Java 变量不应为空时出错'；T

我犯了一个错误

Exception in thread "main" java.lang.NullPointerException
at Proj5.main(Proj5.java:61)

当在第61行（

char z=placeHolder.charAt（counter）

；）中运行程序时，我无法找出是什么原因导致了

null

String answers = new String("112211324135412") ;
    int k = 0 ;                                     
    int[] correct = new int[15] ;                   
    String placeHolder = ("112211324135412") ;
    for ( int i = 1 ; i < tokens.length ; i+=2)     
    {
        int counter = 0 ;                           


        int amountCorrect = 0 ;                     
        placeHolder = tokens[i] ;

    while (counter < 15)                            

    {   



        char z = placeHolder.charAt(counter) ; 
        char c = answers.charAt(counter) ;
        if( z == c) 
        {
            amountCorrect++ ;
        } // end if
        counter++ ;                                 

    }   // end while    

    correct[k] = amountCorrect ;                    
    k++ ;


    }   

String answers=新字符串（“112211324135412”）；
int k=0；
int[]正确=新的int[15]；
字符串占位符=（“112211324135412”）；
for（int i=1；i

这是完整的代码

/** * * * * */

import java.util.*；
导入java.io.*；
公共类项目5
{
公共静态void main（字符串[]args）引发IOException
{
扫描仪s=新的扫描仪（System.in）；
/*这篇文章询问带有ID和答案的文件名，如果不是
输入正确表示无效，并让用户重试。*/
字符串文件检查；
File=null；
做{
System.out.print（“输入文件名：”）；
fileCheck=s.nextLine（）；
文件=新文件（文件检查）；
如果（！file.exists（））
System.out.println（“错误，无效文件”）；
}而（！file.exists（））；
/*这一部分打开与文件的连接，拆分每一行，然后放置
将碎片放入数组（令牌）*/
String[]令牌=新字符串[100]；
令牌=信息（文件检查）；
/*此部分包含答案字符串，并将答案的每个字符与
来自令牌数组的奇数编号元素（来自文件的答案），并跟踪
编号正确，然后放入数组。*/
字符串答案=新字符串（“112211324135412”）；
int k=0；//用于帮助存储到正确的数组中。
int[]correct=new int[15]；//存储量correct可用于与WID进行比较
字符串占位符=（“112211324135412”）；
for（inti=1；iimport java.util.* ;
import java.io.* ;

public class Proj5 
{

    public static void main(String[] args) throws IOException
    {

    Scanner s = new Scanner(System.in) ;


/* This piece asks for the name of the file with the ids and answers and if it is not
entered correctly it says invalid and makes the user try again. */  

    String fileCheck ;
    File file = null ;
    do{
    System.out.print("Enter the name of the File: ") ;
    fileCheck = s.nextLine() ;
    file = new File(fileCheck) ;
    if (!file.exists())
        System.out.println("ERROR, INVALID FILE") ;
        }while (!file.exists()) ;


/* This piece opens connections with the file, splits up each line, and then puts the 
pieces into an array (tokens). */

    String[] tokens = new String[100] ; 
    tokens= information(fileCheck) ;

/* This piece contains the answers string and compares each character of answers to the 
odd numbered elements(answers from file) from the tokens array and keeps track of the 
number correct which is then placed into an array. */   

    String answers = new String("112211324135412") ;
    int k = 0 ;                                     // used to help store into correct array.
    int[] correct = new int[15] ;                   // stores amount correct can be used to compare to WID
    String placeHolder = ("112211324135412") ;
    for ( int i = 1 ; i < tokens.length ; i+=2)     // adds two so it skips WID
    {
        int counter = 0 ;                           // used to pull characters 
                                                    //in the strings

        int amountCorrect = 0 ;                     // keeps track of amount correct
        placeHolder = tokens[i] ;

    while (counter < 15)                            // goes through all characters in 
                                                    //the string of answers
    {   

                    //puts answers into a string so 
                                                    //individual answers can be compared
        char z = placeHolder.charAt(counter) ; 
        char c = answers.charAt(counter) ;
        if( z == c) 
        {
            amountCorrect++ ;
        } // end if
        counter++ ;                                 // moves to next char

    }   // end while    

    correct[k] = amountCorrect ;                    // stores amount correct in array for later use.
    k++ ;


    }   // end for



/* This piece takes "correct" array and loops it through to determine what percentage and 
what letter grade each elements would get and plugs each of those into their own array 
(percentage and letterGrade). */    


    int halfToken = (tokens.length/2) ;                 // only need half the amount in tokens array since we dont need to include WID.
    double[] percentage = new double[halfToken] ;
    int a = 0 ;                                     //used to step through the 3 arrays
    char[] letterGrade = new char[halfToken] ;

    while(a < halfToken)
    {   
    if(correct[a] == 15)
    {
        percentage[a] = 100.0 ;
        letterGrade[a] = 'A' ;
        a++ ;
    }   
    else if(correct[a] == 14)
    {
        percentage[a] = 93.3 ;
        letterGrade[a] = 'A' ;
        a++ ;
    }
    else if(correct[a] == 13)
    {
        percentage[a] = 86.7 ;
        letterGrade[a] = 'B' ;
        a++ ;
    }
    else if(correct[a] == 12)
    {
        percentage[a] = 80.0 ;
        letterGrade[a] = 'B' ;
        a++ ;
    }
    else if(correct[a] == 11)
    {
        percentage[a] = 73.3 ;
        letterGrade[a] = 'C' ;
        a++ ;
    }
    else if(correct[a] == 10)
    {
        percentage[a] = 66.7 ;
        letterGrade[a] = 'D' ;
        a++ ;
    }
    else if(correct[a] == 9)
    {
        percentage[a] = 60.0 ;
        letterGrade[a] = 'D' ;
        a++ ;
    }
    else if(correct[a] == 8)
    {
        percentage[a] = 53.3 ;
        letterGrade[a] = 'F' ;
        a++ ;
    }
    else if(correct[a] == 7)
    {
        percentage[a] = 46.7 ;
        letterGrade[a] = 'F' ;
        a++ ;
    }
    else if(correct[a] == 6)
    {
        percentage[a] = 40.0 ;
        letterGrade[a] = 'F' ;
        a++ ;
    }
    else if(correct[a] == 5)
    {
        percentage[a] = 33.3 ;
        letterGrade[a] = 'F' ;
        a++ ;
    }
    else if(correct[a] == 4)
    {
        percentage[a] = 26.7 ;
        letterGrade[a] = 'F' ;
        a++ ;
    }
    else if(correct[a] == 3)
    {
        percentage[a] = 20.0 ;
        letterGrade[a] = 'F' ;
        a++ ;
    }
    else if(correct[a] == 2)
    {
        percentage[a] = 13.3 ;
        letterGrade[a] = 'F' ;
        a++ ;
    }
    else if(correct[a] == 1)
    {
        percentage[a] = 6.7 ;
        letterGrade[a] = 'F' ;
        a++ ;
    }
    else if(correct[a] == 0)
    {
        percentage[a] = 0.0 ;
        letterGrade[a] = 'F' ;
        a++ ;
    }

    } // end while

/* This piece prints the header for id/#correct/%correct/grade and then cycles in a loop 
prints out each students info. */   


    int f = 0 ;
    int g = 0 ;

    System.out.println("Student ID \t" + "# Correct \t" + "% Correct \t" + "Grade") ;
    while(f < tokens.length)
    {
        System.out.println(tokens[f] + "\t" + correct[g] + "\t" + percentage[g] + "\t" + letterGrade[g]) ;
        f+=2 ;
        g++ ;
    } // end while

/* This piece finds adds up the percentage array and then divides by its length which is 
the class avg grade. It then uses this percentage to determine the letter grade. It 
lastly prints out the total average and letter grade. */    

    double totalAVG = 0.0 ;
    char totalGrade = 'A' ;

    while(f<percentage.length) // calculates average percent
    {
        totalAVG = (totalAVG+percentage[f]) ;
    } // end while

        totalAVG = totalAVG/percentage.length ;


    if(totalAVG >= 90.0)
    {
        totalGrade = 'A' ;
    }
    else if (totalAVG >= 80.0 && totalAVG <=89.9)
    {
        totalGrade = 'B' ;
    }
    else if (totalAVG >= 70.0 && totalAVG <=79.9)
    {
        totalGrade = 'C' ;
    }
    else if (totalAVG >= 60.0 && totalAVG <=69.9)
    {
        totalGrade = 'D' ;
        }       
        else 
            totalGrade = 'F' ;

        System.out.println() ;
        System.out.println("Average: " + totalAVG + "% (" + totalGrade + ")" );

    /* This next piece cycles through the correct array to find the max and then takes that 
    times 2 to get high score, then it prints it out. */    


        int highScore = 0 ;

        for(int h = 0; h<correct.length ; h++) 
        {
            if(correct[h] >= highScore)
            {
                highScore = correct[h] ;
            }


        } // end for

        System.out.println("High Score: " + highScore*2) ;

    /* This piece cycles through the correct array to find the min and then takes that times 
    2 to get the low Score, then it prints it out. */       

        int lowScore = 0 ;

        for(int h = 0; h<correct.length ; h++) 
        {
            if(correct[h] <= lowScore)
            {
                lowScore = correct[h] ;
            }       
        } // end for

        System.out.println("Low Score: " + lowScore*2) ;




    PrintWriter pw = new PrintWriter(new FileWriter("Result.txt")) ;

        for (int t=0 ; t< tokens.length ; t+=2 )
        {   
            g = 0 ;
            pw.println(tokens[t] + "," + correct[g] + percentage[g] + letterGrade[g]) ;
            g++ ;
        }
            pw.println("Average: " + totalAVG + "% (" + totalGrade + ")") ;
            pw.println("High Score: " + highScore*2) ;  
            pw.println("Low Score: " + lowScore*2) ;

        pw.close() ;    


        } // end main

    /*
    *
    * Opens connection to file with ids and answers and returns them split up. 
    * @param (String a) pulls in the filename for use in method
    * @return Returns an array containing the split-up file. */




        public static String[] information(String a) throws IOException
        {   
            Scanner inFile = new Scanner (new File(a)) ; // opens connection with file
            String[] quarters = new String[100] ;
            int index = 0 ;
            while (inFile.hasNext())                        // loops while more lines in file
            {

                String line = inFile.nextLine() ;           // brings in next line to be broken up
                String[] array = line.split(",") ;
                quarters[index] = array[0]  ;           //stores lines into array tokens
                index++ ;
                quarters[index] = array[1] ;
                index++ ;
            }       
                inFile.close() ;                            // close connection to file
                return quarters ;
        } // end information



    } // end class  

    String[] tokens = new String[100] ; 
    tokens= information(fileCheck) ;

for ( int i = 1 ; i < tokens.length ; i+=2)     // adds two so it skips WID
{
    ...
    placeHolder = tokens[i];
    ...
    placeHolder.charAt(...)