英文:
NullPointerException when using .size() in an Arraylist class
问题
public static void main(String[] args) {/* 注意：我有一个名为 "UniversityMember" 的超类，它是 "Instructor" 类的超类 */// 声明我需要的东西ArrayList<UniversityMember> membersList = new ArrayList<>();Scanner read = new Scanner("inputFile.txt"); // 文件包含上述文本// 第一步：每次出现 ", " 标志时拆分行String[] line = read.nextLine().split(", ");// 第二步：将每个值分配给其对应的变量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]);// 检查是否已经注册了该 IDfor (int i = 0; i < membersList.size(); i++) { // 错误发生处if (membersList.get(i) == null) {break;}if (membersList.get(i).id.equals(id)) {System.out.println("该讲师已经注册，ID 已在系统中找到。");System.exit(0);}}// 添加并创建构造函数的新对象membersList.add(new Instructor(name, id, birthDate, gender, degree, specialization, address, availability));System.out.println("讲师已成功添加。");} // 结束主函数
英文:
currently, I'm doing an assignment that deals with the ArrayList class.
at some point, I need to check of the id of the instructor and make sure that the instructor is not added twice to the ArrayList, so I made a for loop to go through all the id that has been registered and get the id and check if it exists already
the problem is when I use the method " .size()" in the loop, the JVM throws NullPointerException
and I don't know why.
==========================================================================
what I need to read is this:
\\name - id - dateOfBirth - gender - degree - speciality - city - availability
Amanda Smith, 102020, 320101200000, M, PhD, Software Engineering, NewYork, true
=======================================================================
this is the code:
public static void main(String[] args) {/* NOTE: I HAVE A CLASS CALLED "UniversityMember" THAT IS A SUPERCLASS FOR "Instructor" CLASS *///declare what I needArrayList<UniversityMember> membersList;Scanner read = new Scanner("inputFile.txt");//the file contains the text above//First: Split the line everytime the sign ", " showsString[] line = read.nextLine().split(", ");//Second: Assign each valuse to its correspondeding variableString 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 alreadyfor (int i = 0; i < membersList.size(); i++) { //ERROR OCCUREif (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 constructormembersList.add(new Instructor(name, id, birthDate, gender, degree, specialization, address, availability));System.out.println("The instructor is successfully added.");}//end main
答案1
得分: 1
问题在于当您在其上调用 .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>();
英文:
The problem is membersList doesn't exist when you call .size() on it
instead of
ArrayList<UniversityMember> membersList;
you need to initialize it
ArrayList<UniversityMember> membersList = new ArrayList<UniversityMember>();
答案2
得分: 1
你需要初始化 ArrayList。就像这样初始化 ArrayList
英文:
You need to initialize the ArrayList.
Like that ArrayList<UniversityMember> membersList = new ArrayList<Method name>();
After that, in the first size() returns 0 and not null. Remember all data structure must be initialize in java.
答案3
得分: 0
你还没有向membersList中添加任何内容，然后却要求获取一个没有内容的东西的大小。以下是正在发生的情况示例：
String str;for(int i = 0; i < str.length(); i++){System.out.println("hey");}
另外，你需要像这样声明ArrayList：
ArrayList<Method name> membersList = new ArrayList<Method name>();
还要别忘了导入ArrayList类：
import java.util.ArrayList;
英文:
You haven't added anything to the membersList then asking for the size for something that has nothing in it.
Example of whats going on
String str;for(int i = 0; i < str.length(); i++){System.out.println("hey");}
also you need to declare the array list like this
ArrayList<Method name> membersList = new ArrayList<Method name>();
also don't forget to import the ArrayList class
import java.util.ArrayList;
答案4
得分: 0
nvm，我发现我没有初始化我的数组（ ╥ω╥ ）我会保留这个问题，以便其他人注意==================================================修复后的代码：```javapublic static void main(String[] args) {/* 注意：我有一个名为“UniversityMember”的类，它是“Instructor”类的超类 *///声明我需要的东西ArrayList<UniversityMember> membersList;Scanner read = new Scanner("inputFile.txt");//文件包含上面的文本/* ===== 修复错误 ======*/membersList = new ArrayList();//首先：每次出现“， ”标记时拆分行String[] line = read.nextLine().split(", ");//其次：将每个值分配给其对应的变量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]);//检查Id是否已注册for (int i = 0; i < membersList.size(); i++) {if (membersList.get(i) == null) {break;}if (membersList.get(i).id.equals(id)) {System.out.println("该讲师已经注册，系统中找到了该ID。");System.exit(0);}}//添加并为构造函数创建一个新对象membersList.add(new Instructor(name, id, birthDate, gender, degree, specialization, address, availability));System.out.println("成功添加了该讲师。");}//end main
英文:
nvm I figured out that I haven't initialized my array ( ╥ω╥ )
I'll keep the question for others to be carefull
==================================================
The code after fixing it:
public static void main(String[] args) {/* NOTE: I HAVE A CLASS CALLED "UniversityMember" THAT IS A SUPERCLASS FOR "Instructor" CLASS *///declare what I needArrayList<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 ", " showsString[] line = read.nextLine().split(", ");//Second: Assign each valuse to its correspondeding variableString 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 alreadyfor (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 constructormembersList.add(new Instructor(name, id, birthDate, gender, degree, specialization, address, availability));System.out.println("The instructor is successfully added.");}//end main
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