这个:

public void Node(String x) {
    data = x;
    next = null;
}

应该是:

   public Node(String x) {
        data = x;
        next = null;
    }

目前您有一个 default 构造函数(不带参数)，它是在没有任何显式构造函数的情况下隐式定义的.

Currently you have a default constructor (taking no arguments), which is implicitly defined in the absence of any explicit constructors.