The State Pattern

The State Pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

The State Pattern allows an object to have many different behaviors that are based on its internal state. Unlike a procedural state machine, the State Pattern represents its states as individual classes, each inheriting from a common interface or abstract class. This will typically result in a greater number of classes in your design. Beside this disadvantage, using explicit classes for each state the code becomes easier to understand, maintain and it is more flexible.

The following class diagram shows an example, where the concrete state classes implement an abstract state interface.

The State Pattern.

The Context gets its behavior by delegating applied actions to the current state object it is composed with. By encapsulating each state into a class, we localize any changes that will need to be made. This way we follow the principle: Encapsulate what varies (the state). The principle: Open for extension but closed for modification is also coverd by the State Pattern. Each state is closed for modification, and yet the Context is open for extension by adding new state classes.

The State and Strategy Patterns have the same class diagram, but they differ in intent. The Strategy Pattern typically configures Context classes with a behavior or algorithm, which can be done through composition during runtime. State Pattern allows a Context to change its behavior as the state of the Context changes.

State transitions can be controlled by the State classes or by the Context classes. It is also possible for State classes to be shared among Context instances.

If we require common methods, that are shared across states, we use an abstract state class. Otherwise it is possible to use an interface. Using an abstract class has the benefit of allowing you to add methods to the abstract class later, without breaking the concrete state implementations.

State Pattern Example

The following example shows a Gumball machine with the following states that will be represented by individual classes:

  • NoQuarterState - The start state where the user hasn’t inserted a quarter.
  • HasQuarterState - After inserting a quarter, we transition to this state.
  • SoldState - This state is reached ff a the user inserted a quarter and truns the crank.
  • SoldOutState - If all gumballs are sold or the machine hasn’t been filled, the machine transitions to this state.

First, we’re going to define a State interface that contains a method for every action in the Gumball Machine.

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

Now we are going to implement a State class for every state of the machine. These classes will be responsible for the behavior of the machine (Context) when it is in the corresponding state. This way we delegate the work to the individual state classes.

The first state that will implement the State interface is NoQuarterState:

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 void refill() { }
 
	public String toString() {
		return "waiting for quarter";
	}
}

Note that not all actions are apropriate for each state. For example, turnCrank() or ejectQuarter() make no sense in the NoQuarterState. Therefore the user will be only informed to insert a quarter and no transition takes place.

Like all states will, this state has a reference to the GumballMachine, the Context, which is used to get and set new states. For example, to transition from this state NoQuarterState to HasQuarterState when the user inserts a coin, the insertQuarter() method uses the getter and setter of the GumballMachine class that is defined next:

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

		this.count = numberGumballs;
 		if (numberGumballs > 0) {
			state = noQuarterState;
		} else {
			state = soldOutState;
		}
	}
 
	public void insertQuarter() {
		state.insertQuarter();
	}
 
	public void ejectQuarter() {
		state.ejectQuarter();
	}
 
	public void turnCrank() {
		state.turnCrank();
		state.dispense();
	}
 
	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;
		System.out.println("The gumball machine was just refilled; it's new count is: " + this.count);
		state.refill();
	}

	void setState(State state) {
		this.state = state;
	}
    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 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();
	}
}

The GumballMachine class instantiates all concrete states and provides all possible action methods to the user. The action methods of this Context class delegate the work to the currently set state, which is stored in the state member. Note that dispense() requires no action method because it is an internal action of the Gumball machine. A user can’t ask the machine to dispense directly. Instead, dispense() is called on the state object inside the turnCrank() action method.

The rest of the states are implemented next. Each state has a reference to the GumballMachine to transition it to a different state. The following is the HasQuarterState:

public class HasQuarterState implements State {
	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...");
		gumballMachine.setState(gumballMachine.getSoldState());
	}

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

The SoldState looks like this:

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 void refill() { }
 
	public String toString() {
		return "dispensing a gumball";
	}
}

Most of the actions are inapropriate in this state except for the internal dispense() action. In this method the machine releases a gumball and transitions to NoQuarterState or SoldOutState depending on the gumball count of the machine.

The final state that needs to be implemented is SoldOutState:

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 void refill() { 
		gumballMachine.setState(gumballMachine.getNoQuarterState());
	}
 
	public String toString() {
		return "sold out";
	}
}

In this state the machine changes its behavior only if it gets refilled.

How to use the GumballMachine is shown in the following test program:

public class GumballMachineTestDrive {

	public static void main(String[] args) {
		GumballMachine gumballMachine = new GumballMachine(2);

		System.out.println(gumballMachine);

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

		System.out.println(gumballMachine);

		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();
		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();
		
		gumballMachine.refill(5);
		gumballMachine.insertQuarter();
		gumballMachine.turnCrank();

		System.out.println(gumballMachine);
	}
}

The output of this test run is:

$ java GumballMachineTestDrive

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 2 gumballs
Machine is waiting for quarter

You inserted a quarter
You turned...
A gumball comes rolling out the slot...

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 1 gumball
Machine is waiting for quarter

You inserted a quarter
You turned...
A gumball comes rolling out the slot...
Oops, out of gumballs!
You can't insert a quarter, the machine is sold out
You turned, but there are no gumballs
No gumball dispensed
The gumball machine was just refilled; it's new count is: 5
You inserted a quarter
You turned...
A gumball comes rolling out the slot...

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 4 gumballs
Machine is waiting for quarter

State Extension

It is easy to extend the Gumball machine with a winner state that will release two gumballs 1 out of 10 times when the crank is turned.

To do this the GumballMachine class needs a new state member winner and add the getter method of this new state:

public State getWinnerState() {
        return winnerState;
}

The Winner state is similar to the SoldState and looks like this:

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() {
		gumballMachine.releaseBall();
		if (gumballMachine.getCount() == 0) {
			gumballMachine.setState(gumballMachine.getSoldOutState());
		} else {
			gumballMachine.releaseBall();
			System.out.println("YOU'RE A WINNER! You got two gumballs for your quarter");
			if (gumballMachine.getCount() > 0) {
				gumballMachine.setState(gumballMachine.getNoQuarterState());
			} else {
            	System.out.println("Oops, out of gumballs!");
				gumballMachine.setState(gumballMachine.getSoldOutState());
			}
		}
	}
 
	public void refill() { }
	
	public String toString() {
		return "despensing two gumballs for your quarter, because YOU'RE A WINNER!";
	}
}

To test the new Winner state we use the following test program:

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);
	}
}

This produce an output that is similar to the following, depending on your luck :)

$ java GumballMachineTestDrive

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 10 gumballs
Machine is waiting for quarter

You inserted a quarter
You turned...
A gumball comes rolling out the slot...
You inserted a quarter
You turned...
A gumball comes rolling out the slot...

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 8 gumballs
Machine is waiting for quarter

You inserted a quarter
You turned...
A gumball comes rolling out the slot...
You inserted a quarter
You turned...
A gumball comes rolling out the slot...

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 6 gumballs
Machine is waiting for quarter

You inserted a quarter
You turned...
A gumball comes rolling out the slot...
You inserted a quarter
You turned...
A gumball comes rolling out the slot...
A gumball comes rolling out the slot...
YOU'RE A WINNER! You got two gumballs for your quarter

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 3 gumballs
Machine is waiting for quarter

You inserted a quarter
You turned...
A gumball comes rolling out the slot...
You inserted a quarter
You turned...
A gumball comes rolling out the slot...

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 1 gumball
Machine is waiting for quarter

You inserted a quarter
You turned...
A gumball comes rolling out the slot...
Oops, out of gumballs!
You can't insert a quarter, the machine is sold out
You turned, but there are no gumballs
No gumball dispensed

Mighty Gumball, Inc.
Java-enabled Standing Gumball Model #2004
Inventory: 0 gumballs
Machine is sold out

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