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.