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Behavorial Pattern
State Pattern
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State Pattern
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Define
State pattern
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An object's behavior change is
represented by its member classes, which share the same super class.
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Where
to use & benefits
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Need to control many states
without using if-else or switch statements.
Use a class to represent a state, not a constant or something else.
Every state has to act in a similar manner.
Every state must be a subclass of the same super class.
Simplify and clarify the program.
Related patterns include
- Flyweight, which explains when and how a state object can
be shared.
- Singleton which is often used with state pattern to ensure
some state change is shared with class itself, not instances of the
class.
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Example
of design pattern
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To show the concept of State
pattern, we use a simple command line program. If a GUI program is
used, a mediator pattern or a flyweight pattern may be applied on it.
Users connect to a database to do some jobs. Users from Management
department may focus on management. Users from Sales department may
focus on sales information. Every connection has to perform similar
functions like open, log and close. Suppose we have an abstract
Connection class and have these functions listed. Thus, every subclass
of Connection must implement these functions. We list three subclasses
Management, Sales and Accounting for example, just to show the State
pattern concept. The Controller class contains each state of
connection. Its behavior is decided by another object, which is a Test
class. All the details have been hidden from the Test class. Suppose we
have a server which is a singleton. Which connection is made depends on
the user. We use a Test class which makes a trigger from command line.
In the real program, the trigger should be made by the user.
The following is a skeleton program.
abstract class Connection {
public abstract void open();
public abstract void close();
public abstract void log();
}
class Accounting extends Connection {
public void open() {
System.out.println("open database for
accounting");
}
public void close() {
System.out.println("close the database");
}
public void log() {
System.out.println("log activities");
}
//...
}
class Sales extends Connection {
public void open() {
System.out.println("open database for
sales");
}
public void close() {
System.out.println("close the database");
}
public void log() {
System.out.println("log activities");
}
public void update() {
//
}
}
class Management extends Connection {
public void open() {
System.out.println("open database for
management");
}
public void close() {
System.out.println("close the database");
}
public void log() {
System.out.println("log activities");
}
//...
}
class Controller {
public static Accounting acct;
public static Sales sales;
public static Management manage;
private static Connection current;
Controller() {
acct = new Accounting();
sales = new Sales();
manage = new Management();
}
public void makeAccountingConnection() {
current = acct;
}
public void makeSalesConnection() {
current = sales;
}
public void makeManagementConnection() {
current = manage;
}
public void open() {
current.open();
}
public void close() {
current.close();
}
public void log() {
current.log();
}
}
class Test {
String con;
Controller controller;
Test(String con) {
controller = new Controller();
//the following trigger should be made by the
user
if(con.equalsIgnoreCase("management"))
controller.makeManagementConnection();
if(con.equalsIgnoreCase("sales"))
controller.makeSalesConnection();
if(con.equalsIgnoreCase("accounting"))
controller.makeAccountingConnection();
controller.open();
controller.log();
controller.close();
}
}
class Server {
public static Test test;
public static void main(String[] args) {
new Test(args[0]);
}
}
When we run the program with different connection, we will invoke the
program in the following printout:
%java Server management
open database for management
log activities
close the database
%java Server accounting
open database for accounting
log activities
close the database
%java Server sales
open database for sales
log activities
close the database
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