changé le type de collection de contactlist à List

ContactList.java

 import java.util.ArrayList;
+import java.util.List;
 import java.io.Serializable;
 
 class ContactList implements Serializable
 {
-    //private Contact[] contacts = {};
-    private ArrayList<Contact> contacts = new ArrayList<>();
+    private List<Contact> contacts = new ArrayList<>();
     private static final long serialVersionUID = 1234567890L;
 
     public ContactList(){}