This is the story of
Antony "The ruthless mafia boss". Antony is ruling city mafia for
over 3 decades now. Every crime in the city happens with his orders.
He has 4 Gangs working
under him. He takes care that the authorities, police etc. are under control.
Now he is 65 years old but still he has not lost his control. Although he don't
go into the details of each and every crime and its execution but his approval
is must.
Every morning people
assemble in his courtyard with their requests. They meet his son who looks into
all the details and decide if the request will go further and to which gang.
For such requests he creates a letter with all the required information and
then they take this letter to Antony. This letter has all the details required
for the execution of crime.
Antony never opens the
envelope. If situation is favourable. Connections with Police and all other
authorities are under control he just says "DO IT". Then the letters
reaches the Gangs who execute the orders.
Nice story, but the
title of this article is 'Command Pattern' and not 'Godfather Returns'. So,
time to put on our Java glasses and read the story again.
I told you this story as
Antony has been using Command Pattern to run his mafia successfully since
ages.
The Gangs (Executors)
Although they don't seem
to be very important in the story but they are the ones who execute everything
and they know "How to do it?".
Here are the 3 Gangs
which specialize in different types of crimes.
Here is sample Gang
Class-
These Gangs only wait
for commands(Letter) from Antony to do the work.
The Letter
Letter plays a very
important role it is a command for the Gangs. And there could be different
types of commands.
- MurderCommand
- KidnapCommand
- StealCommand ....And More
How can a Letter reach
the destination without address?
So, Letter should have
information about the Gang where it should go.
Command object should have the reference of the
Executor Object (Gang object).
Which will do the real
execution.
So, MurderCommand
has Gang3 object. It calls murder() method of Gang3 from its execute() method.
Command has Gang
In this case calling a single method murder() is enough for execution.
But in some cases Command might need to call mutiple methods of one or more executors.
Example- There could be a Macro command to steal, kidnap & murder.
So, execute() method will call steal(), kidnap() and murder() method of Gang1, Gang2 & Gang3 objects.
But in some cases Command might need to call mutiple methods of one or more executors.
Example- There could be a Macro command to steal, kidnap & murder.
So, execute() method will call steal(), kidnap() and murder() method of Gang1, Gang2 & Gang3 objects.
Letter is the part which
travels from beginning till end of this story and thus makes it possible
Command Object helps in separating the creation and execution
logic.
Antony (The Invoker)-
He is the most important
character of this story. As nothing can happen without him. He is the one who
decides when the command will be executed(remember DO IT). He is the invoker of
command.
So, To murder someone
Antony don't need to know how a murder is done(Although he knows). he just need
to say DO IT and rest is done.
In Java terms to execute
the command invoker just need to have Command object whose execute() method it
will call to Invoke the command.
Command pattern separates the logic of creating a request and
executing a request.
Antony has Command
Requestors (or Clients)
These are the people who
came to Antony for getting their work done. Their requests actually changed to
commands when Antony said DO IT.
Their work is to give
all the details required for the execution in the letter. And to hand it over
to Antony. The story starts from them.
Client creates the
Command object and then set this object in Invoker using setter method.
Full Picture
Command Pattern Class Diagram
Command Pattern is required when the command can't be executed as
soon as it is created. And there is a separate entity or condition which
controls the invocation of command(Antony in this case).
There could be more than
one letter with antony when he decides time is appropriate for execution.
Invoker can have more than one command.
