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.

State Pattern Diagram

Implementation

Creating a Context:

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;
		System.out.println("The gumball machine was just refilled; its new count is: " + this.count);
		state.refill();
	}
 
	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();
	}
}

Creating a State interface:

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

Creating Concrete States:

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";
	}
}

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 void refill() { }
	
	public String toString() {
		return "waiting for turn of crank";	
	}
 
}

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";
	}
 
}

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";
	}
}

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!";
	}
}

Testing:

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);
	}
}