java Server
java Client

<message>

Everyone in the chatroom will receive a message like:

<username> say to all: <message>

/login <username>

Note:

1. The username must be unique in a chatroom
2. Whitespace is not allowed in a username
3. All features are all unavailable before login

/to <username> <message>

Send <message> to <username>.

Note:

1. <username> will receive <yourname> say to you: <message>, you will receive You say to <username>: <message> if sending succeed.
2. If <username> is not existed, you will received Can't find user <username>.

/history <begin_index> <end_index>

Note:

1. begin_index and end_index is optional. If they are empty, all chatting history will be printed.

/who

Display all online users.