在 Arraylist class 中使用 .size() 时出现 NullPointerException

Question

目前，我正在做一个处理 ArrayList class 的作业。在某些时候，我需要检查讲师的 id 并确保讲师没有被添加到 ArrayList 中两次，所以我做了一个 for 循环来遍历所有已注册的 id 并获取 id 并检查如果它已经存在

问题是当我在循环中使用方法“.size()”时，JVM 抛出 NullPointerException 我不知道为什么。

============================================= =============================

我需要看的是这个：

\\姓名-身份证号-出生日期-性别-学位-专业-城市-可用性

Amanda Smith，102020，320101200000，硕士，博士，软件工程，纽约，真实

============================================= ==========================

这是代码：

public static void main(String[] args) {

    /* NOTE: I HAVE A CLASS CALLED "UniversityMember" THAT IS A SUPERCLASS FOR "Instructor" CLASS */

    //declare what I need   
    ArrayList<UniversityMember> membersList;
    Scanner read = new Scanner("inputFile.txt");//the file contains the text above

    //First: Split the line everytime the sign ", " shows
    String[] line = read.nextLine().split(", ");

    //Second: Assign each valuse to its correspondeding variable
    String name = line[0];
    String id = line[1];
    long date = Long.parseLong(line[2]);
    Date birthDate = new Date(date);
    char gender = line[3].charAt(0);
    String degree = line[4];
    String specialization = line[5];
    String address = line[6];
    boolean availability = Boolean.parseBoolean(line[7]);

    //check if the Id is registered already
    for (int i = 0; i < membersList.size(); i++) { //ERROR OCCURE
        if (membersList.get(i) == null) {
            break;
        }

        if (membersList.get(i).id.equals(id)) {
            System.out.println("The instructor is registered already, the ID is found in the system.");
            System.exit(0);
        }
    }

    //add and make a new object for the constructor
    membersList.add(new Instructor(name, id, birthDate, gender, degree, specialization, address, availability));
    System.out.println("The instructor is successfully added.");

}//end main

Answer 1

您还没有向 membersList 添加任何内容，然后询问其中没有任何内容的内容的大小。发生了什么事的例子

String str;

for(int i = 0; i < str.length(); i++){
    System.out.println("hey");
}

你还需要像这样声明数组列表

ArrayList<Method name> membersList = new ArrayList<Method name>();

也不要忘记导入 ArrayList class

import java.util.ArrayList;

Answer 2

问题是当您调用 .size() 时 membersList 不存在

而不是

ArrayList<UniversityMember> membersList;

你需要初始化它

ArrayList<UniversityMember> membersList = new ArrayList<UniversityMember>();

Answer 3

nvm 我发现我的数组还没有初始化( ╥ω╥ ) 我会把问题留给其他人小心

============================================= =====

修复后的代码：

    public static void main(String[] args) {

    /* NOTE: I HAVE A CLASS CALLED "UniversityMember" THAT IS A SUPERCLASS FOR "Instructor" CLASS */

    //declare what I need   
    ArrayList<UniversityMember> membersList;
    Scanner read = new Scanner("inputFile.txt");//the file contains the text above

    /* ===== FIXING THE ERROR ======*/
     membersList = new ArrayList();

    //First: Split the line everytime the sign ", " shows
    String[] line = read.nextLine().split(", ");

    //Second: Assign each valuse to its correspondeding variable
    String name = line[0];
    String id = line[1];
    long date = Long.parseLong(line[2]);
    Date birthDate = new Date(date);
    char gender = line[3].charAt(0);
    String degree = line[4];
    String specialization = line[5];
    String address = line[6];
    boolean availability = Boolean.parseBoolean(line[7]);

    //check if the Id is registered already
    for (int i = 0; i < membersList.size(); i++) { 
        if (membersList.get(i) == null) {
            break;
        }

        if (membersList.get(i).id.equals(id)) {
            System.out.println("The instructor is registered already, the ID is found in the system.");
            System.exit(0);
        }
    }

    //add and make a new object for the constructor
    membersList.add(new Instructor(name, id, birthDate, gender, degree, specialization, address, availability));
    System.out.println("The instructor is successfully added.");

}//end main

Answer 4

您需要初始化ArrayList。就像 ArrayList membersList = new ArrayList(); 之后，在第一个 size() returns 0 而不是 null。请记住，所有数据结构都必须在 java.

中初始化

在 Arraylist class 中使用 .size() 时出现 NullPointerException

NullPointerException when using .size() in an Arraylist class

java

size

nullpointerexception