Java聚合中面临空指针异常

Question

在学习Java聚合时，我遇到了NULLPointerException，请帮我找出根本原因。 有三节课

1. 学生
2. 部门

3. 机构

   包裹通讯。JAVA遗产 导入java。util。列表 导入java。io.*

   public class Department {
   
       private String name;
       private List<Student> listofstudents;
   
       Department(String name,List<Student> listofstudents){
   
           this.name =name;
           this.listofstudents = listofstudents;
       }
   
       public List<Student> getStudents(){
   
           return this.listofstudents;
       }
   
   
   }
   

包裹通讯。JAVA继承

public class Student {

    private String name;
    private int id;
    private String Department;

    Student(String name,int id,String Department){

        this.name =name;
        this.id= id;
        this.Department = Department;

    }

}



    package com.java.inheritance;
    import java.util.List;

    public class Institute {

       private String institureName;
       private List<Department> listofDept;

      public Institute(String institureName,List<Department> listofDept){
           this.institureName = institureName;
           this.listofDept = listofDept;
       }

      public List<Department> getDepartments(){
          return this.listofDept;
      }

      // Get total number of Student in a Institute.

      public int gettotalnoofStudents(){
          int numberofStudent = 0;
          List<Student> listofStudent=null;
          for (Department dept:listofDept){
              listofStudent = dept.getStudents();
              for (Student student:listofStudent){

                  numberofStudent++;  
              }

          }

          return numberofStudent;
      }


    } // End of Institute

现在我在另一个班级叫他们MainClass

package com.java.inheritance;
import java.util.List;

public class MainClass {

    public static void main(String[] args) {

        try{  
        List<Student> sciencestudents=null;
        List<Student> artstudnets=null;
        Student s1= new Student("Asfakul",1,"Science");
        Student s2=new Student("Arizul",2,"Science");
        Student s3=new Student("Noor",3,"Arts");

        sciencestudents.add(s1);
        sciencestudents.add(s2);
        artstudnets.add(s3);


        Department d1=new Department("Science",sciencestudents);
        Department d2=new Department("Arts",artstudnets);

        List<Department> departmentlist=null;
        departmentlist.add(d1);
        departmentlist.add(d2);

        Institute i1= new Institute("BEC",departmentlist);
        System.out.println(i1.gettotalnoofStudents());
        }catch(Exception e){
            e.printStackTrace();    
        }
    }
}

但是现在我得到了空点异常。我哪里做错了

Answer 1

在添加列表之前，需要先创建列表

    List<Student> sciencestudents=null;
    List<Student> artstudnets=null;
    Student s1= new Student("Asfakul",1,"Science");
    Student s2=new Student("Arizul",2,"Science");
    Student s3=new Student("Noor",3,"Arts");

    sciencestudents.add(s1);
    sciencestudents.add(s2);
    artstudnets.add(s3);

在这里，您将添加到一个空列表，这将导致异常。尝试创建一个新的ArrayList或LinkedList

List<Student> sciencestudents = new ArrayList<>();
List<Student> artstudents = new ArrayList<>();

同样，你也需要创建一个部门列表

List<Department> departmentlist = new ArrayList<>();