状态模式（State Pattern）中，对象的行为依赖于它的状态（属性），类的行为是随着类自身的状态改变而改变。常用于if...else分支语句，但状态不宜太多。

一、需求举例：

1.当工程师处于忙碌状态时，不接受新的项目开发需求；
2.当工程师处于不忙状态时，接受新的项目开发需求。

二、类图

三、代码实例

1.状态父类：State

package com.lance.state;

public abstract class State {

    public static final String BUSY = "busy";

    public static final String IDLE = "idle";


    protected String state;

    public String getState() {
        return state;
    }

    public void setState(String state) {
        this.state = state;
    }



    public void exe(EngineerContext context) {
        System.out.println("do default.");
    }
}

2.忙工程师类：BusyEngineer

package com.lance.state;

public class BusyEngineer extends State {

    public BusyEngineer(String state) {
        this.state = state;
    }


    @Override
    public void exe(EngineerContext context) {
        System.out.println("Engineer is busy refuse new project requirements.");
        context.setState(this);
    }
}

3.空闲工程师类：IdleEngineer

package com.lance.state;

public class IdleEngineer extends State {

    public IdleEngineer(String state) {
        this.state = state;
    }

    @Override
    public void exe(EngineerContext context) {
        System.out.println("Engineer is idle accept new project requirements.");
        context.setState(this);
    }
}

4.上下文类：EngineerContext

package com.lance.state;

public class EngineerContext {
    private State state;

    public State getState() {
        return state;
    }

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

5.主类：StatePattern

package com.lance.state;

public class StatePattern {

    public static void main(String[] args) {
        System.out.println("==========start==========");

        EngineerContext engineerContext = new EngineerContext();
        State state = new IdleEngineer(State.IDLE);
        state.exe(engineerContext);


        System.out.println("current state:" + engineerContext.getState().getState());

        System.out.println("=======================");


        state = new BusyEngineer(State.BUSY);
        state.exe(engineerContext);

        System.out.println("current state:" + engineerContext.getState().getState());

        System.out.println("==========end==========");

    }
}

输出结果：

==========start==========
Engineer is idle accept new project requirements.
current state:idle
=======================
Engineer is busy refuse new project requirements.
current state:busy
==========end==========