Question

Take the following generics example

import java.util.List; import java.util.ArrayList;  public class GenericsTest {     private List<Animal> myList;      public static void main(String args[]) {         new GenericsTest(new ArrayList<Animal>()).add(new Dog());     }      public GenericsTest(List<Animal> list) {         myList = list;     }      public void add(Animal a) {       myList.add(a);     }      public interface Animal {}     public static class Dog implements Animal {}     public static class Cat implements Animal {} } 

It works fine. But as you know, you cannot construct it with

new GenericsTest(new ArrayList<Dog>()); 

because, as you know, the add(Animal) would make possible to add Cats. The suggested way of solving this problem, i.e. wildcarding does not work either, because, yes, you can change every List<Animal> in List<? extends Animal> but it has the same problem: you can create the GenericsTest with List<Cat> and then add Dogs.

So my question is: is there a convenient way to write this class once, and then use it for all the possible Animals? Of course it should solve straightforwardly the above mentioned problem.

Answer 1

If I understand what you’re trying to do then you need to put the generic type at class level:

public class GenericsTest<T extends Animal> {   private List<T> myList;    public static void main(String args[])   {     new GenericsTest<Dog>(new ArrayList<Dog>());   }    public GenericsTest(List<T> list)   {     myList = list;   }    public void add(T a)   {     myList.add(a);   } }