State Pattern

Definition-Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

//UML Class Diagram

When to use?

• Suppose an object is always in one of several

known states

• The state an object is in determines the behavior

of several methods

• Could use if/case statements in each method

Compared to Strategy Pattern

Strategy Pattern typically configures context classes with a behavior or algorithm. But State Pattern allows a context to change its behavior as the state of the context changes.

Watch out: Using the State Pattern will typically result in a greater number of classes in your designed.

Notes

• Can use singletons for instances of each state class

– State objects don’t encapsulate state, so can be shared

• Easy to add new states

– New states can extend other states

• Override only selected functions

Example:

//Class Diagram

//Code

//Main.java

class  TcpConnection
... {
    TcpState tcpCurrentState;
    TcpEstablishedState establishedState;
    TcpListenState tcpListenState;
    TcpClosedState tcpClosedState;
    public TcpConnection()
    ...{
        establishedState=new TcpEstablishedState(this);
        tcpListenState=new TcpListenState(this);
        tcpClosedState=new TcpClosedState(this);
        tcpCurrentState=tcpClosedState;
    }
    public void Open()
    ...{
        tcpCurrentState.Open();
    }
    public void Close()
    ...{
        tcpCurrentState.Close();
    }
    public void Acknowledge()
    ...{
        tcpCurrentState.Acknowledge();
    }
    public void SetState(TcpState tcpState)
    ...{
        tcpCurrentState = tcpState;
        
    }
    public TcpState GetEstablishState()
    ...{
        return this.establishedState;
    }
    public TcpState GetListenState()
    ...{
        return this.tcpListenState;
    }
    public TcpState GetClosedState()
    ...{
        return this.tcpClosedState;
    }
}
interface  TcpState  ... {
    void Open();
    void Close();
    void Acknowledge();
}
class  TcpEstablishedState  implements  TcpState
... {
    TcpConnection tcpConn;
    public TcpEstablishedState(TcpConnection tcpConn)
    ...{
        this.tcpConn = tcpConn;
    }
    public void Open()
    ...{
        System.out.println("the connection has been established!");
    }
    public void Close()
    ...{
        System.out.println("Closing...");
        tcpConn.SetState(tcpConn.GetClosedState());
    }
    public void Acknowledge()
    ...{
        System.out.println("acknowledge...and then listening ...");
        tcpConn.SetState(tcpConn.GetListenState());
    }
}
class  TcpListenState  implements  TcpState
... {
    TcpConnection tcpConn;
    public TcpListenState(TcpConnection tcpConn)
    ...{
        this.tcpConn = tcpConn;
    }
    public void Open()
    ...{
        System.out.println("the connection has been established!");
    }
    public void Close()
    ...{
        System.out.println("Closing...");
        tcpConn.SetState(tcpConn.GetClosedState());
    }
    public void Acknowledge()
    ...{
        System.out.println("acknowledge...continued listen...");
    }
}
class  TcpClosedState  implements  TcpState
... {
    TcpConnection tcpConn;
    public TcpClosedState(TcpConnection tcpConn)
    ...{
        this.tcpConn = tcpConn;
    }
    public void Open()
    ...{
        System.out.println("the connection is establishing...");
        tcpConn.SetState(tcpConn.GetEstablishState());
    }
    public void Close()
    ...{
        System.out.println("the connection has been Closed.Don't close again.");
    }
    public void Acknowledge()
    ...{
        System.out.println("the connection has been Closed so that it could not acknowledge any msg");
    }
}

 

public   class  Main  ... {

    public static void main(String[] args) ...{
        // TODO Auto-generated method stub
        TcpConnection tcpConn=new TcpConnection();
        
        //right
        tcpConn.Open();
        tcpConn.Acknowledge();
        tcpConn.Acknowledge();
        tcpConn.Acknowledge();
        tcpConn.Close();
        
        //error
        tcpConn.Close();
        
        //right
        tcpConn.Open();
        tcpConn.Close();
        
        
    }

}

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