java - 具有2d数组和对象的空指针

您好，我正在制作一个创建进度报告的多个类的项目。但是，我正在测试方法，但尚未完成该项目，并且遇到了空指针异常。看一下代码，看看是否可以帮助我。请记住，并不是所有方法都只能先着重解决我的问题。我也有一个单独的驱动程序文件，除非另外需要，否则我认为它与发布无关。

学生班：

public class Student {
private String name;
private char grade;
private double average;
private int[] scores = new int[5];

// Constructor
public Student() {
    this.name = name;
    this.grade = grade;
    this.average = average;
    this.scores = scores;
}


// Get the Name.
public String getName() {
    return name;
}

// Set the Name.
public void setName(String name) {
    this.name = name;
}

// Get the Grade.
public char getGrade() {
    return grade;
}

// Set the Grade.
public void setGrade(char grade) {
    this.grade = grade;
}

// Get the Average.
public double getAverage() {
    return average;
}

// Set the Average.
public void setAverage(double average) {
    this.average = average;
}

// Get the Scores.
public int[] getScores() {
    return scores;
}

// Set the Scores.
public void setScores(int[] scores) {
    this.scores = scores;
}

// Determine the average of the five test scores for each student
public void calculateAverage(){

}

public void calculateGrade(){

}
}

ProgressReport类（即时消息获取空指针异常）：

public class ProgressReport {
// Create array to hold sections and students.
Student[][] sectionArray = new Student[2][];

// Constructor.
public ProgressReport() {

}

// Get sectionArray.
public Student[][] getSectionArray() {
    return sectionArray;
}

// Set sectionArray.
public void setSectionArray(Student[][] sectionArray) {
    this.sectionArray = sectionArray;
}

// Read the input file.
public void readInputFile() throws FileNotFoundException{
    String line;
    int studentNo;
    // Open file
    File inFile = new File("file.in");
    // Create scanner for reading.
    Scanner scanner = new Scanner(inFile);
    // While inFile has more lines.
    while(scanner.hasNext()){
        // Read the next line.
        line = scanner.nextLine();
        // Trim line.
        line = line.trim();
        //Parse line into int.
        studentNo = Integer.parseInt(line);
        // For the number of students in section 1 extract data.
        for(int i = 0; i<= studentNo; i++){
            //Create new student.
            sectionArray[0][i] = new Student(); **THIS IS WHERE I GET NULL POINTER EXCEPTION**
            // Read next line.
            line = scanner.nextLine();
            // Create String Tokenizer using a space as the delimiter.
            StringTokenizer strTokenizer = new StringTokenizer(line," ");
            // While the String Tokeizer has more tokens get data.
            while(strTokenizer.hasMoreTokens()){
                // Extract name
                String name = strTokenizer.nextToken();
                // Set name
                sectionArray[0][i].setName(name);

                int[] scores = new int[5];
                // Extract scores.
                int score1 = Integer.parseInt(strTokenizer.nextToken());
                int score2 = Integer.parseInt(strTokenizer.nextToken());
                int score3 = Integer.parseInt(strTokenizer.nextToken());
                int score4 = Integer.parseInt(strTokenizer.nextToken());
                int score5 = Integer.parseInt(strTokenizer.nextToken());
                //Put scores in scores array.
                scores[0] = score1;
                scores[1] = score2;
                scores[2] = score3;
                scores[3] = score4;
                scores[4] = score5;
                // Set scores.
                sectionArray[0][i].setScores(scores);

            }
        }
    }
}

// Generate a report.
public void generateReport(){
    System.out.println("Progress Report\n");
    System.out.println("Section 1");
    System.out.println(sectionArray[0][0].getName());
}

// Sort by name.
public void sortByName(){

}

// Binary search.
public Student binarySearch(int section, String searchName){
    return null;

}

}

我并没有要求任何人完成我的工作，只是请解释为什么我会得到一个空指针异常。

最佳答案

一旦知道Student的数量为

    studentNo = Integer.parseInt(line);

    // initialize the Array
    sectionArray[0] = new Student[studentNo];

    // For the number of students in section 1 extract data.
    for(int i = 0; i<= studentNo; i++){

您一直将sectionArray用作sectionArray[0][*]。我不确定您是否真的需要二维数组。将其初始化为new Student[2][];表示您有时也会使用sectionArray[1][*]。

如果以后再做；您还需要初始化sectionArray[1]。