class OutContent {
    int m = 0;

    public void output(NodeList nodeList) {
        int size = nodeList.getLength();
        for (int k = 0; k < size; k++) {
            Node node = nodeList.item(k);
            if (node.getNodeType() == Node.TEXT_NODE) {
                                //父类型Node强转成子类型Text，怎么不报错？
                Text textNode = (Text) node;
                String content = textNode.getWholeText();
                m++;
                System.out.print(content);
            }
            if (node.getNodeType() == Node.ELEMENT_NODE) {
                                //这里也是样的问题
                Element elementNode = (Element) node;
                String name = elementNode.getNodeName();
                System.out.print(name);
                NodeList nodes = elementNode.getChildNodes();
                output(nodes);
            }
        }
    }

package com.test;

class A {

    void aMthod() {

        System.out.println("A method");

    }

}

class B extends A {

    void bMethod1() {

        System.out.println("B method 1");

    }

    void bMethod2() {

        System.out.println("B method 2");

    }

}

public class C {

    public static void main(String[] args) {
        A a2 = new A();

        B b2 = (B) a2; // 向下转型，编译无错误，运行时将出错

        b2.aMthod();

        b2.bMethod1();

        b2.bMethod2();

    }

}