How can elements be added to a wildcard generic collection?

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Use this instead:

1  public List<? extends Foo> getFoos()
2  {
3    List<Foo> foos = new ArrayList<Foo>(); /* Or List<SubFoo> */
4    foos.add(new SubFoo());
5    return foos;
6  }

Once you declare foos as List<? extends Foo>, the compiler doesn’t know that it’s safe to add a SubFoo. What if an ArrayList<AltFoo> had been assigned to foos? That would be a valid assignment, but adding a SubFoo would pollute the collection.

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