Design pattern (12) - state model

State mode allows the object to change its behavior when the internal state changes, the object appears to modify its class.

Direct look like some obscure definitions, we look at examples, then it will come back to the definition.

We now simulate a candy machine operating state, its state changes as shown in FIG.

Find all state:

• No 25 cents
• 25 cents
• Candy sold out
• Sold candy

Now we need to use the code to achieve these states. We can create an instance variable to hold the current state, then define the value for each state:

final static int SOLD_OUT=0; 
final static int NO_QUARTER=1; 
final static int HAS_QUARTER=2; 
final static int SOLD=3; 

int state =SOLD_OUT;

But apparently it can be expected that this approach will bring a downside, when you need to extend the status, modify the code will become very difficult. For example, we need to add a function for the candy machine, "sold candy" state has a 10% chance will lead to fall two candy instead of one, which is obviously a need to increase the state, with the original implementation let us into a state of confusion can not get out.

To this end, we should try to localize the behavior of each state, to comply with "change package" principle, the behavior of each state are placed in each class, so long as each state implements its own action on it. We do is

• We can define a State Interface, in this interface, the operation of each candy machine has a corresponding method.
• Then for each state class implementing a state machine. These classes will be responsible for the behavior of the machine in the corresponding state.
• Finally, the delegate to the state class action.

State Interface

public interface State {
 
	public void insertQuarter();
	public void ejectQuarter();
	public void turnCrank();
	public void dispense();
}

Each state corresponds to a state class

import java.util.Random;

public class HasQuarterState implements State {
	Random randomWinner = new Random(System.currentTimeMillis());
	GumballMachine gumballMachine;
 
	public HasQuarterState(GumballMachine gumballMachine) {
		this.gumballMachine = gumballMachine;
	}
  
	public void insertQuarter() {
		System.out.println("You can't insert another quarter");
	}
 
	public void ejectQuarter() {
		System.out.println("Quarter returned");
		gumballMachine.setState(gumballMachine.getNoQuarterState());
	}
 
	public void turnCrank() {
		System.out.println("You turned...");
		int winner = randomWinner.nextInt(10);
		if ((winner == 0) && (gumballMachine.getCount() > 1)) {
			gumballMachine.setState(gumballMachine.getWinnerState());
		} else {
			gumballMachine.setState(gumballMachine.getSoldState());
		}
	}

    public void dispense() {
        System.out.println("No gumball dispensed");
    }
 
	public String toString() {
		return "waiting for turn of crank";
	}
}

public class NoQuarterState implements State {
    GumballMachine gumballMachine;
 
    public NoQuarterState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
 
	public void insertQuarter() {
		System.out.println("You inserted a quarter");
		gumballMachine.setState(gumballMachine.getHasQuarterState());
	}
 
	public void ejectQuarter() {
		System.out.println("You haven't inserted a quarter");
	}
 
	public void turnCrank() {
		System.out.println("You turned, but there's no quarter");
	 }
 
	public void dispense() {
		System.out.println("You need to pay first");
	} 
 
	public String toString() {
		return "waiting for quarter";
	}
}

public class SoldOutState implements State {
    GumballMachine gumballMachine;
 
    public SoldOutState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
 
	public void insertQuarter() {
		System.out.println("You can't insert a quarter, the machine is sold out");
	}
 
	public void ejectQuarter() {
		System.out.println("You can't eject, you haven't inserted a quarter yet");
	}
 
	public void turnCrank() {
		System.out.println("You turned, but there are no gumballs");
	}
 
	public void dispense() {
		System.out.println("No gumball dispensed");
	}
 
	public String toString() {
		return "sold out";
	}
}

public class SoldState implements State {
    GumballMachine gumballMachine;
 
    public SoldState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
       
	public void insertQuarter() {
		System.out.println("Please wait, we're already giving you a gumball");
	}
 
	public void ejectQuarter() {
		System.out.println("Sorry, you already turned the crank");
	}
 
	public void turnCrank() {
		System.out.println("Turning twice doesn't get you another gumball!");
	}
 
	public void dispense() {
		gumballMachine.releaseBall();
		if (gumballMachine.getCount() > 0) {
			gumballMachine.setState(gumballMachine.getNoQuarterState());
		} else {
			System.out.println("Oops, out of gumballs!");
			gumballMachine.setState(gumballMachine.getSoldOutState());
		}
	}
 
	public String toString() {
		return "dispensing a gumball";
	}
}

public class WinnerState implements State {
    GumballMachine gumballMachine;
 
    public WinnerState(GumballMachine gumballMachine) {
        this.gumballMachine = gumballMachine;
    }
 
	public void insertQuarter() {
		System.out.println("Please wait, we're already giving you a Gumball");
	}
 
	public void ejectQuarter() {
		System.out.println("Please wait, we're already giving you a Gumball");
	}
 
	public void turnCrank() {
		System.out.println("Turning again doesn't get you another gumball!");
	}
 
	public void dispense() {
		System.out.println("YOU'RE A WINNER! You get two gumballs for your quarter");
		gumballMachine.releaseBall();
		if (gumballMachine.getCount() == 0) {
			gumballMachine.setState(gumballMachine.getSoldOutState());
		} else {
			gumballMachine.releaseBall();
			if (gumballMachine.getCount() > 0) {
				gumballMachine.setState(gumballMachine.getNoQuarterState());
			} else {
            	System.out.println("Oops, out of gumballs!");
				gumballMachine.setState(gumballMachine.getSoldOutState());
			}
		}
	}
 
	public String toString() {
		return "despensing two gumballs for your quarter, because YOU'RE A WINNER!";
	}
}

Here is our candy native code

public class GumballMachine {
 
	State soldOutState;
	State noQuarterState;
	State hasQuarterState;
	State soldState;
	State winnerState;
 
	State state = soldOutState;
	int count = 0;
 
	public GumballMachine(int numberGumballs) {
		soldOutState = new SoldOutState(this);
		noQuarterState = new NoQuarterState(this);
		hasQuarterState = new HasQuarterState(this);
		soldState = new SoldState(this);
		winnerState = new WinnerState(this);

		this.count = numberGumballs;
 		if (numberGumballs > 0) {
			state = noQuarterState;
		} 
	}
 
	public void insertQuarter() {
		state.insertQuarter();
	}
 
	public void ejectQuarter() {
		state.ejectQuarter();
	}
 
	public void turnCrank() {
		state.turnCrank();
		state.dispense();
	}

	void setState(State state) {
		this.state = state;
	}
 
	void releaseBall() {
		System.out.println("A gumball comes rolling out the slot...");
		if (count != 0) {
			count = count - 1;
		}
	}
 
	int getCount() {
		return count;
	}
 
	void refill(int count) {
		this.count = count;
		state = noQuarterState;
	}

    public State getState() {
        return state;
    }

    public State getSoldOutState() {
        return soldOutState;
    }

    public State getNoQuarterState() {
        return noQuarterState;
    }

    public State getHasQuarterState() {
        return hasQuarterState;
    }

    public State getSoldState() {
        return soldState;
    }

    public State getWinnerState() {
        return winnerState;
    }
 
	public String toString() {
		StringBuffer result = new StringBuffer();
		result.append("\nMighty Gumball, Inc.");
		result.append("\nJava-enabled Standing Gumball Model #2004");
		result.append("\nInventory: " + count + " gumball");
		if (count != 1) {
			result.append("s");
		}
		result.append("\n");
		result.append("Machine is " + state + "\n");
		return result.toString();
	}
}

Test code as follows

public class GumballMachineTestDrive {

	public static void main(String[] args) {
		GumballMachine gumballMachine = 
			new GumballMachine(10);

		System.out.println(gumballMachine);

		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();
		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();

		System.out.println(gumballMachine);

		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();
		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();

		System.out.println(gumballMachine);

		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();
		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();

		System.out.println(gumballMachine);

		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();
		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();

		System.out.println(gumballMachine);

		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();
		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();

		System.out.println(gumballMachine);
	}
}

Test results

Now we go back to define the beginning of the article state mode, the first part of the description of the "state model allows an object to change its behavior change in the internal state" with quite a lot of meaning, because this model will be packaged into a separate class status, and the action delegate to represent the current state of the object behavior as the internal state is changed.

The second part of the definition of "objects appear to modify its class," What does it mean? From the customer's perspective: If you use an object that can completely change its behavior, then you will find that this object is actually from another class is instantiated come. However, in practice, we are using a combination of different states by simple reference to the object to create the illusion of class change.

The picture shows the state pattern class

State model and strategy mode

We think of the strategy pattern is a flexible alternative except inheritance. If you inherit defines the behavior of a class, you will be trapped by this behavior, even to modify it very difficult. With the strategy mode, you can change the behavior of different combinations of objects.

We would like to state mode alternative is to place a number of conditions no longer judged in context. By the behavior of the package into the state object, you can change the behavior of context by simply changing the state of the object in the context.