Read and save list of Object using list of interface

I have seen someone's code and unable to understand how it works.

As the code save List of object in list of Interfaces and can read it back.

Please explain how it works. And can we save Object in interface (where Object implements it). AFAIK interface doesn't have members. How this code save and retrieve please explain.

ContactBO implementing BusinessObject

public class ContactBO implements BusinessObject {

    private String id, fullName, department, ;

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getFullName() {
        return fullName;
    }

    public void setFullName(String fullName) {
        this.fullName = fullName;
    }

    public String getDepartment() {
        return department;
    }

    public void setDepartment(String department) {
        this.department = department;
    }

    @Override
    public boolean isSection() {
        // TODO Auto-generated method stub
        return false;
    }

}

Where BussinessObject

public interface BusinessObject {
    boolean isSection();
}

Saving list of Contact code

List<ContactBO> contactList = new Gson().fromJson(jsonArray.toString(),
                new TypeToken<List<ContactBO>>() {}.getType());
        List<BusinessObject> list = new ArrayList<BusinessObject>();
        list.addAll(contactList);

read List of ContactBO using List of Interface

List<ContactBO> contactListItems = (List<ContactBO>) (List<?>) result.list;//where list is public List<BusinessObject> list;