状态模式

Intent

• Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
• An object-oriented state machine
• Wrapper + polymorphic wrappee + collaboration

Structure

state by keith

2. 代码:

public class State {
    public static void main(String[] args) {
        State state = new State();
        state.test();
    }

    private void test() {
        Context context = new Context();
        context.request();
        context.request();
    }

    class Context {
        BaseState state;

        public Context() {
            this.state = new StartState(this);
        }
        public void setState(BaseState state){
            this.state=state;
        }
        public void request() {
            state.handle();
        }
    }
    // base state
    interface BaseState {
        void handle();
    }
    // change the state internally
    class StartState implements BaseState {

        Context context;

        public StartState(Context context) {
            this.context = context;
        }

        @Override
        public void handle() {
            System.out.println("start it.");
            context.setState(new StopState());
        }
    }
    class StopState implements BaseState {

        @Override
            public void handle() {
            System.out.println("stop it.");
        }
    }
}

3. Output

start it.
stop it.

Notice:

• 状态模式中，状态本身会包含Context的引用，从而实现状态迁移 ，但策略模式则没有Context的引用;
• 策略模式是由客户端进行处理的，而状态的改变Context或者State对象都可以进行。

Refenrence

1. Design Patterns
2. 设计模式
3. 状态模式和策略模式的相似与不同