java Lists getting class attributes

Question

Im unable to get the mark attribute of Student and dont understand why.

public class StudentTest
{
    public static int numberPassed(List students)
    {
        int count =0;
        for( int i = 0; i < students.size();i++  )
        {
        System.out.println(students.get(i).mark);
        }
        return count;
}

`List students` - you erased the type. Every element of the list is treated as `Object` — UnholySheep, Aug 12 '17 at 16:49
@cᴏʟᴅsᴘᴇᴇᴅ why introduce the overhead of casting?. just use generics in the first place. — Ousmane D., Aug 12 '17 at 16:51
incomplete code. where is 'students' declaration and initiation? — Jacek Cz, Aug 12 '17 at 16:53

score 0 · Accepted Answer · edited Aug 12 '17 at 17:05

In Java, every List holds objects, so operation list.get(i) will return an Object. If you want to do some action on Student class instance, you need to check if this object is a Student class instance., then cast it to Student class.

If your lists holds only students, you can add a generic type to it like so: List<Student> stidents. Then operation list.get(i) will return a Student instance. But this goes both ways, you can only add Student objects to this list.

In short, you have two choices:

Use generic list type:

public static int numberPassed(List<Student> students){
      ...
}

or cast:

Student student = (Student)students.get(i);
System.out.println(student.mark);

java Lists getting class attributes

1 Answers1