构建不使用Java中的ArrayList将包含多个变量的文本文件读入数组的方法

Question

public class Main2{
public static void main(String[] args) throws FileNotFoundException {
    String filename = "students.txt";
    Student[] record = readData(filename);
    record = new Student [record.length];

    for(int i = 0; i < record.length; i++){

        System.out.println(record[i]);

    }
}

//Method for read data from the .txt file into an array
private static Student[] readData(String filename){

    int arraysize = 0;
    try{
        Scanner s1 = new Scanner(new File(filename));

       //Count elements in the file
       while(s1.hasNext()){
           arraysize++;
           s1.next();
       }

       //Create an array and copy the elements into it
        Student[] studentRecord = new Student[arraysize];
        Scanner s2 = new Scanner(new File(filename));

        for(int i = 0; i < arraysize; i++){
            studentRecord[i].name = s2.next();
            studentRecord[i].physic = s2.nextInt();
            studentRecord[i].chemistry = s2.nextInt();
            studentRecord[i].math = s2.nextInt();
            studentRecord[i].average = (double)(studentRecord[i].physic + studentRecord[i].chemistry + studentRecord[i].math)/3;
            // double roundedAvg = Math.round(average * 100.0) / 100.0;
            // studentRecord[i].average = Math.round((physic + chemistry + math)/3 * 100.0)/100.0;

        }
        return studentRecord;
    }

    catch(FileNotFoundException e){

    }
    return null;
}

Jack  77      84       93
Tod   84      86       80
Name marksP marksC   marksM

Exception in thread "main" java.lang.NullPointerException: Cannot assign field "name" because "<local3>[<local5>]" is null
    at Main2.readData(Main2.java:40)
    at Main2.main(Main2.java:10)

public Student(String name, int m1, int m2, int m3, double avg){
       this.name = name;
       int.mark1 = m1;
        ...
 }

我做错了什么？如何修复代码？当我在Student类中初始化时，名称不应为null。

Answer 1

在从文件中读取数据的for循环中，StudentRecord还不包含实际的Student对象。到目前为止，它只是一个占位符，可以容纳指定数量的student对象。因此，您需要实际创建student对象来存储在指定的数组中。

为此，请读取数据，将其暂时存储在变量中，然后通过已有的构造函数创建每个student对象。

所以它实际上应该是这样的：

for(int i = 0; i < arraysize; i++){
    String name = s2.next();
    int physic = s2.nextInt();
    int chemistry = s2.nextInt();
    int math = s2.nextInt();
    double average = (double)(physic + chemistry + math) / 3;
    Student myStudent = new Student (name, physic, chemistry, math, average);
    studentRecord[i] = myStudent;
}

while(s1.hasNextLine()){
    arraysize++;
    s1.nextLine();
}