55079

Why does undefined Generics types of Collections defaults to Object even if bound to other class?

class Test<G extends String>{ public G test(){return null;} public List<G> tests(){return new ArrayList<>();} } public void doTest(Test t){ //works fine String str = t.test(); //Compile error: expected String found Object str = t.tests().iterator().next(); }

I would like the last line to return a String instance instead of Object, as the type G was bound to subclass String. There is any other way than casting ?

Answer1:

No, the type as been erased. You're running into two different rules for type erasure - from JLS section 4.6:

    <li>The erasure of a parameterized type (§4.5) G is |G|.</li> <li>The erasure of a type variable (§4.4) is the erasure of its leftmost bound.</li> </ul>

    So the erasure of List<G> is List, but the erasure of G is String - that's why the first assignment works.

    All you need to do to get this code to compile is use a wildcard in your parameter:

    public void doTest(Test<?> t)

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