[在Arraylist类中使用.size（）时为NullPointerException

Question

当前，我正在做一个处理ArrayList类的任务。在某个时候，我需要检查讲师的ID，并确保没有将讲师两次添加到ArrayList中，所以我进行了一个for循环以遍历所有已注册的ID并获取该ID并进行检查如果已经存在

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

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

我需要读的是这个：

\\名称-ID-dateOfBirth-性别-学位-专业-城市-可用性

[Amanda Smith，102020，320101200000，M，PhD，Software Engineering，NewYork，true

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

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

问题是，当您在其上调用.size（）时，membersList不存在

而不是

ArrayList<UniversityMember> membersList;

您需要初始化它

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

Answer 2

您需要初始化ArrayList。像那样ArrayList MembersList = new ArrayList（）;之后，在第一个size（）中返回0而不是null。请记住，所有数据结构必须在java中初始化。

Answer 3

您尚未将任何内容添加到membersList，然后要求其中不包含任何内容的大小。发生什么事的例子

String str;

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

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类

import java.util.ArrayList;

Answer 4

nvm，我发现我还没有初始化我的数组（╥ω╥）我将保留此问题，以供其他人谨慎

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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