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我一直在做一個項目，在這個項目中我用Backus-Naur Form文法表示文件並用它生成句子。下面是BNF文件我工作過的：神祕的空指針
<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
幾乎一切工作正常，與隨時隨地的異常的字母「a」是通過規則集介紹。發生這種情況時，我收到以下錯誤：
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)
我一直在努力追查並找到此錯誤的原因，但一直未能如願。因此，我正在尋求一些可能有更多經驗的人的建議，告訴我我的錯誤在哪裏，以便我能夠理解是什麼造成了錯誤，並避免在將來重複類似的錯誤。
我的程序代碼如下：
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;
}
}
和我的測試工具如下：
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));
}
}
任何幫助或建議將不勝感激;
謝謝！
編輯：線95中，106和116關聯到
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);
啊，那難以捉摸的NullPointerException。 95線是什麼？ – Bozho 2011-02-28 21:39:33
當我嘗試運行它時，它工作正常，你能發佈導致錯誤的句子.txt嗎？當我得到NPE幫助的時候，我做的事情是在地方添加「assert ___！= null」，以幫助隔離具體到來的內容爲空。 – 2011-02-28 21:44:23
試圖從錯誤消息中同步代碼和行號，但它不起作用 - 不匹配。某些其他版本的文件產生了錯誤信息。請標記「第95行」。 （我猜這是這樣的：'int rand = randomNumber（grammar.length）;'） – 2011-02-28 21:48:41