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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，并遇到了空指针异常。看看代码，看看你是否可以帮助我。请记住，所有方法都没有完成，只是首先尝试关注我的问题。我还有一个单独的驱动程序文件，我发现它与发布无关，除非另有需要。

学生班级：

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（我得到空指针异常的地方）：

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;

}

}

我不是要求任何人完成我的工作，只是解释一下为什么我得到一个空指针异常。

4

4 回答 4

3

一旦您知道Students的数量，您需要初始化第二维

    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++){

你一直用你的sectionArrayas sectionArray[0][*]。我不确定你是否真的需要二维数组。将其初始化为new Student[2][];表明您sectionArray[1][*]在某个时间点也会使用它。

如果你以后这样做；您还需要初始化sectionArray[1]。

于 2013-10-13T18:58:38.347 回答
1

如果你做这样的事情

String[][] array = new String[2][];

它将创建一个包含两个空元素的数组，因此它与

String[][] array = {null,null};

并且由于您正在调用此类数组，因此它与引发 NPEsectionArray[0][i]的调用相同。null[i]

于 2013-10-13T19:06:52.800 回答
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您需要像这样指定两个维度：Student[][] sectionArray = new Student[2][2];或像这样初始化第二个维度：sectionArray[0] = new Student[students];和sectionArray[1] = new Student[students];.

于 2013-10-13T19:06:16.830 回答
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好吧，您在这里为学生类使用数组数组。

对于（数组数组）的每个数组，您需要使用其所需数量的元素来启动每个数组。

这里：在这一行之前......你得到空指针异常的地方，

sectionArray[0][i] = new Student();

您需要使用这样的新关键字启动数组 sectionArray[0]。

sectionArray[0]= new Student[studentNo];             // add this line to you code

那么你使用的代码就会出现。

于 2013-10-13T19:35:41.800 回答