最初我的文本文件有行：
12
Steve Jobs 9 final 91
Bill Gates 6 midterm 90
James Gosling 3 midterm 100
James Gosling 3 final 100
Dennis Ritchie 5 midterm 94
Steve Jobs 9 midterm 95
Dennis Ritchie 5 final 100
Jeff Dean 7 midterm 100
Bill Gates 6 final 96
Jeff Dean 7 final 100
Sergey Brin 27 final 97
Sergey Brin 22 midterm 99
方法public static Exam [] readAllExams（Scanner s）创建并返回一个对象数组。 请注意字符串的第一个字母＆＃39; final＆＃39;和期中＆＃39;转换为char。因此，当我运行readAllExams（）时，结果是：
Steve Jobs 9 f 91
Bill Gates 6 m 90
James Gosling 3 m 100
James Gosling 3 f 100
Dennis Ritchie 5 m 94
Steve Jobs 9 m 95
Dennis Ritchie 5 f 100
Jeff Dean 7 m 100
Bill Gates 6 f 96
Jeff Dean 7 f 100
Sergey Brin 27 f 97
Sergey Brin 22 m 98
但是，当我运行公共静态考试[] collateExams（考试[]考试）时，它会在此行中为我提供NullPointerException：if(exams[i].getExamType()=='m')。 smb可以解释/帮助我吗？ 请注意，getExamType（）会返回一个字符。
import java.io.*;
import java.util.*;
class P2 {
public static void main(String [] args) throws FileNotFoundException
{
Scanner data = new Scanner(new File("Exam.txt"));
Exam[] readObjects = readAllExams(data);
Exam[] collateObjects = collateExams(readObjects);
System.out.println("1) Initially the list of exams of students is: ");
System.out.println();
for(int i = 0; i < readObjects.length; i++)
{
System.out.println(readObjects[i]);
}
System.out.println("Sorted list: ");
for (int i = 0; i < collateObjects.length; i++)
{
System.out.println(collateObjects[i]);
}
}
public static Exam[] readAllExams(Scanner s) throws ArrayIndexOutOfBoundsException
{
String firstName = "";
String lastName = "";
int ID = 0;
String examType = "";
char examTypeCasted;
int score = 0;
int index = 0;
Exam[] object = new Exam[s.nextInt()];
while(s.hasNext())
{
//Returns firtsName and lastName
firstName = s.next();
lastName = s.next();
//Returns ID number
if(s.hasNextInt())
{
ID = s.nextInt();
}
else
s.next();
//Returns examType which is 'M' or 'F'
examType = s.next();
examTypeCasted = examType.charAt(0);
if(s.hasNextInt())
{
score = s.nextInt();
}
//Exam[] object = new Exam[s.nextInt()];
object[index] = new Exam(firstName, lastName, ID, examTypeCasted, score);
//System.out.println();
index++;
}
readExam(s);
return object;
}
public static Exam readExam(Scanner s)
{
String firstName = "";
String lastName = "";
int ID = 0;
String examType = "";
char examTypeCasted = 0;
int score = 0;
while (s.hasNext())
{
//Returns firtsName and lastName
firstName = s.next();
lastName = s.next();
//Returns ID number
if(s.hasNextInt())
{
ID = s.nextInt();
}
//Returns examType which is 'M' or 'F'
examType = s.next();
examTypeCasted = examType.charAt(0);
if(s.hasNextInt())
{
score = s.nextInt();
}
}
Exam temp = new Exam(firstName, lastName, ID, examTypeCasted, score);
return temp;
}
public static Exam[] collateExams(Exam[] exams)
{
Exam [] r = new Exam[exams.length];
r = exams;
int [] position = new int[exams.length];
for(int i = 0;i < position.length;i++)
position[i]=-1;
int index = 0;
for(int i = 0; i < exams.length; i++)
{
if(exams[i].getExamType()=='m')
{
if(position[exams[i].getID()]==-1)
{
r[index*2]=new Exam(r[i].getFirstName(), r[i].getLastName(), r[i].getID(), r[i].getExamType(), r[i].getScore());
position[exams[i].getID()] = 2*index;
}
else r[2*position[exams[i].getID()] - 1]=new Exam(r[i].getFirstName(), r[i].getLastName(), r[i].getID(), r[i].getExamType(), r[i].getScore());
}
else
{
if(position[exams[i].getID()]==-1)
{
r[index*2+1] = new Exam(r[i].getFirstName(), r[i].getLastName(), r[i].getID(), r[i].getExamType(), r[i].getScore());
position[exams[i].getID()] = 2*index+1;
}
else r[2*position[exams[i].getID()]+1]=new Exam(r[i].getFirstName(), r[i].getLastName(), r[i].getID(), r[i].getExamType(), r[i].getScore());
}
index++;
}
return r;
/*Exam [] r = new Exam[100];
r = exams;
int[] position = new int[100];
int index = 0;
for(int i = 0; i < exams.length; i++)
{
if(r[i].getExamType() == 'm')
{
r[index] = new Exam(r[index].getFirstName(), r[index].getLastName(),
r[index].getID(), r[index].getExamType(), r[index].getScore());
position[index] = r[index].getID();
index+=2;
/*if((r[index+1].getExamType() == 'F') && (position[index+1] == r[i].getID()))
{
r[index+1] = new Exam(r[index].getFirstName(), r[index].getLastName(),
r[index].getID(), r[index].getExamType(), r[index].getScore());
}
/*if(position[i].getID() == r[i].getID())
r[i] = new Exam(r[i].getFirstName(), r[i].getLastName(),
r[i].getID(), r[i].getExamType(), r[i].getScore());
}
else if((r[index+1].getExamType() == 'f') && position[index] == r[i].getID())
{
r[index + 1] = new Exam(r[index].getFirstName(), r[index].getLastName(),
r[index].getID(), r[index].getExamType(), r[index].getScore());
}
}
return r;*/
}
}
答案 0 :(得分：1)
当我运行你的代码时，我没有获得空指针异常，但是当你执行r * 2时，索引超出范围异常。这不会起作用，因为你为你的数组分配12个点，迭代12个项目，然后告诉它在迭代时转到n * 2。它迅速超越了界限。还有一些其他问题将ID设置为数组索引等。我为您清理了一些代码并遵循了您的意图（分配两个位置，假设检查数量包括两个ID，一个M和一个F）。请注意，您的文本文件还包含Sergey Brin的ID中的错误，该错误打破了这一假设。这可能不是你的最终产品，但它得到你在那里工作的东西：
public static void main(String [] args) throws FileNotFoundException
{
Scanner data = new Scanner(new File("exams.txt"));
Exam[] readObjects = readAllExams(data);
System.out.println("1) Initially the list of exams of students is: ");
System.out.println();
for(int i = 0; i < readObjects.length; i++)
{
System.out.println(readObjects[i]);
}
Exam[] collateObjects = collateExams(readObjects);
System.out.println("Sorted list: ");
for (int i = 0; i < collateObjects.length; i++)
{
System.out.println(collateObjects[i]);
}
}
public static Exam[] readAllExams(Scanner s) throws ArrayIndexOutOfBoundsException
{
int index = 0;
Exam[] object = new Exam[s.nextInt()];
while(s.hasNext())
{
object[index] = readExam(s);
index++;
}
return object;
}
public static Exam readExam(Scanner s)
{
String firstName = "";
String lastName = "";
int ID = 0;
String examType = "";
char examTypeCasted = 0;
int score = 0;
//Returns firtsName and lastName
firstName = s.next();
lastName = s.next();
//Returns ID number
if(s.hasNextInt())
{
ID = s.nextInt();
}
//Returns examType which is 'M' or 'F'
examType = s.next();
examTypeCasted = examType.charAt(0);
if(s.hasNextInt())
{
score = s.nextInt();
}
return new Exam(firstName, lastName, ID, examTypeCasted, score);
}
public static Exam[] collateExams(Exam[] exams)
{
Exam [] r = new Exam[exams.length];
System.arraycopy(exams, 0, r, 0, exams.length);
int [] position = new int[exams.length];
for(int i = 0;i < position.length;i++)
position[i]=-1;
for(int i = 0; i < exams.length; i++)
{
int nextPos = determineNextPosition(position, exams[i]);
r[nextPos] = new Exam(exams[i].getFirstName(), exams[i].getLastName(), exams[i].getID(), exams[i].getExamType(), exams[i].getScore());
}
return r;
}
private static int determineNextPosition(int[] posArray, Exam exam)
{
int position = -1;
int id = exam.getID();
// see if there is a spot for the ID already
for (int i = 0; i < posArray.length; i++)
{
if (posArray[i] == id)
{
position = i;
break;
}
}
if (position < 0)
{
// if a spot wasnt found, make one
for (int i = 0; i < posArray.length; i+=2)
{
if (posArray[i] == -1)
{
posArray[i] = id;
posArray[i+1] = id;
position = i;
break;
}
}
}
if (position < 0) throw new RuntimeException("Something went really wrong");
if (exam.getExamType() == 'm')
{
return position;
}
else
{
return position + 1;
}
}