状态模式

状态模式，当一个对象的内在状态改变时允许改变其行为，这个对象看起来像是改变了其类。

状态模式可以将复杂的判断逻辑简化，将庞大的判断逻辑分隔开，对单个判断的修改不会影响到其他判断。并且在客户端调用时，也只需要执行Context向外暴露的类即可，不需要知道到底有多少判断逻辑。

State类

interface State {
    void writeName(StateContext context, String name);
}

class LowerCaseState implements State {
    @Override
    public void writeName(StateContext context, String name) {
        System.out.println(name.toLowerCase());
        context.setState(new MultipleUpperCaseState());
    }
}

class MultipleUpperCaseState implements State {
    /* Counter local to this state */
    private int count = 0;

    @Override
    public void writeName(StateContext context, String name) {
        System.out.println(name.toUpperCase());
        /* Change state after StateMultipleUpperCase's writeName() gets invoked twice */
        if (++count > 1) {
            context.setState(new LowerCaseState());
        }
    }
}

context类

class StateContext {
    private State state;
    
    public StateContext() {
        state = new LowerCaseState();
    }

    /**
     * Set the current state.
     * Normally only called by classes implementing the State interface.
     * @param newState the new state of this context
     */
    void setState(State newState) {
        state = newState;
    }

    public void writeName(String name) {
        state.writeName(this, name);
    }
}

客户端代码

public class StateDemo {
    public static void main(String[] args) {
        StateContext context = new StateContext();

        context.writeName("Monday");
        context.writeName("Tuesday");
        context.writeName("Wednesday");
        context.writeName("Thursday");
        context.writeName("Friday");
        context.writeName("Saturday");
        context.writeName("Sunday");
    }
}

最后展示内容

 monday
 TUESDAY
 WEDNESDAY
 thursday
 FRIDAY
 SATURDAY
 sunday