我收到错误
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，我无法弄清楚出了什么问题并导致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++ ;
}
这是完整代码。
/ ** * * * * * /
import 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
答案 0 :(得分：1)
我之前的回答是不正确的。我误读了你的代码。
问题是您从null数组中获取了tokens个值，我只是没有正确地找出原因。
你的代码开头是这样的：
String[] tokens = new String[100] ;
tokens= information(fileCheck) ;
第一行分配一个100个元素的字符串数组......第二行抛弃你刚分配的数组（呵呵？）并用information方法创建的新数组替换它。
在information方法中，您分配另一个100个元素的字符串数组，并用从该文件中读取的东西填充它。这是问题开始的地方。你看，如果你只阅读（比方说）30个令牌，其余70个元素将是null。
然后在main方法中执行此操作：
for ( int i = 1 ; i < tokens.length ; i+=2) // adds two so it skips WID
{
...
placeHolder = tokens[i];
...
placeHolder.charAt(...)
所以问题是你假设数组中的所有标记都不为空。但实际上，填写令牌的方法并不能保证填满所有的插槽。
我会留给你找出解决方案。 （这是你的功课！）但这里有几个想法：