四元素數組對象setter方法？

Question

我編寫了一個名爲TestStudent的程序，它創建了Student對象的四個元素數組。提示用戶輸入學號，姓名，部門和分類等級。下面的代碼：

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; 
     } 

     } 

} 

的預期結果是：

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 

但出現錯誤：

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

我想，這個問題似乎是studentList[i].setIdStudent(input.nextInt());線。有關如何解決這個問題的任何建議？

Answer 1

1）進入管線28

2）有你有這在目前有一個空值
一定的參考，當您嘗試調用它的方法或訪問其字段
一個（類變量）。

3）確保你想
使用前初始化參考，所以它不是空當你需要它有
一個非空值。