我尝试在 StringBuilder 中使用 setCharAt 方法,但遇到空指针异常。有没有一种方法可以将值添加到我制作的 StringBuilder 数组中,这样我就不会收到这些错误。
通过研究,我找到了 .append() 方法,但我什至不确定它是如何工作的。
import java.util.*; // Allows for the input of a scanner method.
import java.io.*; // Allows for the inputting and outputting of a data file.
import java.lang.*; // Allows for the use of String Methods.
//////////////////////////////////////////////////////////////////////////////////////
public class TESTY
{
static Scanner testanswers;
static PrintWriter testresults;
public static void main(String[] args) throws IOException
{
testanswers = new Scanner(new FileReader("TestInput.dat"));
testresults = new PrintWriter("TestOutput.dat");
String StudentID;
String answers;
// Reads first two lines first to know how many records there are.
String answerKey = testanswers.nextLine();
int count = Integer.parseInt(testanswers.nextLine());
// Allocate the array for the size needed.
String[][] answerArray = new String[count][];
for (int i = 0; i < count; i++)
{
String line = testanswers.nextLine();
answerArray[i] = line.split(" ", 2);
}
for(int row = 0; row < answerArray.length; row++)
{
for(int col = 0; col < answerArray[row].length; col++)
{
System.out.print(answerArray[row][col] + " ");
}
System.out.println();
}
gradeData(answerArray, answerKey);
testanswers.close();
testresults.close();
}
///////////////////////////////////////////////////////////
//Method: gradeData
//Description: This method will grade testanswers showing
//what was missed, skipped, letter grade, and percentage.
///////////////////////////////////////////////////////////
public static double gradeData(String[][] answerArray, String answerKey)
{
String key = answerKey;
double Points = 0;
StringBuilder[] wrongAnswers = new StringBuilder[5];
String studAnswers;
for(int rowIndex = 0; rowIndex < answerArray.length; rowIndex++) /// Counting rows
{
studAnswers = answerArray[rowIndex][1].replace(" ", "S"); ///Counts rows, Col stay static index 1
for(int charIndex = 0; charIndex < studAnswers.length(); charIndex++)
{
if(studAnswers.charAt(charIndex) == key.charAt(charIndex))
{
Points += 2;
}
else if(studAnswers.charAt(charIndex) == 'S')
{
Points --;
}
else if(studAnswers.charAt(charIndex) != key.charAt(charIndex))
{
for(int i = 0; i < wrongAnswers.length; i++)
{
wrongAnswers[i].setCharAt(charIndex, 'X');
}
Points -= 2;
}
}
System.out.println(Points);
}
return Points;
}
}
错误发生在第 91 行:
wrongAnswers[i].setCharAt(charIndex, 'X');
最佳答案
您已经声明了一个 StringBuilder
数组,但尚未初始化任何插槽,因此它们仍然是 null
。
初始化它们:
StringBuilder[] wrongAnswers = new StringBuilder[5];
for (int i = 0; i < wrongAnswers.length; i++)
{
wrongAnswers[i] = new StringBuilder();
}
此外,使用 setCharAt
在这里不起作用,因为最初 StringBuilder
中没有任何内容。根据您想要的内容,您可能只需要调用 append
,或者您最初可能需要一个充满空格的字符串,以便您可以将特定字符设置为 'X'
>.
关于java - 空指针异常字符串生成器,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/23505635/