创建一个方法来打印 java 中的子 arrayList 项目

Question

I have an abstract Person class which has 3 children: Admin , Employee and Student . There is only one admin object.

I also have a class which it's "name" is Statics and holds all public data I need, like this:

public class Statics {  

private static Person currentLoginUser;
private static ArrayList<Person> people = new ArrayList<>();
private static ArrayList<Student> students = new ArrayList<>();
private static ArrayList<Employee> employees = new ArrayList<>();
//We don't need ArrayList<Admin> because there is only one admin in the whole program

//Adder and getter methods:
//...
}

You may ask what ArrayList<Person> in the code is? When I create an object, I add it to both "it's own type arraylist" and " ArrayList<Person> ". So ArrayList<Person> has everybody.

As I said, I only have 1 admin:

Person admin = new Admin(); //`Admin` extends `Person`

I want to create a method (in Admin class) which takes an arrayList of Person as input and prints it's data. So I did this:

//Admin class:
public void printList(ArrayList<Person> people){
//Do something
}

Let's assume admin wants to see the list of the students: I call it like this:

ArrayList<Student> s = Statics.getStudents();
((Admin)admin).printList(s); //admin object was created by `Person` class so I have to cast it to (Admin) to use `Admin`'s own methods.

It (eclipse) says that:

The method printList(ArrayList< Person >) in the type Admin is not applicable for the arguments (ArrayList< Student >)

I tried to cast it to person:

((Admin)admin).printList((ArrayList<Person>)s);

this time I got this error:

Cannot cast from ArrayList< Student > to ArrayList< Person >

In this link the answer is to pass the main arrayList and check if it's object's are from type "Student" or not, then print it's value but I don't want to check the whole Person arrayList everytime, I just want a method, which takes and arrayList of Person 's children ( Employee , Student , etc) and prints them.

Answer 1

Your problem is with generics. Even though Student extends Person , List<Student> does not extend List<Person> . That's why java generics has the wildcard which is represented with a ?(question mark).

If you use List<? extends Person> List<? extends Person> that means that each element in the list is an instance of either Person or any class that extends Person .

Refer to Generics, Inheritance, and Subtypes in the Generics lesson of the Learning the Java Language trail in Oracle's java tutorials.

Answer 2

Change printList method signature to:

public void printList(ArrayList<? extends Person> people)

Explanation :

Changing the signature will allow you to pass an ArrayList parameterized with Person or its subclasses.

In java the ? extends T ? extends T construct is known as Upper Bounded Wildcards .

When using the people inside the printList method you will be able to "consume" ( get ) people from the list (but not to "produce" ( add ) them). "Consume" and "produce" are meant here in the PECS sense.

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For more information about generics see Generics (Java tutorial)

Answer 3

As mentioned by @Abra problem is with generics. You can use generics in your code to fix your problem.

Create Admin class as a generic class and create printList method with List<T> list argument so that it will accept any type of object. eg Person, Employee, Student etc.

This is just an example so please change your code accordingly.

public class Admin<T> {

  public void printList(List<T> list) {
   // Print list here
  }
}

    Statics stat = new Statics();
    Admin admin = new Admin<Student>();

    // Students
    admin.printList(stat.getStudents());
    // Employees
    admin.printList(stat.getEmployees());
    // People
    admin.printList(stat.getPeople());