Question

尝试制作一个简单的基于控制台的地址簿。第一种方法是我的“加载”方法，它从文本文件中获取数据。但是，它会抛出这样的错误：

Exception in thread "main" java.lang.NullPointerException
    at pack.AddressBook.load(AddressBook.java:18)
    at pack.AddressBook.main(AddressBook.java:33)

我的AddressBook类看起来像这样：

public class AddressBook {

public static Student [] load() throws FileNotFoundException{
    Scanner input  = new Scanner(System.in);
    System.out.println("Which file do you want to access?");
    String filename = input.nextLine();
    Scanner fileScanner = new Scanner(new File(filename));
    Student [] studentArray = new Student[1];
    int i = 0;
    while(fileScanner.hasNextLine()){
        studentArray[i].firstName = fileScanner.nextLine();
        studentArray[i].lastName = fileScanner.nextLine();
        studentArray[i].age = fileScanner.nextLine();
        studentArray[i].grade = fileScanner.nextLine();
        i++;
        System.out.println(i);
    }
    return null;
    input.close();
    fileScanner.close();
    return studentArray;
}


public static void main(String[] args) throws entryError, FileNotFoundException {
    Student [] studentsInCurrentBook = load();
}


}

我的学生班看起来像这样（如果重要的话）：

public class Student {
public String firstName;
public String lastName;
public String age;
public String grade;

public Student() throws entryError{
    throw new entryError("Please provide at least a first and last name.");
}

public Student(String firstName, String lastName){
    this.firstName = firstName;
    this.lastName = lastName;
    age = "unknown";
    grade = "unknown";
}

public Student(String firstName, String lastName, String age){
    this.firstName = firstName;
    this.lastName = lastName;
    this.age = age;
    grade = "unknown";
}

public Student(String firstName, String lastName, String grade, char i){
    this.firstName = firstName;
    this.lastName = lastName;
    age = "unknown";
    this.grade = grade;
}

public Student(String firstName, String lastName, String age, String grade){
    this.firstName = firstName;
    this.lastName = lastName;
    this.age = age;
    this.grade = grade;
}

}

student.txt文件的格式如下：

firstname
lastname
age
grade

谢谢！

Answer 1

Student [] studentArray = new Student[1];
int i = 0;
while(fileScanner.hasNextLine()){
    studentArray[i].firstName = fileScanner.nextLine();
    studentArray[i].lastName = fileScanner.nextLine();
    studentArray[i].age = fileScanner.nextLine();
    studentArray[i].grade = fileScanner.nextLine();
    i++;
    System.out.println(i);
}

因此，您创建一个大小为1的空数组，然后尝试在其i元素中设置字段。在开始访问studentArray [i]字段之前，您需要创建一个学生进入您的数组。在您这样做之前，null是您阵列中的唯一内容，这就是您获得NullPointerException的原因。

然后你需要确保你的数组有足够的长度来容纳你添加的所有学生（所以你最好使用ArrayList）。

Answer 2

基本上你没有在studentArray中添加任何学生你只创建了一组Student个对象，然后尝试访问索引Student的{{1}}这当然是空的。您需要做的是添加i然后添加studentArray[i] = new Student();循环或只是执行此操作：

while

或者这个：

studentArray[i] = new Student(fileScanner.nextLine(), fileScanner.nextLine(), fileScanner.nextLine(), fileScanner.nextLine());

等等......希望这是有道理的

使用Java Filescanner的NullPointerException

2 个答案: