如何更好地构造可观察对象链(如果first返回null，则退出)

// 定义抽象处理者类
abstract class Handler {
    protected Handler nextHandler;

    public void setNextHandler(Handler nextHandler) {
        this.nextHandler = nextHandler;
    }

    public abstract void handleRequest(String request);
}

// 具体处理者类
class ConcreteHandler1 extends Handler {
    public void handleRequest(String request) {
        if (request.equals("first")) {
            System.out.println("处理请求：" + request);
        } else if (nextHandler != null) {
            nextHandler.handleRequest(request);
        }
    }
}

class ConcreteHandler2 extends Handler {
    public void handleRequest(String request) {
        if (request.equals("second")) {
            System.out.println("处理请求：" + request);
        } else if (nextHandler != null) {
            nextHandler.handleRequest(request);
        }
    }
}

class ConcreteHandler3 extends Handler {
    public void handleRequest(String request) {
        if (request.equals("third")) {
            System.out.println("处理请求：" + request);
        } else if (nextHandler != null) {
            nextHandler.handleRequest(request);
        }
    }
}

// 客户端代码
public class Main {
    public static void main(String[] args) {
        // 创建处理者对象
        Handler handler1 = new ConcreteHandler1();
        Handler handler2 = new ConcreteHandler2();
        Handler handler3 = new ConcreteHandler3();

        // 构造可观察对象链
        handler1.setNextHandler(handler2);
        handler2.setNextHandler(handler3);

        // 发送请求
        handler1.handleRequest("first");
        handler1.handleRequest("second");
        handler1.handleRequest("third");
        handler1.handleRequest("fourth");
    }
}