2011-02-28 181 views
2

我一直在做一个项目,在这个项目中我用Backus-Naur Form文法表示文件并用它生成句子。下面是BNF文件我工作过的:神秘的空指针

<s>::=<np> <vp> 
<np>::=<dp> <adjp> <n>|<pn> 
<pn>::=John|Jane|Sally|Spot|Fred|Elmo 
<adjp>::=<adj>|<adj> <adjp> 
<adj>::=big|fat|green|wonderful|faulty|subliminal|pretentious 
<dp>::=the|a 
<n>::=dog|cat|man|university|father|mother|child|television 
<vp>::=<tv> <np>|<iv> 
<tv>::=hit|honored|kissed|helped 
<iv>::=died|collapsed|laughed|wept 

几乎一切工作正常,与随时随地的异常的字母“a”是通过规则集介绍。发生这种情况时,我收到以下错误:

Exception in thread "main" java.lang.NullPointerException at GrammarSolver.generate(GrammarSolver.java:95) at GrammarSolver.generate(GrammarSolver.java:109) at GrammarSolver.generate(GrammarSolver.java:116) at GrammarSolver.generate(GrammarSolver.java:116) at GrammarSolver.(GrammarSolver.java:51) at GrammarTest.main(GrammarTest.java:19)

我一直在努力追查并找到此错误的原因,但一直未能如愿。因此,我正在寻求一些可能有更多经验的人的建议,告诉我我的错误在哪里,以便我能够理解是什么造成了错误,并避免在将来重复类似的错误。

我的程序代码如下:

import java.util.*; 
import java.util.regex.*; 

class GrammarSolver { 

    //Create output variable for sentences 
    String output = ""; 

    //Create a map for storing grammar 
    SortedMap<String, String[]> rules = new TreeMap<String, String[]>(); 

    //Create a queue for managing sentences 
    Queue<String> queue = new LinkedList<String>(); 

    /** 
    * Constructor for GrammarSolver 
    * 
    * Accepts a List<String> then processes it splitting 
    * BNF notation into a TreeMap so that "A ::= B" is 
    * loaded into the tree so the key is A and the data 
    * contained is B 
    * 
    * @param  grammar  List of Strings with a set of 
    *       grammar rules in BNF form. 
    */ 
    public GrammarSolver(List<String> grammar){ 
     //Convert list to string 
     String s = grammar.toString(); 

     //Split and clean 
     String[] parts = s.split("::=|,"); 
     for(int i = 0; i < parts.length; i++){ 
      parts[i] = parts[i].trim(); 
      parts[i] = parts[i].replaceAll("\\[|]", ""); 
      //parts[i] = parts[i].replaceAll("[ \t]+", ""); 

     } 
     //Load into TreeMap 
     for(int i = 0; i < parts.length - 1; i+=2){ 
      String[] temp = parts[i+1].split("\\|"); 
      rules.put(parts[i], temp); 
     } 

     //Debug 
     String[] test = generate("<s>", 2); 
     System.out.println(test[0]); 
     System.out.println(test[1]); 
    } 

    /** 
    * Method to check if a certain non-terminal (such as <adj> 
    * is present in the map. 
    * 
    * Accepts a String and returns true if said non-terminal is 
    * in the map, and therefore a valid grammar. Returns false 
    * otherwise. 
    * 
    * @param  symbol  The string that will be checked 
    * @return  boolean  True if present, false if otherwise 
    */ 
    public boolean grammarContains(String symbol){ 
     if(rules.keySet().toString().contains(symbol)){ 
      return true; 
     }else{ 
      return false; 
     } 
    } 

    /** 
    * Method to generate sentences based on BNF notation and 
    * return them as strings. 
    * 
    * @param  symbol  The BNF symbol to be generated 
    * @param  times  The number of sentences to be generated 
    * @return  String  The generated sentence 
    */ 
    public String[] generate(String symbol, int times){ 
     //Array for output 
     String[] output = new String[times]; 

     for(int i = 0; i < times; i++){ 
      //Clear array for run 
      output[i] = ""; 

      //Grab rules, and store in an array 
      lString[] grammar = rules.get(symbol); 

      //Generate random number and assign to var 
      int rand = randomNumber(grammar.length); 

      //Take chosen grammar and split into array 
      String[] rules = grammar[rand].toString().split("\\s"); 

      //Determine if the rule is terminal or not 
      if(grammarContains(rules[0])){ 
       //System.out.println("grammar has more grammars"); 
       //Find if there is one or more conditions 
       if(rules.length == 1){ 
        String[] returnString = generate(rules[0], 1); 
        output[i] += returnString[0]; 
        output[i] += " "; 
       }else if(rules.length > 1){ 
        for(int j = 0; j < rules.length; j++){ 
         String[] returnString = generate(rules[j], 1); 
         output[i] += returnString[0]; 
         output[i] += " "; 
        } 
       } 
      }else{ 
       String[] returnArr = new String[1]; 
       returnArr[0] = grammar[rand];; 
       return returnArr; 
      } 
      output[i] = output[i].trim(); 
     } 
     return output; 
    } 

    /** 
    * Method to list all valid non-terminals for the current grammar 
    * 
    * @return  String  A listing of all valid non-terminals 
    *       contained in the current grammar that 
    *       can be used to generate words or 
    *       sentences. 
    */ 
    String getSymbols(){ 
     return rules.keySet().toString(); 
    } 

    public int randomNumber(int max){ 
     Random rand = new Random(); 
     int returnVal = rand.nextInt(max); 
     return returnVal; 
    } 
} 

和我的测试工具如下:

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

public class GrammarTest { 
    public static void main(String[] args) throws FileNotFoundException { 
     Scanner console = new Scanner(System.in); 
     System.out.println(); 

     // open grammar file 
     Scanner input = new Scanner(new File("sentence.txt")); 

     // read the grammar file and construct the grammar solver 
     List<String> grammar = new ArrayList<String>(); 
     while (input.hasNextLine()) { 
      String next = input.nextLine().trim(); 
      if (next.length() > 0) 
       grammar.add(next); 
     } 
     GrammarSolver solver = 
      new GrammarSolver(Collections.unmodifiableList(grammar)); 
    } 

} 

任何帮助或建议将不胜感激;

谢谢!

编辑:线95中,106和116关联到

94 //generate random number and assign to var 
95  int rand = randomNumber(grammar.length); 
... 
105//Find if there is one or more conditions 
106 if(rules.length == 1){ 
... 
115 for(int j = 0; j < rules.length; j++){ 
116 String[] returnString = generate(rules[j], 1); 
+1

啊,那难以捉摸的NullPointerException。 95线是什么? – Bozho 2011-02-28 21:39:33

+0

当我尝试运行它时,它工作正常,你能发布导致错误的句子.txt吗?当我得到NPE帮助的时候,我做的事情是在地方添加“assert ___!= null”,以帮助隔离具体到来的内容为空。 – 2011-02-28 21:44:23

+0

试图从错误消息中同步代码和行号,但它不起作用 - 不匹配。某些其他版本的文件产生了错误信息。请标记“第95行”。 (我猜这是这样的:'int rand = randomNumber(grammar.length);') – 2011-02-28 21:48:41

回答

2

作为第一步,我将确保

字符串[]语法= rules.get(符号);

不返回空值。这将消除像“grammar.length”和“grammar [rand] .toString()”这样的可疑表达式。下一步是仔细检查所有其他的解除引用为空。

+0

我同意@mazaneicha,然后继续进一步验证变量的空值为 – 2011-03-01 01:43:02

+0

非常感谢你们。当我意识到“a”正在通过递归过程时发现问题实际上源于grammarContains方法并使用报告误报的String.contains(),因为可以找到“a”所以我redid的方法,一切都很好! – Suki 2011-03-01 02:44:19

0

这并不直接回答你的问题,但我建议你使用一个IDE与一个集成的调试,如Eclipse

使用调试器可以让您在发生异常时了解变量的状态。这将允许您解决这样的问题,而无需等待我们尝试找出您的代码。

+0

感谢您的建议!有没有可以使用Eclipse调试器的指南或教程? – Suki 2011-02-28 22:30:18

+0

这[YouTube视频](http://www.youtube.com/watch?v=WeSitNPAExg)看起来不错。 – GavinH 2011-02-28 22:45:52

0

rules不包含您的终端(一),似乎。尝试rules.get("a")时失败,因为它返回null

我也推荐使用例如蚀进行调试 - 可以很容易地逐步执行堆栈帧时崩溃:-)