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我从文件中读取单词并将其放入双链表中我使用 .equals() 方法来检查单词是否在双链表中并且该方法工作正常但是当我使用 compareto() 方法对单词进行排序时它显示了这些异常：- 

 Exception in thread "main" java.lang.NullPointerException
    at DoubleLinkedList.insert(DoubleLinkedList.java:49)
    at FileSystemIndexer.indexFile(FileSystemIndexer.java:88)
    at FileSystemIndexer.IndexDirectory(FileSystemIndexer.java:18)
    at TestPhase1.main(TestPhase1.java:15)

方法 ：-

  import java.io.*;
import java.util.*;



public class FileSystemIndexer {

    private  DoubleLinkedList<Index> invertedIndex;

    public  FileSystemIndexer (){
        invertedIndex=new DoubleLinkedList<Index>();
    }

    public void IndexDirectory(File dir){

         File[] files =dir.listFiles();
         for(int i=0 ; i<files.length; i++){
             indexFile(files[i]);    //FileSystemIndexer.java:18
         }

    }

    public void indexFile(File file ){
        Scanner input = null;
        try{
            input = new Scanner(file);
        }
        catch(Exception e) {
            System.out.println(e);
        }

            String c;
            input.useDelimiter(",|\\.|:|\\s+");
        while(input.hasNext()) {
        c=input.next();
        if(c.equals("")){
            c=input.next();


        }else{
        //System.out.println("here: " + c);

         if(invertedIndex.empty()){

        invertedIndex.insert( new Index(c,file));
         }else{
             invertedIndex.FindFirst();
             while(!invertedIndex.last()){

                 if(invertedIndex.retrieve().getWord().compareTo(c)==-1){
                     System.out.println("here: " + c);
                     invertedIndex.FindPrevious();
                     invertedIndex.insert(new Index(c,file));
                 }else if(invertedIndex.retrieve().getWord().compareTo(c)==1){
                     System.out.println("here: " + c);
                     invertedIndex.findNext();  
            invertedIndex.insert(new Index(c,file));  //FileSystemIndexer.java:88
                 }
                     else if(invertedIndex.retrieve().getWord().compareTo(c)==0){ 
                         System.out.println("here: " + c);
                         invertedIndex.retrieve().getFiles().insert(file);
                     }

                 /*
                 if(invertedIndex.retrieve().getWord().equals(c)){
                     invertedIndex.retrieve().getFiles().insert(file);

                 }

                 invertedIndex.findNext();
             } 
             if(invertedIndex.retrieve().getWord().equals(c)){
                 invertedIndex.retrieve().getFiles().insert(file);

             }else {
                 Index i=new Index(c,file);
                 invertedIndex.insert(i);
             }

         }/**/


        }                       if(invertedIndex.retrieve().getWord().compareTo(c)==-1){
             invertedIndex.FindPrevious();
             invertedIndex.insert(new Index(c,file)); 
         }else if(invertedIndex.retrieve().getWord().compareTo(c)==1){
             invertedIndex.findNext();
             invertedIndex.insert(new Index(c,file));
         }
             else if(invertedIndex.retrieve().getWord().compareTo(c)==0){ 
                 invertedIndex.retrieve().getFiles().insert(file);
             }


                }
            }
        }
        input.close();

       //}
        //catch (Exception e){
            //  System.out.println(e);
            //}



        }

双链表中的插入方法：-

public void insert(T val ){
        Node<T> tmp= new Node<T> (val);  
        if(empty()){
            current=head=tmp;
        }else{
            tmp.next=current.next; //DoubleLinkedList.java:49
            tmp.previous=current;
            if(current.next !=null)
                current.next.previous=tmp;
            current.next=tmp;
            current=tmp;
        }
    }

我对双链表的实现： 

 public class DoubleLinkedList<T> {
    private Node<T> head;
    private Node<T> current ;

    public DoubleLinkedList(){
        head=current=null;
    }
    public boolean empty(){
        return head==null;

    }
    public boolean last(){
        return current.next==null;

    }
    public boolean first(){
        return current.previous==null;
    }

    public boolean full(){
         return false ;
    }
    public void FindFirst(){
        current=head;
    }
    public void findNext(){
        current=current.next;
    }
    public void FindPrevious(){
        current=current.previous;
    }
    public T retrieve(){
        return current.data;

    }



    public void update(T val ){
        current.data=val;
    }

    public void insert(T val ){
        Node<T> tmp= new Node<T> (val);  
        if(empty()){
            current=head=tmp;
        }else{
            tmp.next=current.next;
            tmp.previous=current;
            if(current.next !=null)
                current.next.previous=tmp;
            current.next=tmp;
            current=tmp;
        }/*
           if(empty()){
                current=head=tmp;
            }else{
                tmp.previous = current;
                tmp.next = current.next;
                if(current.next != null)
                    current.next.previous = tmp;
                current.next = tmp;
            }*/
        }


    public void remove(){
        if(current==head){
            head=head.next;
            if(head!=null)
                head.previous=null;
        }else{
            current.previous.next=current.next;
            if(current.next !=null)
                current.next.previous=current.previous;
        }

        if(current.next==null)
            current=head;
        else
                current=current.next ;
    }

    public void remove2(){

        if(current==head)
            head=head.next;
        else
            current.previous.next=current.next;

        if(current.next !=null)
            current.next.previous=current.previous;
        if(current.next==null)
            current=head;
        else
            current=current.next;

    }

}
4

1 回答 1

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如果您的插入意图是在列表末尾添加一个新节点，则插入代码必须更改如下...

    public void insert(T val ){
        Node<T> tmp= new Node<T> (val);  
        if(empty()){
            current=head=tmp;
        }else{
            tmp.previous=current;
            current.next=tmp;
            current=tmp;
        }
    }

但是如果你想在中间的某个地方插入 Val，那么你的代码就可以了，但你不能将你的 current 设置为 tmp。因此，必须从您的代码中删除最后一行。

    public void insert(T val ){
        Node<T> tmp= new Node<T> (val);  
        if(empty()){
            current=head=tmp;
        }else{
            tmp.previous = current;
            tmp.next = current.next;
            if(current.next != null)
                current.next.previous = tmp;
            current.next = tmp
        }
    }

如果它不起作用，那么您必须提供 DoubleLinkedList 的实现，否则将无法提供任何解决方案。

于 2012-11-28T17:36:06.433 回答