古格洛-R

删除无用符号和无用产生式——对某一文法产生式集中的无用符号和无用产生式进行删除，达到化简文法的目的。

import java.util.*;

public class RemoveUnuseful {
 public static void main(String[] args){
  Wenfa G=new Wenfa();
  G.CreatGenerate();
  System.out.println("原始文法如下：");
  G.output();
  System.out.println("删除无用符号和产生式之后结果：");
  SuanFas.Suanfa1(G);
  G.output();
 }
}

class SuanFas{
 
 
 public static void Suanfa1(Wenfa G1){//删除无用符号和无用产生式
  SuanFas.Suanfa11(G1);
  SuanFas.Suanfa12(G1);  
 } 
 private static void Suanfa11(Wenfa G1){//删除无用符号和无用产生式Part1
  String N="";
  boolean isAdd=false;
  for(int i=0;i<G1.pNum;i++){
   if(!Belong(G1.p[i].left,N)&&(BelongS(G1.p[i].right,G1.T)||G1.p[i].right==null)){
    N=N+G1.p[i].left;
   }
  }
  
  do{
   isAdd=false;
   for(int i=0;i<G1.pNum;i++){
    if(!Belong(G1.p[i].left,N)&&(BelongS2(G1.p[i].right,G1.T,N))){
     N=N+G1.p[i].left;
     isAdd=true;
    }
   }
  }while(isAdd);
  
  G1.N=N;
  int renum=0;
  for(int i=0;i<G1.pNum;i++){
   if(Belong(G1.p[i].left,N)&&BelongS2(G1.p[i].right,N,G1.T)){
    continue;
   }else{
    G1.p[i]=null;
    renum++;
   }
   
  }
  generate[] p=new generate[G1.pNum-renum];
  int j=0;
  for(int i=0;i<G1.pNum;i++){
   if(G1.p[i]!=null) {
    p[j]=G1.p[i];
    j++;
   }   
  }

  G1.p=p;
  G1.pNum=j;
 } 
 private static void Suanfa12(Wenfa G1){//删除无用符号和无用产生式Part2
  String N="";
  String T="";
  N=N+G1.start;
  boolean isAdd=false;
 
  do{
   isAdd=false;
   for(int i=0;i<G1.pNum;i++){
       if(Belong(G1.p[i].left,N)){
        for(int j=0;j<G1.p[i].right.length();j++){
         if(!Belong(G1.p[i].right.charAt(j),T)&&Belong(G1.p[i].right.charAt(j),G1.T)){
          T=T+G1.p[i].right.charAt(j);
          isAdd=true;
         }else if(!Belong(G1.p[i].right.charAt(j),N)&&Belong(G1.p[i].right.charAt(j),G1.N)){
          N=N+G1.p[i].right.charAt(j);
          isAdd=true;
         }
        }
       }
      }
  }while(isAdd);
  G1.N=N;
  G1.T=T;
  
  int renum=0;
  for(int i=0;i<G1.pNum;i++){
   if(!Belong(G1.p[i].left,N)) G1.p[i]=null;
   else if(!BelongS2(G1.p[i].right,N,T)){
    G1.p[i]=null;
    renum++;
   }
  }
  generate[] p=new generate[G1.pNum-renum];
  int j=0;
  for(int i=0;i<G1.pNum;i++){
   if(G1.p[i]!=null){
    p[j]=G1.p[i];
    j++;
   }
  }

  G1.p=p;
  G1.pNum=j;
 }
 
 private static boolean Belong(char a,String b){  
  for(int i=0;i<b.length();i++){
   if(a==b.charAt(i))return true;
  }
  return false;
 }
 private static boolean BelongS(String a,String b){
  int len=0;
  for(int i=0;i<a.length();i++){
   for(int j=0;j<b.length();j++){
    if(a.charAt(i)==b.charAt(j)){
     len++;
     break;
    }     
   }   
  }
  if(a.length()==len) return true;
  else return false;
 }
 private static boolean BelongS2(String a,String b,String c){
  for(int i=0;i<b.length();i++){
   for(int j=0;j<c.length();j++){
    if(b.charAt(i)==c.charAt(j)){
     System.out.println("Error!");
     return false;
    }     
   }   
  }
  int len=0;
  for(int i=0;i<a.length();i++){
   for(int j=0;j<b.length();j++){
    if(a.charAt(i)==b.charAt(j)){
     len++;
     break;
    }     
   }   
  }
  for(int i=0;i<a.length();i++){
   for(int j=0;j<c.length();j++){
    if(a.charAt(i)==c.charAt(j)){
     len++;
     break;
    }     
   }   
  }
  if(a.length()==len) return true;
  else return false;
 }
}

class Wenfa{
 public Wenfa(){
  Scanner in=new Scanner(System.in);
  System.out.print("请输入非终结符号集合(连续输入不需分隔)：");
  N=in.next();
  System.out.print("请输入终结符号集合(连续输入不需分隔)：");
  T=in.next();
  System.out.print("请输入起始符号：");
  start=in.next().charAt(0);
  System.out.print("请输入文法规则P的数量：");
  pNum=in.nextInt(); 
  System.out.println();
 }
 
 public void CreatGenerate(){
  Scanner in=new Scanner(System.in);
  p=new generate[pNum];
  for(int i=0;i<pNum;i++){
   p[i]=new generate();
  }
  for(int i=0;i<pNum;i++){
   System.out.print("请输入第"+(i+1)+"个规则的左部：");
   p[i].left=in.nextLine().charAt(0);
   System.out.print("请输入第"+(i+1)+"个规则的右部：");
   p[i].right=in.nextLine();
   System.out.println();
  }
  in.close();
 }
 
 public void output(){
  System.out.println("G=({"+N+"},{"+T+"},P,"+start+")");
  System.out.print("P: ");
  for(int i=0;i<pNum;i++){
   if(p[i]!=null) System.out.print(p[i].left+"->"+p[i].right+"   ");
  } 
  System.out.print("\n");
 }
 
 char start;
 String N;
 String T;
 int pNum;
 generate[] p;
}

class generate{
 public char left;
 public String right;
}

正在上编译原理这门课，一些编程实验的代码也就放上来吧。代码只反映大概思想，一些比如容错性等细节没时间做具体处理。程序我是这样写了但不敢说没有差错，仅供参考。

词法分析——对输入的一段C-程序代码，进行单词分析，构造Token序列，删除无用字符如空格，注释等。

import java.io.*;

public class WordAnalysis{
 public static void main(String[] args) throws IOException{ 
  FileReader read=new FileReader("code.txt");
  BufferedReader reader=new BufferedReader(read);
  String line=reader.readLine();//这行的字符串
  int lineNum=1;                //行号
  int curStart=0;               //词的开始位置
  int now=0;                    //当前的序号
  char current;                 //当前的字符
  String str="";
  Token curToken=new Token(0,"","",null);
  Token head=curToken;
  
  while(line!=null){//逐行分析 
   for(now=0;now<line.length();now++){//当前行分析   
    current=line.charAt(now);
    
    //判断数字
    if(isNum(current)){
     while(isNum(current)){
      now++;
      if(now>=line.length()) break;
      if(!isNum(line.charAt(now))&&line.charAt(now)!=' '
       &&!isSpcial(line.charAt(now))){
       System.out.println("Error: 非标准数字类型单词！(Line:"+lineNum+")");
       return;
      }
      current=line.charAt(now);      
     }
     str=line.substring(curStart,now);
     curToken.nextToken=new Token(lineNum,str,"NUM",null);
     curToken=curToken.nextToken;
    }
    //判断字符
    if(isChar(current)){
     while(isChar(current)){
      now++;
      if(now>=line.length()) break;
      if(!isChar(line.charAt(now))&&line.charAt(now)!=' '
       &&!isSpcial(line.charAt(now))){
       System.out.println("Error: 非标准字符类型单词！");
       return;
      }
      current=line.charAt(now);      
     }
     str=line.substring(curStart,now);
     if(str.equals("else")){      
      curToken.nextToken=new Token(lineNum,str,"ELSE",null);
      curToken=curToken.nextToken;
     }
     else if(str.equals("if")){      
      curToken.nextToken=new Token(lineNum,str,"IF",null);
      curToken=curToken.nextToken;
     }
     else if(str.equals("int")){      
      curToken.nextToken=new Token(lineNum,str,"INT",null);
      curToken=curToken.nextToken;
     }
     else if(str.equals("return")){      
      curToken.nextToken=new Token(lineNum,str,"RETURN",null);
      curToken=curToken.nextToken;
     }
     else if(str.equals("while")){      
      curToken.nextToken=new Token(lineNum,str,"WHILE",null);
      curToken=curToken.nextToken;
     }
     else if(str.equals("void")){      
      curToken.nextToken=new Token(lineNum,str,"VOID",null);
      curToken=curToken.nextToken;
     }
     else{      
      curToken.nextToken=new Token(lineNum,str,"ID",null);
      curToken=curToken.nextToken;
     }      
     curStart=now+1;
    }    
    //判断标点
    if(current==';'){
     str=";";
     curToken.nextToken=new Token(lineNum,str,"PUNCTUATION",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    else if(current==','){
     str=",";
     curToken.nextToken=new Token(lineNum,str,"PUNCTUATION",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    //判断空格
    else if(current==' '){
     curStart=now+1;
    }
    //判断+-号
    else if(current=='+'){
     str="+";
     curToken.nextToken=new Token(lineNum,str,"OPERATOR",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    else if(current=='-'){
     str="-";
     curToken.nextToken=new Token(lineNum,str,"OPERATOR",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    //判断({[号
    else if(current=='('){
     str="(";
     curToken.nextToken=new Token(lineNum,str,"LPAREN",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    else if(current=='['){
     str="[";
     curToken.nextToken=new Token(lineNum,str,"LPAREN",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    else if(current=='{'){
     str="{";
     curToken.nextToken=new Token(lineNum,str,"LPAREN",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    else if(current==')'){
     str=")";
     curToken.nextToken=new Token(lineNum,str,"RPAREN",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    else if(current==']'){
     str="]";
     curToken.nextToken=new Token(lineNum,str,"RPAREN",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    else if(current=='}'){
     str="}";
     curToken.nextToken=new Token(lineNum,str,"RPAREN",null);
     curToken=curToken.nextToken;
     curStart=now+1;
    }
    //判断<>=号
    else if(current=='='){
     if(line.charAt(now+1)=='='&&now<line.length()){
      now++;
      str="==";
      curToken.nextToken=new Token(lineNum,str,"COMPARE",null);
      curToken=curToken.nextToken;
      curStart=now+1;
     }else{
      str="=";
      curToken.nextToken=new Token(lineNum,str,"ASSIGNMENT",null);
      curToken=curToken.nextToken;
      curStart=now+1;
     }
    }
    else if(current=='>'){
     if(line.charAt(now+1)=='='&&now<line.length()){
      now++;
      str=">=";
      curToken.nextToken=new Token(lineNum,str,"COMPARE",null);
      curToken=curToken.nextToken;
      curStart=now+1;
     }else{
      str=">";
      curToken.nextToken=new Token(lineNum,str,"COMPARE",null);
      curToken=curToken.nextToken;
      curStart=now+1;
     }
    }
    else if(current=='<'){
     if(line.charAt(now+1)=='='&&now<line.length()){
      now++;
      str="<=";
      curToken.nextToken=new Token(lineNum,str,"COMPARE",null);
      curToken=curToken.nextToken;
      curStart=now+1;
     }else{
      str="<";
      curToken.nextToken=new Token(lineNum,str,"COMPARE",null);
      curToken=curToken.nextToken;
      curStart=now+1;
     }
    }
    else if(current=='!'){
     if(line.charAt(now+1)=='='&&now<line.length()){
      now++;
      str="!=";
      curToken.nextToken=new Token(lineNum,str,"COMPARE",null);
      curToken=curToken.nextToken;
      curStart=now+1;
     }else{
      System.out.println("Error:!");
      break;
     }
    }
    //判断*/号
    else if(current=='*'){
     if(line.charAt(now+1)=='/'&&now<line.length()){
      System.out.println("Error:*/");
      break;
     }else{
      str="*";
      curToken.nextToken=new Token(lineNum,str,"OPERATOR",null);
      curToken=curToken.nextToken;
      curStart=now+1;
     }
    }
    else if(current=='/'){
     cango:
     if(line.charAt(now+1)=='*'&&now<line.length()){
      now=now+2;
      while(line!=null){
       while(now<line.length()){
        if(line.charAt(now)=='*'&&line.charAt(now+1)=='/'){
         now=now+1;
         break cango;
        }else{
         now++;
        }
       }
       line=reader.readLine();
       lineNum++;
       now=0;
      }
      System.out.println("Error:/*");
      break;
     }else{
      str="/";
                     curToken.nextToken=new Token(lineNum,str,"OPERATOR",null);
         curToken=curToken.nextToken;
      curStart=now+1;      
     }   
    }
    
   }//当前行分析结束      
   line=reader.readLine();
   lineNum++; 
   curStart=0;
  }
  //结束处理
  curToken.nextToken=new Token(-1,"end","END",null);
  curToken=curToken.nextToken;
  Token all=head.nextToken;
  while(all!=null){  
   all.outprint();
   all=all.nextToken;
            System.out.println();
   
  }
  read.close();
  reader.close();
 }

    public static boolean isNum(char x){
     if(x>='0'&&x<='9') return true;
     else return false;
    }
    public static boolean isChar(char x){
     if(x>='A'&&x<='Z'||x>='a'&&x<='z') return true;
     else return false;
    }
    public static boolean isSpcial(char x){
     char[] spe={'+','-','*','/','<','>','=','!',';',',','(',')','[',']','{','}'};
     for(int i=0;i<spe.length;i++){
  if(x==spe[i]) return true;
     }
     return false;
    }
}

class Token {
 public Token(int lineNum,String word,String types,Token nextToken){ 
  this.lineNum=lineNum;
  this.word=word;
  this.types=types;
  this.nextToken=nextToken;
 }
 
 public void outprint(){ 
  System.out.print("{"+lineNum+","+word+","+types+"} ");
 }
 
 public Token nextToken; 
 private int lineNum;
 private String word;
 private String types;
}

NullPointerException，为什么报这种错误，这种报错肯定是java代码的错。

[ 所谓空指针异常，是因为用空（null）去调用属性或方法。
null表示没有这个对象，既然没有这个对象，那么去调用他的属性和方法，就会报异常。
<--主要有以下几种原因：
1、使用了未初始化的变量（虽然已经声明）
2、使用了未初始化的对象（虽然已经声明）
3、使用了关键字或已存在的类名作变量对象方法或类名。

当应用程序试图在需要对象的地方使用 null 时，抛出该异常。
<--这种情况包括：
调用 null对象的实例方法。
访问或修改null对象的字段。
将null作为一个数组，获得其长度。
将null作为一个数组，访问或修改其时间片。
将null作为Throwable值抛出。]