1.定义#

当一个对象内在状态改变时允许其改变行为，这个对象看起来像改变了其类。每个状态一个实现类，状态的切换实则为类之间的切换。不同的状态类负责本状态的逻辑和状态转换。委托统一的上下文类来控制整个过程。

2.类图#

类图

3.实现#

3.1抽象状态类##

public abstract class State {
    protected  Context context;

    public void setContext(Context _context) {
        this.context = _context;
    }

    public abstract void handler1();

    public abstract void handler2();

}

3.2具体状态类##

public class ConcreteState1 extends State {

    @Override
    public void handler1() {
        System.out.println("处于状态1");
    }

    @Override
    public void handler2() {
        System.out.println("转移向状态2");
        super.context.setState(Context.concreteState2);
        super.context.handler2();
    }
}

public class ConcreteState2 extends State {
    @Override
    public void handler2() {
        System.out.println("处于状态2");
    }

    @Override
    public void handler1() {
        System.out.println("转移向状态1");
        super.context.setState(Context.concreteState1);
        super.context.handler1();
    }
}

3.3上下文类##

public class Context {
    private State state;
    public static final ConcreteState1 concreteState1 = new ConcreteState1();
    public static final ConcreteState2 concreteState2 = new ConcreteState2();

    public void setState(State state) {
        this.state = state;
        this.state.setContext(this);
    }

    public void handler1() {
        this.state.handler1();
    }

    public void handler2() {
        this.state.handler2();
    }

}

3.4客户端类##

public class Client {
    public static void main(String args[]) {
        Context context = new Context();
        context.setState(Context.concreteState1);
        context.handler2();
        context.handler1();
    }
}