写在前头：希望你了解到的不仅是模式，而学会用OOP的眼光分析问题，解决问题

关键字：封装变化

描述：避免if与switch语句。分离抽象出状态，再通过组合建立联系

原则：往往需要修改的，就是需要封装的，需要明确职责的；就需要考虑单一职责和开放封闭原则。

示例：pull & push改变把手行为颜色

//状态接口
public interface State {
	//
	public void pull(PaintBoard paintBoard);
	public void push(PaintBoard paintBoard);
	public String name();
}

//红色状态
public class RedState implements State {

	@Override
	public void pull(PaintBoard paintBoard) {
		
		paintBoard.setState(new YellowState());
	}
	@Override
	public void push(PaintBoard paintBoard) {
		
		paintBoard.setState(new GreenState());
	}
	@Override
	public String name() {
		return "RED";
	}

}

//绿色状态
public class GreenState implements State {

	@Override
	public void pull(PaintBoard paintBoard) {
		
		paintBoard.setState(new RedState());
	}

	@Override
	public void push(PaintBoard paintBoard) {
		
		paintBoard.setState(new YellowState());
	}

	@Override
	public String name() {
		return "Green";
	}

}

//黄色状态
public class YellowState implements State {

	@Override
	public void pull(PaintBoard paintBoard) {
		
		paintBoard.setState(new GreenState());
	}

	@Override
	public void push(PaintBoard paintBoard) {
		
		paintBoard.setState(new RedState());
	}

	@Override
	public String name() {
		return "Yellow";
	}

}

public class PaintBoard {
	//默认为红色
	private State state;
	public PaintBoard() {
		this.state=new RedState();
	}
	//有参构造 可以指定初始颜色
	public PaintBoard(State state) {
		this.state=state;
	}
	//委托给状态对象处理，以后添加修改，可以不用修改此段代码
	public void push() {
		state.push(this);
	}
	//委托给状态对象处理，以后添加修改，可以不用修改此段代码
	public void pull() {
		state.pull(this);
	}
	public void setState(State state) {
		this.state=state;
	}
	//查看当前状态颜色
	public String getCurrentClolor() {
		return state.name();
	}
	
	public static void main(String[] args) {
		
		//无参构造
		PaintBoard paintBoard= new PaintBoard();
		System.out.println("Origin Color:"+paintBoard.getCurrentClolor());
		//pull测试
		for(int i=0; i<3; i++) {
			System.out.print(paintBoard.getCurrentClolor());
			paintBoard.pull();
			System.out.println("->pull->"+paintBoard.getCurrentClolor());
		}
		System.out.println();
		//push测试
		for(int i=0; i<3; i++) {
			System.out.print(paintBoard.getCurrentClolor());
			paintBoard.push();
			System.out.println("->pull->"+paintBoard.getCurrentClolor());
		}
	}
}