线程“主”中的异常java.lang.NullPointerException-不明白为什么？

Question

I'm still a beginner with Java and writing a student record programme at the moment, and im stuck with these errors, and can't work out why.

Exception in thread "main" java.lang.NullPointerException
at Results.Results(Results.java:11)
at Main.main(Main.java:21)

This is where the error is in my main class, on line 21 (second line below).

public class Main 
{
    public static void main(String[] args) 
    {

        StudentNames nameInputobject = new StudentNames();
        nameInputobject.StudentNames();


        Subject subjectInputobject = new Subject();
        subjectInputobject.Subject();

        StudentNumber numberInputobject = new StudentNumber();
        numberInputobject.StudentNumber();      

        StudentYear yearInputobject = new StudentYear();
        yearInputobject.StudentYear();  

        Results resultsInputobject = new Results();
        resultsInputobject.Results();   // <<<error here




    }
}    

and the error in my results class

    import java.util.ArrayList;
import java.util.Scanner;

public class Results 
{
    public static void Results()
    {

        StudentYear obj=new StudentYear();
        ArrayList<Integer> studentYear=obj.getStudentYear();
        int year=studentYear.get(0); // <<<<error here 
        ArrayList<Integer> results = new ArrayList<Integer>();
        Scanner sc = new Scanner(System.in); 
        System.out.println("Please enter the results for "+year+":");
        results.add(sc.nextInt());
        System.out.println(results);

        }
    }

the results class gets the studentYear value from an arraylist in another class code here.

import java.util.ArrayList;
import java.util.Scanner;

public class StudentYear 
{
    private ArrayList<Integer> studentYear;
    public void StudentYear()
    {
        studentYear = new ArrayList<Integer>();
        Scanner sc = new Scanner(System.in); 
        System.out.println("Please enter the year the student has most recently completed: ");
        studentYear.add(sc.nextInt());
        System.out.println(studentYear);
        }
        public ArrayList<Integer> getStudentYear()
        {
            return studentYear;
            }
}

And i'm using Arraylists as i plan on making the programme loop in the end to hold multiple student values

Answer 1

Note that the studentYear variable is an instance of ArrayList<Integer> . This means it's a collection of the object Integer . This is a "boxed" wrapper around the primitive data type int .

An Integer object can be null, however an int cannot.

Given you have not added anything to this ArrayList, the the element at position zero will be null. When the VM attempts to convert this null instance of Integer to the primtive int it is throwing a NullPointerException .