【发布时间】：2011-02-28 21:37:44
【问题描述】：
我一直在从事一个项目，在该项目中，我使用 Backus–Naur 形式语法符号获取一个文件并用它生成句子。这是我正在处理的 BNF 文件：
<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
除了通过规则集引入字母“a”的任何时候，几乎一切都运行良好。发生这种情况时，我收到以下错误：
线程“main”中的异常 java.lang.NullPointerException 在 GrammarSolver.generate(GrammarSolver.java:95) 在 GrammarSolver.generate(GrammarSolver.java:109) 在 GrammarSolver.generate(GrammarSolver.java:116) 在 GrammarSolver.generate(GrammarSolver.java:116) 在 GrammarSolver.(GrammarSolver.java:51) 在 GrammarTest.main(GrammarTest.java:19)
我一直在尝试追踪和定位此错误的原因，但一直未能如愿。因此，我正在寻求可能有更多经验的人的建议，以向我展示我的错误在哪里，以便我了解导致它的原因，并避免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;
}
}
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));
}
}
任何帮助或提示将不胜感激；
谢谢！
编辑：第 95、106 和 116 行与
相关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);
【问题讨论】：
-
啊，那个难以捉摸的 NullPointerException。第 95 行是什么？
-
当我尝试运行它时，它运行良好，你能发布一个导致错误的 sentence.txt 吗？当我得到 NPE 有帮助时，我会做的事情是在一些地方添加“assert ___！= null”，以帮助隔离具体的内容为 null。
-
尝试从错误消息中同步代码和行号，但它不起作用 - 不匹配。该文件的某些其他版本产生了错误消息。请标记“第 95 行”。 （我猜是这个：
int rand = randomNumber(grammar.length);） -
我添加了一个编辑来匹配行号和行。非常感谢到目前为止的帮助！
-
语法对我来说是空的，但只有当 sentence.txt 文件不是格式正确的语法时才会发生这种情况。