Self-type

[Alex Buckley]

Because the "B this" term in your method signature is a receiver 
parameter. Per JLS8 8.4.1, a receiver parameter "represents the object 
for which the method is invoked" and "exists solely to allow the type of 
the represented object to be denoted in source code, so that the type 
may be annotated." The only type permitted for a receiver parameter is 
the enclosing class, i.e., a self type but not with the generality you 
desire.

Alex

On 6/16/2015 2:23 AM, Timo Kinnunen wrote:
> How come this simple and obvious thing doesn’t work?
>
>
> <B> B init(B this, T value) {
>
>      this.item = value;
>
>      return this;
>
> }
>
>
> To me that says: “whatever type you used as a receiver, you get the same back”, as simple as that. The equivalent static method version works as expected but isn’t as convenient to use for consumers:
>
>
> static <B extends Box<T>, T> B init2_OK(B thiz, T value) {
>
>      thiz.item = value;
>
>      return thiz;
>
> }
>
>
> This similar construct works as well but requires extra discipline from the consumer:
>
>
> <B> B init3_OK(B thiz, T value) {
>
>      this.item = value;
>
>      return thiz;
>
> }
>
>
> This is the closest I have gotten to a workaround, but it is obviously not completely type-safe:
>
>
> <B extends Box<T>> B init4_OKISH(T value) {
>
>      this.item = value;
>
>      return thiz();
>
> }
>
> private <B extends Box<T>> B thiz() {
>
>      @SuppressWarnings("unchecked")
>
>      B thiz = (B) this;
>
>      return thiz;
>
> }
>
>
> A little help from the language, please?
>
>
>
>
>