Question

我编写了一个名为TestStudent的程序，它创建了Student对象的四个元素数组。用户提示输入学生ID，姓名，部门和分类级别。这是代码：

import java.util.Scanner;

public class TestStudent {

public static void main(String[] args) {
    //Create a Scanner object
    Scanner input = new Scanner(System.in);

    //Create a four element array of Student object 
    Student[] studentList = new Student[4];

    for (int i = 0; i < studentList.length; i++) {
        int j = i + 1;
        //Prompt user to enter student matrix number
        System.out.println("Enter student " + j + " id : ");            
        studentList[i].setIdStudent(input.nextInt());

        //Prompt user to enter student name
        System.out.println("Enter student " + j + " name : ");
        studentList[i].setName(input.nextLine());

        //Prompt user to enter student department
        System.out.println("Enter student " + j + " department : ");
        studentList[i].setDepartment(input.nextLine());

        //Prompt user to enter student classification level
        System.out.println("Enter student " + j + " classification : ");
        studentList[i].setClassification(input.next());

        System.out.println("\n");
    }

    //Print result
    System.out.println("Id Student  Name            Department      Classification");
    System.out.println("******************************************************************************");
    for (int i = 0; i < studentList.length; i++) {
    System.out.println(studentList[i].getIdStudent() + "    " + studentList[i].getName() + "    " + studentList[i].getDepartment() + "      " + studentList[i].getClassification());
    }

}

    public class Student {
      //Student matrix number
      private int idStudent;

      //Student name
      private String name;

      //Student department
      private String department;

      //Student classification level
      private String classification;

      //Construct a default Student object
      public Student() {
        idStudent = 0;
        name = " ";
        department = " ";
        classification = " ";
      }

      //Construct a Student object
      public Student(int idStudent, String name, String department, String classification) {
        this.idStudent = idStudent;
        this.name = name;
        this.department = department;
        this.classification = classification;
       }

       //Return student matrix number
       public int getIdStudent() {
        return idStudent;
       }

       //Return student name
       public String getName() {
        return name;
       }

       //Return student department
       public String getDepartment() {
        return department;
       }

       //Return student classification level
       public String getClassification() {
        return classification;
       }

       //Set student matrix number
       public void setIdStudent(int newIdStudent) {
        newIdStudent = idStudent;
       }

       //Set student name
       public void setName(String newName) {
        newName = name;
       }

       //Set student department
       public void setDepartment(String newDepartment) {
        newDepartment = department;
       }

       //Set student classification level
       public void setClassification(String newClassification) {
        newClassification = classification;
       }

         }

}

预期输出为：

    Id Student     Name           Department        Classification
  ********************************************************************************
  1140           Will Gerard    Music             Junior
  1152           Julia Ross     Architecture      Freshman
  1130           Fred Huan      Medic             Senior
  1137           Clara Whist    Aviation          Sophomore

但是发生了错误：

Enter student 1 id : 
1140
Exception in thread "main" java.lang.NullPointerException
    at Asg2.TestStudent.main(TestStudent.java:28)

我认为问题似乎是studentList[i].setIdStudent(input.nextInt());行。有关如何解决这个问题的任何建议？

Answer 1

1）转到第28行。

2）你有一些具有空值的参考文献在您尝试在其上调用方法或访问时其中一个字段（类变量）。

3）确保在尝试之前初始化该参考使用它，所以当你需要它时它不是空的非空值。

四元素数组对象的setter方法？

1 个答案: