在一个Java类内定义了一个接口，然后将接口作为本类的一个方法的参数，当调用这个方法时，本应传入接口的位置传入了本类的另一个方法（这个方法和接口中的唯一方法的参数类型对应）。程序如下所示。我很不清楚这样使用的原理，接口和方法完全是两个东西，在调用的过程中发生了什么转换实现了这种方式？希望哪位大神指点一下，感谢。

包含接口的类（其中定义了函数式接口、和参数为接口的方法、以及调用时传入的方法）：

public class ClientMessage {
    private static final Logger logger = LoggerFactory.getLogger(ClientMessage.class);

    public static HashMap<Integer, Transfer> tranferHandlerMap = new HashMap<>();
    public static HashMap<Class<?>, Integer> msg2ptoNum = new HashMap<>();

    @FunctionalInterface
    public interface Transfer{
        void process(Message msg, ClientConnection conn) throws IOException;
    }

    public static void registerTransferHandler(Integer ptoNum, Transfer tranfer, Class<?> cla) {
        if (tranferHandlerMap.get(ptoNum) == null)
            tranferHandlerMap.put(ptoNum, tranfer);
        else {
            logger.error("pto has been registered in transeerHandlerMap, ptoNum: {}", ptoNum);
            return;
        }

        if(msg2ptoNum.get(cla) == null)
            msg2ptoNum.put(cla, ptoNum);
        else {
            logger.error("pto has been registered in msg2ptoNum, ptoNum: {}", ptoNum);
            return;
        }
    }

    public static void processTransferHandler(Message msg, ClientConnection conn) throws IOException {
        logger.info("MessageName {}", msg.getClass());
        int ptoNum = msg2ptoNum.get(msg.getClass());
        Transfer transferHandler = tranferHandlerMap.get(ptoNum);

        if(transferHandler != null) {
            transferHandler.process(msg, conn);
        }
    }

    public static void transfer2Logic(Message msg, ClientConnection conn) {
        ByteBuf byteBuf = null;
        if(conn.getUserId() == null ) {
            logger.error("User not login.");
            return;
        }

        if(msg instanceof Chat.CPrivateChat) {
            byteBuf = Utils.pack2Server(msg, ParseMap.getPtoNum(msg), conn.getNetId(), Internal.Dest.Logic, conn.getUserId());
        }

        GateLogicConnectionHandler.getGatelogicConnection().writeAndFlush(byteBuf);
    }

    public static void transfer2Auth(Message msg, ClientConnection conn) {
        ByteBuf byteBuf = null;
        if(msg instanceof Auth.CLogin) {
            String userId = ((Auth.CLogin) msg).getUserid();
            byteBuf = Utils.pack2Server(msg, ParseMap.getPtoNum(msg), conn.getNetId(), Internal.Dest.Auth, userId);
            ClientConnectionMap.registerUserid(userId, conn.getNetId());
        } else if(msg instanceof Auth.CRegister) {
            byteBuf = Utils.pack2Server(msg, ParseMap.getPtoNum(msg), conn.getNetId(), Internal.Dest.Auth, ((Auth.CRegister) msg).getUserid());
        }

        GateAuthConnectionHandler.getGateAuthConnection().writeAndFlush(byteBuf);

    }
}

调用方法时的代码：

ClientMessage.registerTransferHandler(1000, ClientMessage::transfer2Auth, Auth.CLogin.class);
ClientMessage.registerTransferHandler(1001, ClientMessage::transfer2Auth, Auth.CRegister.class);
ClientMessage.registerTransferHandler(1003, ClientMessage::transfer2Logic, Chat.CPrivateChat.class);

ClientMessage.processTransferHandler(message, conn);