创建对象时出现空指针异常

Question

我想知道有人可以帮我找出这个空指针异常.我正在制作一个程序来查找英语单词的所有字谜.我创建了一个类EnglishDictionary,当给出英文单词及其第一个字母的长度时,扫描包含所有英文单词的文本文件,并选择添加长度相同且包含ArrayList的第一个字母的文本文件.这是代码:

import java.io.FileReader;
import java.io.BufferedReader;
import java.io.IOException;
import java.util.ArrayList;

public class EnglishDictionary {

    public EnglishDictionary(String firstLetter, int wordLength) {
        ArrayList<String> words = new ArrayList<String>();
        processDict(firstLetter, wordLength);
    }

    public void processDict(String firstLetter, int wordLength) {
        try {
            FileReader fReader = new FileReader("englishWords.txt");
            BufferedReader reader = new BufferedReader(fReader);
            while (true) {
                String line = reader.readLine();
                if (line == null) {
                    break;
                } else {
                    if (line.length() == wordLength) {
                        words.add(line);
                    }
                }
            }
        } catch (IOException e) {
            System.out.println("File does not exist.");
        }
    }

    public ArrayList<String> getWords() {
        return words;
    }

    public void printWords() {
        for (String word : words) {
            System.out.println(word);
        }
    }

    private ArrayList<String> words;

}

如您所见,我还没有添加检查第一个字母是否在所选英语单词中的功能.当我从另一个类创建一个EnglishDictionary对象时,我得到一个空指针异常.它说:

Exception in thread "main" java.lang.NullPointerException
    at EnglishDictionary.processDict(EnglishDictionary.java:23)
    at EnglishDictionary.<init>(EnglishDictionary.java:10)
    at Main.main(Main.java:6)

我似乎无法弄清楚这是从哪里来的.其他人可以看到吗？提前致谢.

Answer 1

words没有正确初始化.您正在初始化隐藏CLASS变量字的LOCAL变量字.

在您的构造函数中执行此操作:

public EnglishDictionary(String firstLetter, int wordLength) {
  //ArrayList<String> words = new ArrayList<String>();  // Creates a local variable called 'words'; class member does NOT get initialized.
  words = new ArrayList<String>(); // THIS initializes the class variable.
  processDict(firstLetter, wordLength);

}