设计模式：状态模式（State Pattern）

/**
 * 状态模式。
 * @author Bright Lee
 */
public class StatePattern {

	public static void main(String[] args) {
		Context context = new Context();
		
		context.request();
		context.request();
	}

}

class Context {
	
	private State stateA;
	private State stateB;
	private State currentState;
	
	public Context() {
		this.stateA = new ConcreteStateA(this);
		this.stateB = new ConcreteStateB(this);
		this.currentState = stateA;
	}
	
	public void request() {
		currentState.handle();
	}
	
	public State getStateA() {
		return stateA;
	}
	
	public State getStateB() {
		return stateB;
	}
	
	void setCurrentState(State currentState) {
		this.currentState = currentState;
	}
	
}

interface State {
	
	void handle();
	
}

class ConcreteStateA implements State {
	
	private Context context;
	
	public ConcreteStateA(Context context) {
		this.context = context;
	}

	public void handle() {
		System.out.println("现在是状态A，马上转换到状态B。");
		context.setCurrentState(context.getStateB());
	}
	
}

class ConcreteStateB implements State {
	
	private Context context;
	
	public ConcreteStateB(Context context) {
		this.context = context;
	}

	public void handle() {
		System.out.println("现在是状态B，马上转换到状态A。");
		context.setCurrentState(context.getStateA());
	}
	
}

运行结果：
现在是状态A，马上转换到状态B。
现在是状态B，马上转换到状态A。