OnJava8-Examples/generics/CheckedList.java

// generics/CheckedList.java
// (c)2016 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Using Collection.checkedList()
import typeinfo.pets.*;
import java.util.*;

public class CheckedList {
  @SuppressWarnings("unchecked")
  static void oldStyleMethod(List probablyDogs) {
    probablyDogs.add(new Cat());
  }
  public static void main(String[] args) {
    List<Dog> dogs1 = new ArrayList<>();
    oldStyleMethod(dogs1); // Quietly accepts a Cat
    List<Dog> dogs2 = Collections.checkedList(
      new ArrayList<>(), Dog.class);
    try {
      oldStyleMethod(dogs2); // Throws an exception
    } catch(Exception e) {
      System.out.println("Expected: " + e);
    }
    // Derived types work fine:
    List<Pet> pets = Collections.checkedList(
      new ArrayList<>(), Pet.class);
    pets.add(new Dog());
    pets.add(new Cat());
  }
}
/* Output:
Expected: java.lang.ClassCastException: Attempt to insert
class typeinfo.pets.Cat element into collection with
element type class typeinfo.pets.Dog
*/