小编sev*_*een的帖子

我写了一个这样的简单函数:

private static void write(String Swrite) throws IOException {
   if(!file.exists()) {
      file.createNewFile();
   }
   FileOutputStream fop=new FileOutputStream(file);
   if(Swrite!=null)
      fop.write(Swrite.getBytes());
   fop.flush();
   fop.close();
}

每次我调用它,它都会重写,然后我就会得到最后写的项目.如何将其更改为不重写？的file变量是全局定义为File.

我决定实现摘要List<Node>.这是它的一部分:

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

    public class myNodeList implements NodeList{


    Node root = null;
    int length = 0;

    public myNodeList() {}   
    public void addNode(Node node) {  
        if(root == null)   
        {
            root = node;  
               }
          else          
         root.appendChild(node);   
        length++;   
        System.out.println("this is the added node " +node);
    }      
     } 

但是当我尝试add一个节点时,它给了我以下异常:

Exception in thread "main" org.w3c.dom.DOMException: HIERARCHY_REQUEST_ERR: An attempt was made to insert a node where it is not permitted. 
at com.sun.org.apache.xerces.internal.dom.NodeImpl.insertBefore(NodeImpl.java:478)
at com.sun.org.apache.xerces.internal.dom.NodeImpl.appendChild(NodeImpl.java:235)
at pageparsertest.myNodeList.addNode(myNodeList.java:27)

这是因为Node root = …

每当我运行以下代码时它给了我,NullPointerException.虽然我检查是否不为null然后添加,但仍然给出了eception.the file是一个纯文本(.txt) 什么可能是错的？

BufferedReader  br2 = new BufferedReader(new FileReader(file));
ArrayList<String> keArrayList=null;
for(int i=0;br2.readLine()!=null;i++)
{
  String  letter= br2.readLine();
  if (letter!=null)
    keArrayList.add(i,letter);
}