继承的空指针异常（Java）

Question

在我正在编写的程序中，我有以下代码来为单词创建正则表达式并从一系列不同的ArrayList中检索与之匹配的所有单词。

public ArrayList<String> solveWord(String str)
{
    String regex = "";
    String[] strArr = str.split("");
    for (String temp : strArr)
    {
        if (temp.equals(" "))
        {
            regex = regex + "[a-z]";
        }
        else
        {
            String s = "[" + temp + "]";
            regex = regex + s;
        }

    }
    int x = str.length();
    ArrayList<String> m = null;
    System.out.println("2ns");
     switch (x) 
     {
     case 1:  break;
     case 2:  m = let2;
              break;
     case 3:  m = let3;
            break;
     case 4:  m = let4;
            break;
     case 5:  m = let5;
            break;
     case 6:  m = let6;
            break;
     case 7:  m = let7;
            break;
     case 8:  m = let8;
            break;
     case 9:  m = let9;
            break;
     case 10:  m = let10;
            break;
     case 11:  m = let11;
            break;
     case 12:  m = let12;
            break;
     case 13:  m = let13;
            break;
     case 14:  m = let14;
            break;
     case 15:  m = let15;
            break;
     case 16:  m = let16;
            break;
     }   
    System.out.println(" ns");
    ArrayList<String> trans = retrieve(str,m,regex);
    return trans;
}

private ArrayList<String> retrieve(String word, ArrayList<String> arr, String reg)
{
    ArrayList<String> ex = arr;
    ArrayList<String> matches = new ArrayList<String>();
    for (String temp : ex)
    {
        if (temp.matches(reg))
        {
            matches.add(temp);
        }
    }
    return matches;
}

当我从另一个类调用求解（String str）时，行"strArr=str.split (""); 抛出一个空指针异常。我遇到的另一个问题是，当我自己测试这个类时，行if（temp.matches（reg））给出了以下错误。

Exception in thread "main" java.util.regex.PatternSyntaxException: Unclosed character     class near index 12
[][h][a-z][h]
            ^
at java.util.regex.Pattern.error(Unknown Source)
at java.util.regex.Pattern.clazz(Unknown Source)
at java.util.regex.Pattern.sequence(Unknown Source)
at java.util.regex.Pattern.expr(Unknown Source)
at java.util.regex.Pattern.compile(Unknown Source)
at java.util.regex.Pattern.<init>(Unknown Source)
at java.util.regex.Pattern.compile(Unknown Source)
at java.util.regex.Pattern.matches(Unknown Source)
at java.lang.String.matches(Unknown Source)
at Words.fillInTheBlanks.retrieve(fillInTheBlanks.java:238)
at Words.fillInTheBlanks.solveWord(fillInTheBlanks.java:228)
at Words.tester.main(tester.java:12)

有人能帮忙吗？

Answer 1

这里的拆分函数问题是；您需要使用一些字符串进行拆分，而该字符串不能只是“”。如果您想按空格拆分，请写“”。

Answer 2

正则表达式中不能有空方括号[]。在生成正则表达式之前，必须检查temp是否为空：

if (temp.equals(" "))
{
    regex = regex + "[a-z]";
}
else if (temp.length() > 0)
{
    String s = "[" + temp + "]";
    regex = regex + s;
}

对于NullPointerException，它发生在String[]strArr=str.split ("");行的唯一可能性是str为空。