Java的State模式

 State模式：

    允许一个对象在其状态改变时，改变它的行为。看起来对象似乎修改了它的类。

    例子：

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    public interface State {
        public void handle(Context ctx);
    }

    public class Context {

        private State _state;

        public Context(State state) {
            _state = state;
        }

        public void request() {
            if (_state != null) {
                _state.handle(this);
            }
        }

        public void ChangeState(State s) {
            if (_state != null) {
                _state = null;
            }
            _state = s;
        }
    }

    public class ConcreteStateA implements State {

        public void handle(Context ctx) {
            System.out.println("handle by ConcreteStateA");
            if (ctx != null) {
                ctx.ChangeState(new ConcreteStateB());
            }
        }
    }

    public class ConcreteStateB implements State {

        public void handle(Context ctx) {
            System.out.println("handle by ConcreteStateB");
            if (ctx != null) {
                ctx.ChangeState(new ConcreteStateA());
            }
        }
    }

    public class StateClient {

        public static void main(String[] args) {

            State state = new ConcreteStateA();
            Context context = new Context(state);
            context.request();
            context.request();
            context.request();
            context.request();
        }
    }