Question

在Java中学习聚合时，我遇到了 NULLPointerException ，请帮我确定根本原因。有3个班级。

学生
系。
机构。

package com.java.inheritance; import java.util.List; import java.io。*;
```
iTLB-loads
```

包com.java.inheritance;

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;
    }


}

现在我在另一个类MainClass中调用它们。

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

但此时我得到 NULLpointerException 。我哪里做错了。

Answer 1

您需要在添加列表之前创建列表。

ArrayList

这里你要添加一个空列表，这会导致异常。尝试创建新的LinkedList或List<Student> sciencestudents = new ArrayList<>(); List<Student> artstudents = new ArrayList<>();。

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

同样，您还需要创建一个部门列表。

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在Java聚合中面对NULL指针异常

1 个答案: