我不知道如何从字符串、整数或变量中创建一个特定的数组

Posted 2023-02-19

【中文标题】我不知道如何从字符串、整数或变量中创建一个特定的数组

【英文标题】：I cannot figure out how to make a specific array from strings, integers, or variables

【发布时间】：2016-11-23 02:31:48

【问题描述】：

这些是我的错误：

Exception in thread "main" java.lang.NullPointerException
at Student.<init>(Student.java:22)
at Roster.<init>(Roster.java:35)
at Roster.main(Roster.java:15))

我知道问题是因为我有一个包含 3 个成绩的数组（学生姓名字符串中的最后 3 个）没有正确存储。

import java.util.*;
import java.lang.*;
public class Roster 
 
    static String[] students = "1,John,Smith,[e-mail],20,88,79,59", 
                                "2,Suzan,Erickson,Erickson_1990@gmailcom,19,91,72,85",
                                "3,Jack,Napoli,The_lawyer99yahoo.com,19,85,84,87",
                                "4,Erin,Black,[e-mail],22,91,98,82",
                                "5,Michael,Brauer,[e-mail],30,99,98,96";

    static ArrayList<Student> myStudents = new ArrayList<Student>();

    public static void main(String[] args) 
     
        new Roster(); 
     

    public Roster() 
     
        for (int i = 0; i < students.length; i++) 
         
            String y = students[i];
            String[] variable = y.split(",");

            String id = variable[0];
            String first = variable[1];
            String last = variable[2];
            String email = variable[3];            
            int age = Integer.parseInt(variable[4]);
            int grade1 = Integer.parseInt(variable[5]);
            int grade2 = Integer.parseInt(variable[6]);
            int grade3 = Integer.parseInt(variable[7]);
            int[] grades = new int[]grade1, grade2, grade3;

            //Here is the first issue that I am having: Nothing stores to grades.

            Student x = new Student(id, first, last, email, age, grades);

            myStudents.add(x); 
        

        print_all();

        print_invalid_emails(); 
        for (Student y : myStudents)
        
            print_average_grade(y.getId());
         
     

    public static void print_all()
    
        for (Student x : myStudents) 
         
            System.out.println(x);  
        
    

    public static void print_invalid_emails()
    
        for (Student x : myStudents)
        
            String email = x.getEmail(); 
            if (!email.contains("@"))
            
                System.out.println("INVALID EMAIL: " + email);
            
            else if (!email.contains("."))
            
                System.out.println("INVALID EMAIL: " + email);
             
            else if((email.contains(" "))) 
            
                System.out.println("INVALID EMAIL: " + email);
            
        

        remove("3");
        remove("3");
    

    public static void remove(String id) 
     
        for (int i = 0; i < myStudents.size(); i++) 
         
            if (myStudents.get(i).getId().equals(id)) 
             
                myStudents.remove(i);
                System.out.println("STUDENT ID: (" + id + ")  *NOT FOUND*");
                return;
             
         
        System.out.println("STUDENT ID: (" + id + ")  !DELETED!");
    

    public static void print_average_grade(String id)
    
        for (Student y : myStudents)
        
            if(y.getId().equals(id)) 
            
                double total = 100;
                double average = total / 3;
                System.out.println("STUDENT ID: (" + id + ")  " + average);
            
        
     


public class Student 
 
    String id;  
    String first;
    String last;
    String email;   
    int age;
    int[] grades;

    public Student(String id, String first, String last, String email, int age, int[] grades) 
    
        this.id = id;
        this.first = first;
        this.last = last;
        this.email = email;
        this.age = age;
        this.grades = grades;
    

    //This is the other problem. I am trying to make an array

    public int[] gradeArray = grades[1], grades[2], grades[3];

    public String getFirst() 
    
        return first; 
     
    public void setFirst(String first) 
     
        this.first = first; 
     
    public String getLast() 
    
        return last;
     
    public void setLast(String last)
    
        this.last = last;
     
    public String getEmail()
    
        return email;
     
    public void setEmail(String email)
    
        this.email = email;
      
    public int getAge()
    
        return age;
     
    public void setAge(int age)
    
        this.age=age;
     
    public String getId() 
     
        return id; 
     
    public void setId(String id) 
     
        this.id = id; 
    
    public int[] getGradeArray()
    
        return gradeArray;
    
    public void myGrades(int[] grades)
    
        this.grades = grades;
       
    //Print to output
    public String toString() 
    
        return "STUDENT: ID-" + id + "  AGE-" + age + "  NAME-" + last + ", " +  first; 
     

【参考方案1】：

在这里，您将三个等级存储到一个（从 0 开始的）int 数组中：

int[] grades = new int[]grade1, grade2, grade3;

但是您尝试访问超出数组范围的 Grades[3]。

public int[] gradeArray = grades[1], grades[2], grades[3];

另外值得注意的是：Erickson_1990@gmailcom 将无法通过您的有效电子邮件测试。不确定这是出于测试原因，还是只是一个错字。