suggest helper method for this Java Generics piece of code please

Question

I accidentally wrote this piece of code .

   static List<Integer> foo(List<Integer> fillthis,List<? super Integer> readlist){
      for(Object i:readList){
        if(i instanceof Integer)
            fillthis.add(i);
      }
      return fillthis;
    }

As you can see, it basically takes a list of Integer and another List of Integer or any of its supertype, say a List of Object; Then from the 'readList' it extracts every int value and puts it into the given int List 'fillthis' I used List because even a List or List can contain an integer

Ofcourse, it won't compile because compiler won't let an unverified value from 'readList' into a pure int List 'fillthis' . So it ends up giving me typical error :

actual argument Object cannot be converted to Integer by method invocation conversion

But, tend to think of it, this is a perfectly reasonable piece of code if only one can get around this with the help of helper methods Can someone please help ? I have tried my wits ends

THANK YOU VERY MUCH :)

EDIT ------------------

Thanks Meindratheal, but can anyone suggest a helper method for this one , incase if I don't wish to typecast because a helper method like

static <T,U super T> List<T> foo(List<T> fillthis,List<U> readList);

won't work as generics don't allow 'super' in type parameter list. it only allows extends also ,is there even a helpermethod for this one.

Please do suggest :)

UPDATE

Braj, I don't know how it affects what kind of list I am passing! Ofcourse, I could follow your advice and pass on a List and TBH, this code isn't a piece of any serious software design BUT DEAR, this very simple question does challenge our knowledge of java generics

Maybe read my code again above and I guess you know already what my question is

Ok fine, lets say I have

public static void main(String[] args){
   List<Integer> lint=Arrays.asList(1,2,3);
   List<Object> lobj=new List<Object>();
   lobj.add(new Object());
   lobj.add(new Integer(4)); /*will compile perfectly. had I used List<? super Integer> instead of List<Integer>, I can't pass lobj to extract this value of 4. Clear?? */
   foo(lint,lobj);
}

The question is simple :Can we create a helper method to make this method work exactly like it is ??

Hi @Kayaman thanks for taking interest in answering my question :) The thing is it takes first parameter as List of Integer and second parameter is list of Integer OR list of any supertype of Integer[say a Number or Object] because a list of Number or Object can also contain an integer value I hope its clear now — Sarabjeet, May 03 '14 at 14:55

score 1 · Answer 1 · answered May 03 '14 at 14:50

1

You have to cast i to Integer:

static List<Integer> foo(List<Integer> fillthis,List<? super Integer> readlist){
    for(Object i:readList){
        if(i instanceof Integer)
            fillthis.add((Integer) i);
    }
    return fillthis;
}

answered May 03 '14 at 14:50

Meindratheal

158
2
6

Meindratheal , thanks for answering. I am sorry I couldn't give your answers a +1 as I don't have required points. also I am wondering now what Kayaman meant by 'not good generic code' ? is he referring to my code or yours ? @Kayaman please answer :D – Sarabjeet May 03 '14 at 14:59

Braj · Answer 2 · 2014-05-03T15:26:37.003

0

use List<Number> instead of List<? super Integer> to make it more clear.

Simply use in this way.

static List<Integer> foo(List<Integer> fillthis, List<Number> readlist) {
    for (Number i : readlist) {
        if (i instanceof Integer)
            fillthis.add(i.intValue());
    }
    return fillthis;
}

edited May 03 '14 at 15:26

answered May 03 '14 at 15:19

Braj

46,415
5
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What do you understand by `? super Integer`? Why are you using it? What type of values do you have in `readlist`? – Braj May 03 '14 at 15:28
Hi Braj,well I used List super Integer> in place of List because List super Integer> basically means List – Sarabjeet May 03 '14 at 15:30
@Sarabjeet OK your requirement is clear now. I hope you got the answer too. – Braj May 03 '14 at 15:31
actually I didn't the way I wanted to HELPER METHOD ANYONE ? :) – Sarabjeet May 03 '14 at 15:43
Dear, I already knew that even before asking for help here You keep asking me to narrow down the scope of my function. I am just asking you if you can suggest me a helper method for same :D I hope you're aware of helper methods in java generics – Sarabjeet May 03 '14 at 15:55
@Sarabjeet Let me try to find out. Working on it. – Braj May 03 '14 at 15:57
Braj, what you think ? is a helper method for my method possible ?? like for instance this simple example `void foo(List> i) {` `i.set(0, i.get(0));` `}` will not compile So helper method : `private void fooHelper(List l) { l.set(0, l.get(0)); }` and invoked by original method as `void foo(List> i) { fooHelper(i); }` **Yes please work on it. Hopefully waiting for your answer :)** – Sarabjeet May 03 '14 at 15:58
Can you provide some sample code. What type of List are you passing in helper method? Add it in your post. – Braj May 03 '14 at 16:03

suggest helper method for this Java Generics piece of code please

2 Answers2