双重链接的 Lisl 不断收到空指针错误

Question

我查看了之前的所有示例，看不到我做错了什么。出于某种原因，我真的很挣扎于空指针异常，我只是无法绕过它们。

public class DLBDictionary implements DictionaryInterface {
//Store Strings in an Node
public DLBNode firstNode;   
public class DLBNode 
{
    public char value;
    public DLBNode nextValue;
    public DLBNode nextLetter;
    public DLBNode(){
        this.value = '/';
        this.nextValue = null;
        this.nextLetter = null;
    }
    public DLBNode(char value){
        this.value = value;
        this.nextValue = null;
        this.nextLetter = null;         
    }   
}
public DLBDictionary() {
    DLBNode firstNode = new DLBNode('/');   
}
// Add new String to end of list.  If String should come before
// previous last string (i.e. it is out of order) sort the list.
// We are keeping the data sorted in this implementation of
// DictionaryInterface to make searches a bit faster.
public boolean add(String s) {
    int charIndex = 0;              
    while(charIndex<=s.length())
    {
        char currentChar = s.charAt(charIndex);         
        boolean added = false;
        while(!added)
        {
            if(firstNode.value == '/')
            {
                firstNode.value = currentChar;
                added = true;
            }
            else if(firstNode.value == currentChar)
            {
                if(firstNode.nextLetter == null)
                {
                    DLBNode newNode = new DLBNode(); 
                    firstNode.nextLetter = newNode;
                    firstNode = firstNode.nextLetter;
                }
                else
                {
                    firstNode = firstNode.nextLetter;
                }
                added = true;
            }
            else
            {
                firstNode = firstNode.nextValue;
            }
        }           
        charIndex++;
    }
    DLBNode tempNode = new DLBNode('^'); 
    firstNode.nextLetter = tempNode;
    return true;
}

我放弃了其余代码，但最后一个 if 语句是我得到异常的地方。 这对我来说毫无意义！ 我没有在构造函数中将 firstNode 的值初始化为"/"吗？所以firstNode.getValue应该返回'/'，而不是空指针异常。

Answer 1

你应该这样做this.firstNode = new DLBNode（）;在 DLBDictionary 的构造函数中。您实际上是在创建一个新对象，而不是初始化您的第一个节点。希望对您有所帮助。

Answer 2

You reset firstNode with several statements in the loop:
 firstNode = firstNode.nextValue;

因此，第一节点 == null 会导致 NPE。char 值与它无关，无论如何它都会初始化为值0x00的字符。