Java DOM 解析器转换错误

Question

好的，这就是一个简单的决策树，广度和深度优先的搜索程序。在我的打印树方法中，我使用与搜索方法中相同的元素转换过程，当我单独运行打印树方法时，我没有收到任何错误，当我取消注释我的搜索方法时，我在 main 中遇到异常，并且不能在深度优先和广度优先方法中都会抛出异常，但树仍然会打印。在我在方法中获得根子级之后，该异常特别来自于强制转换行。我需要新的眼光来看待这个问题，无论如何，我对在 java 中使用 xml 是全新的，所以我抓狂了。

xml:(评论说我错误地假设所有子节点都是元素，实际上我假设所有节点都是节点，而不是元素，并且它们可以转换为元素，抱歉)

<root>
 <node behavior="Idle" response="">
      <node behavior="" response="Use Computer"/>
      <node behavior="" response="Patrol"/>
 </node>
 <node behavior="Incoming Projectile">
      <node behavior="" response="Evade"/>
 </node>
 <node behavior="Combat" response="">
      <node behavior="Melee" response="">
           <node behavior="" response="Flee"/>
           <node behavior="" response="Attack"/>
      </node>
      <node behavior="Ranged" response="">
           <node behavior="" response="Weapon 1"/>
           <node behavior="" response="Weapon 2"/>
           <node behavior="" response="Weapon 3"/>
      </node>
 </node>
</root>

这是当我使用搜索运行打印树时程序的输出，就像我说的，如果我不使用它运行搜索，我就不会得到第一个异常:

Specify BehaviorIdle
behavior= Idle
Exception in thread "main"      response= Use Computer
        response= Patrol
behavior= Incoming Projectile
        response= Evade
behavior= Combat
    behavior= Melee
        response= Flee
        response= Attack
    behavior= Ranged
        response= Weapon 1
        response= Weapon 2
        response= Weapon 3
java.lang.ClassCastException:com.sun.org.apache.xerces.internal.dom.DeferredTextImpl cannot be cast to org.w3c.dom.Element
at XmlTree.breadthFirst(XmlTree.java:58)
at decisiontree.main(decisiontree.java:34)

主要:

import java.util.*;

import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;

public class decisiontree {

public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    ArrayList <org.w3c.dom.Node> queue = new ArrayList<org.w3c.dom.Node>();
    int dCounter = 0;
    int bCounter = 0;

    System.out.print("Type in path for xml file: ");
    String path = scan.nextLine();

    System.out.print("Specify Behavior");
    String target = scan.nextLine();

    scan.close();


    XmlTree dTree = new XmlTree();
    Document doc =dTree.xmlIn(path);


    NodeList thisList = doc.getElementsByTagName("root");
    Node root = thisList.item(0);

    dTree.printTree(doc);

    String dResponse = dTree.depthFirst(root, target, dCounter);
    String bResponse = dTree.breadthFirst(root, queue, target, bCounter);

    System.out.println("Behavior: "+ target +"\n"+"Depth First Response: " + dResponse +
                         "\n" + "Breadth First Response: "+ bResponse + "\n" + "Depth First Took "+
                        dCounter + " jumps"+"\n"+"Breadth First Took "+ bCounter +" jumps");

    }

}

打印树:

public void printTree(Document doc){

    NodeList rootList = doc.getElementsByTagName("root");
    org.w3c.dom.Node root = rootList.item(0);
    Element rootElement = (Element) root;
    NodeList nodeList = rootElement.getElementsByTagName("node");

    for(int i = 0; i < nodeList.getLength(); i++){
        org.w3c.dom.Node nodes = nodeList.item(i);
        Element nodeElement = (Element) nodes;
        if(nodeElement.getAttribute("behavior") != ""){
            if(nodeElement.getParentNode() != rootElement){
                System.out.println("    behavior= " + nodeElement.getAttribute("behavior"));
            }else{
                System.out.println("behavior= " + nodeElement.getAttribute("behavior"));
            }
        }else if(nodeElement.getAttribute("behavior") == ""){
            if(nodeElement.getParentNode() != rootElement.getChildNodes()){
                System.out.println("        response= " + nodeElement.getAttribute("response"));
            }else{
                System.out.println("    response= " + nodeElement.getAttribute("response"));
            }
        }
    }
}

深度优先:

public String depthFirst(org.w3c.dom.Node root, String target, int dCounter){

    if (root.getChildNodes() == null){
        return "no tree";
    }
    dCounter++;
    NodeList nL = root.getChildNodes(); 
    for(int i = 0;i < nL.getLength();i++){
        org.w3c.dom.Node node = nL.item(i); 
        Element nodeElement = (Element) node;   //ERROR ON THIS LINE
        if(nodeElement.getAttribute("behavior") == target){
            while(nodeElement.hasChildNodes()){
                NodeList newNL = nodeElement.getChildNodes();
                Random rand = new Random();
                int x = rand.nextInt(newNL.getLength());
                node = newNL.item(x);
                Element newElement = (Element) node;
                nodeElement = newElement;
                dCounter++;
            }
        String response = nodeElement.getAttribute("response");
        return response;
        }else{
            depthFirst(node, target, dCounter);
        }
    }

    return null;
}

广度优先:

public String breadthFirst(Node root, ArrayList<org.w3c.dom.Node> q, String target, int bCounter){

    if(!root.hasChildNodes()){
        return "no tree";
    }
    bCounter++;
    NodeList nL = root.getChildNodes();
    for(int i = 0; i < nL.getLength(); i++){
        q.add(nL.item(i));
    }
    Node node = q.get(0);
    Element nodeElement = (Element) node;   // ERROR ON THIS LINE

    if(nodeElement.getAttribute("behavior") != target){
        q.remove(0);
        Node newNode = q.get(0);
        Element newElement = (Element) newNode;
        breadthFirst(newElement, q, target, bCounter);
    }
    if(nodeElement.getAttribute("behavior") == target){
        while(nodeElement.hasChildNodes()){
            NodeList newNL = node.getChildNodes();
            Random rand = new Random();
            int x = rand.nextInt(newNL.getLength());
            Element newElement = (Element) newNL.item(x);;
            nodeElement = newElement;
            bCounter++;
        }
    String response = nodeElement.getAttribute("response");
    return response;
    }
    return null;
}

Answer 1

查看 XML 的前两行:

<root>
 <node behavior="Idle" response="">

看起来 <root> 的第一个子节点是 <node>元素，但事实并非如此。

第一个 child 实际上是 Text对象，其字符内容为"\n " (或者可能是 "\r\n " ，具体取决于 XML 文档使用哪个平台的行结尾)，表示开头 <root> 之间的文本标签和第一个 <node> 的开头标签。第二个 child 是<node>元素。

最简单的解决方法是执行 if (nodes instanceof Element)在继续之前检查。如果您熟悉 XPath，我会使用它，因为代码会更干净。

警告:使用 == 比较字符串和!=最终会失败。在Java中，必须使用方法调用来比较String对象。您需要替换它:

nodeElement.getAttribute("behavior") == ""

这样:

nodeElement.getAttribute("behavior").isEmpty()

同样，您需要替换它:

nodeElement.getAttribute("behavior") != ""

这样:

!nodeElement.getAttribute("behavior").isEmpty()

一般来说，如果要将字符串与空字符串以外的任何值进行比较，则需要使用 equals 方法。例如:

nodeElement.getAttribute("behavior").equals(target)