设计模式读书笔记-状态模式

状态模式和策略模式是双胞胎，在出生时才分开，策略模式是围绕可互换的算法来创建成功的业务，而状态模式是通过改变对象内部的状态来帮助对象控制行为。
状态模式允许在对象内部改变他的状态的时候改变行为，将行为委托不同的状态。
我们来看一个例子：糖果机例子，插入硬币，根据状态决定是否有糖果卖给用户。
public class GumballMachine {

State soldOutState;
State noQuarterState;
State hasQuarterState;
State soldState;

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);

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 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();
}
}
定义一个状态接口，不同状态去实现这个接口，以后增加一个状态就是增加一个实现状态的接口。
public interface State {

public void insertQuarter();
public void ejectQuarter();
public void turnCrank();
public void dispense();
}
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 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";
}
}

测试类：
public class GumballMachineTestDrive {[/b]
[/b]
public static void main(String[] args) {[/b]
GumballMachine gumballMachine = new GumballMachine(5);[/b]
System.out.println(gumballMachine);[/b]
gumballMachine.insertQuarter();[/b]
gumballMachine.turnCrank();[/b]
System.out.println(gumballMachine);[/b]
gumballMachine.insertQuarter();[/b]
gumballMachine.turnCrank();[/b]
gumballMachine.insertQuarter();[/b]
gumballMachine.turnCrank();[/b]
System.out.println(gumballMachine);[/b]
}[/b]
}[/b]
[/b]
状态模式的好处就是将状态的行为的控制委托给状态，可以灵活的添加。