Why am I getting a ClassCastException when converting an arrayList of objects to an array of the same object?

Question

So I'm not too experienced with these sort of things, and its also been a long day so I'm probably missing something obvious, but this is what is causing my error. Here is the error message in its entirety along with the lines that cause the error.

Exception in thread "main" java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class [LenumAssignment.Student; ([Ljava.lang.Object; is in module java.base of loader 'bootstrap'; [LenumAssignment.Student; is in unnamed module of loader 'app')

ArrayList<Student> s = nameObtain();
Student[] students = (Student[]) s.toArray();

Could you please provide a few more details? How is nameObtain() definied? Somewhere you try to convert "Object" to "Student" and that fails. — YuTendo
– YuTendo, Commented Oct 23, 2020 at 22:18
Possible duplicate of this. toArray returns an Object[], so you'll need s.toArray(new Student[0]) or something like that — user
– user, Commented Oct 23, 2020 at 22:22

Nowhere Man · Accepted Answer · 2020-10-23 22:58:27Z

1

Method List::toArray() returns Object[] which cannot be simply cast to Student[] (explained here)

So you have ~~two~~ three options:

Get Object[] arr and cast its elements to Student
Use typesafe List::toArray(T[] arr)
Use typesafe Stream::toArray method.

List<Student> list = Arrays.asList(new Student(), new Student(), new Student());
Object[] arr1 = list.toArray();
        
for (Object a : arr1) {
    System.out.println("student? " + (a instanceof Student) + ": " + (Student) a);
}
        
Student[] arr2 = list.toArray(new Student[0]);
        
System.out.println(Arrays.toString(arr2));

Student[] arr3 = list.stream().toArray(Student[]::new);
System.out.println(Arrays.toString(arr3));

edited Oct 23, 2020 at 22:58

answered Oct 23, 2020 at 22:51

Nowhere Man

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