java - NullPointerException java 错误

Question

我仍在开始学习 OOP，并且我的代码中不断出现此错误；说线程“main”java.lang.NullPointerException中的异常

public class SlumbookDriver{
    public static void main(String args[]){
       Slumbook[] contacts = new Slumbook[19];
       ... // index is an int and is the value of the index of the array
       ... // i feed it to a function "void viewEntry" that just shows
           // the other attributes of the class Slumbook
       viewEntry(index, contacts);
    }
 }

然后我有函数viewEntry

public static void viewEntry(int index, Slumbook[] contacts){
    Scanner sc = new Scanner(System.in);
    if(index == 0){
        System.out.println("Array is empty");
    }
    else{
        String id = contacts[index].getIdNo();
        System.out.println("Please enter ID number");
        String idNo = sc.next();    
        if(id != idNo){
            while(id != idNo && index != -1){
                index--;
                id = contacts[index].getIdNo();
            }
            if(index == -1){
                System.out.println("ID does not exist");
                return; //terminate action since the ID number does not exist
            }
        }   
        System.out.println(contacts[index].viewDetails());
    }
}

Answer 1

您只是初始化数组

   Slumbook[] contacts = new Slumbook[19];

但不是它的元素，因此当您在如下语句中访问数组元素时，您将得到一个 NullPointerException :

    String id = contacts[index].getIdNo();

当你创建一个对象数组时，数组中的对象并没有被初始化，你需要在使用它们之前使用new运算符来初始化它们。像这样的事情:

   contacts[index] = new Slumbook();