Java设计模式(十二)：状态设计模式

谁践踏了优雅 2022-03-30 15:30 215阅读 0赞

# 1.应用场景 #

在软件开发过程中，应用程序可能会根据不同的情况作出不同的处理。最直接的解决方案是将这些所有可能发生的情况全都考虑到。然后使用if… ellse语句来做状态判断来进行不同情况的处理。但是对复杂状态的判断就显得“力不从心了”。随着增加新的状态或者修改一个状体（if else(或switch case)语句的增多或者修改）可能会引起很大的修改，而程序的可读性，扩展性也会变得很弱。维护也会很麻烦。那么我就考虑只修改自身状态的模式。

例子1：按钮来控制一个电梯的状态，一个电梯开们，关门，停，运行。每一种状态改变，都有可能要根据其他状态来更新处理。例如，开门状体，你不能在运行的时候开门，而是在电梯定下后才能开门。

例子2：我们给一部手机打电话，就可能出现这几种情况：用户开机，用户关机，用户欠费停机，用户消户等。 所以当我们拨打这个号码的时候：系统就要判断，该用户是否在开机且不忙状态，又或者是关机，欠费等状态。但不管是那种状态我们都应给出对应的处理操作。

# 2. 概念 #

> 允许对象在内部状态改变时改变它的行为，对象看起来好像修改了它所属的类。

这个模式将状态封装成了独立的类，并且将动作委托到代表当前状态的对象上，我们知道行为会随着内部状态的改变而改变。

# 3.Class Diagram #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIxMTI1MTgz_size_16_color_FFFFFF_t_70]

Context（上下文）是一个类，它可以拥有一些内部状态。

State接口定义了一个所有具体状态的共同接口，任何状态都实现这个相同的接口，这样一来，状态之间可以互相替换。

# 4. Implementation #

糖果销售机有多种状态，每种状态下销售机有不同的行为，状态可以发生转移，使得销售机的行为也发生改变。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzIxMTI1MTgz_size_16_color_FFFFFF_t_70 1]

public interface State { 
        public void insertQuarter();
        public void ejectQuarter();
        public void turnCrank();
        public void dispense();
    
        public void refill();
    }
    
    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 NoQuarterStete implements State { 
        private GumballMachine gumballMachine;
    
        public NoQuarterStete(GumballMachine gumballMachine) { 
            this.gumballMachine = gumballMachine;
        }
    
        @Override
        public void insertQuarter() { 
            System.out.println("You inserted a quarter");
            gumballMachine.setState(gumballMachine.hasQuarterState);
        }
    
        @Override
        public void ejectQuarter() { 
            System.out.println("You haven't inserted a quarter.");
        }
    
        @Override
        public void turnCrank() { 
            System.out.println("You turned, but there's no quarter. ");
        }
    
        @Override
        public void dispense() { 
            System.out.println("you need to pay first");
        }
    
        @Override
        public void refill() { 
        }
    }
    
    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 { 
    	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";
    	}
    }
    
    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();
    	}
    }
    
    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);
    	}
    }

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