode, allowing the NFC device itself to act as an NFC card. The emulated NFC card can then be accessed by an external NFC reader, such as an NFC point-of-sale terminal.