GoF设计模式-状态模式

     状态模式(重要):
         对象行为型模式，系统中某个对象具有多个状态，这些状态之间可以相互转换，而且不同的状态具有不同的行为。将对象的状态从该对象中抽取出来作为状态类
         状态模式的结构：拥有状态的对象类，抽象状态类，具体状态类
     
     状态模式的适用场景：
         1.对象在不同状态下具有不同的行为
         2.代码中包含大量与对象状态有关的条件语句
     
     状态模式的优点：
         1.将状态之间的转换和不同状态下的行为封装在状态类中，提高了代码的可维护性
         2.增加新的状态比较方便，只需改变对象的状态即可改变对象的行为
     
     状态模式的缺点：
         1.增加了系统中类和对象的个数
         2.状态模式的结构和实现较为复杂
         3.增加新的状态类和修改状态的行为需要修改源代码，对开闭原则的支持不太好

状态模式的例子：

拥有状态的对象类

public class Account {
	
	private AccountState accountState;
	private String owner;
	private double balance;
	
	public Account(String owner, double balanceInit) {
		this.owner = owner;
		this.balance = balanceInit;
		this.accountState = new NormalState(this);
	}
	
	//存款
	public void deposit(double amount) {
		this.accountState.deposit(amount);
	}
	
	//取款
	public void withdraw(double amount) {
		this.accountState.withdraw(amount);
	}
	
	public void setState(AccountState accountState){
		this.accountState = accountState;
	}
	
	public AccountState getState(){
		return this.accountState;
	}
	
	public double getBalance() {
		return balance;
	}

	public void setBalance(double balance) {
		this.balance = balance;
	}
	
}

抽象状态类

public abstract class AccountState {
	protected Account account;
	public abstract void checkState();
	
	public void deposit(double amount) {
		this.account.setBalance(this.account.getBalance() + amount);
		checkState();
	}

	public void withdraw(double amount) {
		this.account.setBalance(this.account.getBalance() - amount);
		checkState();
	}
}

具体状态类

public class NormalState extends AccountState{
	
	public NormalState(Account account) {
		this.account = account;
	}
	
	public NormalState(AccountState accountState) {
		this.account = accountState.account;
	}

	@Override
	public void checkState() {
		double balance = this.account.getBalance();
		if (balance >= -2000 && balance <= 0) {
			this.account.setState(new OverdraftState(this));
		}else if(balance < -2000) {
			this.account.setState(new RestrictedState(this));
		}
	}
}

public class OverdraftState extends AccountState{

	public OverdraftState(AccountState accountState) {
		this.account = accountState.account;
	}

	@Override
	public void checkState() {
		double balance = this.account.getBalance();
		if (balance > 0) {
			this.account.setState(new NormalState(this));
		}else if (balance < -2000) {
			this.account.setState(new RestrictedState(this));
		}
	}

}

public class RestrictedState extends AccountState{

	public RestrictedState(AccountState accountState) {
		this.account = accountState.account;
	}

	@Override
	public void checkState() {
		double balance = this.account.getBalance();
		if (balance > 0) {
			this.account.setState(new NormalState(this));
		}else if (balance >= -2000 && balance <=0 ) {
			this.account.setState(new OverdraftState(this));
		}
	}

}

测试类

public class Main {
	public static void main(String[] args) {
		Account account = new Account("xfg", 0);
		account.deposit(5000);
		System.out.println(account.getState());
		
		account.withdraw(6000);
		System.out.println(account.getState());
		
		account.withdraw(2000);
		System.out.println(account.getState());
	}
}

     状态模式的应用：
         1.工作流和游戏软件中