How can elements be added to a wildcard generic collection?

Question

Why do I get compiler errors with this Java code?

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

Where 'SubFoo' is a concrete class that implements Foo, and Foo is an interface.

Errors I get with this code:

• On Line 3: "Cannot instantiate ArrayList<? extends Foo>"
• On Line 4: "The method add(capture#1-of ? extends Foo) in the type List<capture#1-of ? extends Foo> is not applicable for the arguments (SubFoo)"

Update: Thanks to Jeff C, I can change Line 3 to say "new ArrayList<Foo>();". But I'm still having the issue with Line 4.

Answer 1

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.