How can elements be added to a wildcard generic collection?
generics, java
Solution
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.
Problem
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.