Passing generic list to supertype without explicit casting

Question

I use an API which has this method:

public void doSomething(List<Object> list);

On our application side we have only one certain class say 'MyClass' that should be passed to this api method.

So for this restriction I created a method which will call the API:

public void myMethod(List<MyClass>list){ api.doSomething(list); }

Of course it doesnt compile and I cant use wild cards because I cant touch the api code. Right now I am casting the MyClass generic to Object by hand.

Is there a better solution for this?

Not sure if it will work, you can either have myMethod(List<?> list) or myMethod(List list). They will probably receive a warning but not an error, at least the second one. — orirab
– orirab, Commented Jul 19, 2019 at 8:22

josejuan · Accepted Answer · 2019-07-19 08:46:31Z

No, the API definition is "wrong" (if you want to avoid casting). A best API definition could be:

// *external* API static class API { public void doSomething(List<? extends Object> list) { throw new IllegalStateException("not implemented"); } }

Now, you can call with any ? extending Object:

List<String> xs = new ArrayList<>(); new API().doSomething(xs);

As a side note, any object extend Object then ? extends Object is redundant. This API could be rewritten as:

public void doSomething(List<?> list)

But in general, you will have something like:

public void doSomething(List<ApiDataModel> list)

Then, the proper way is again:

public void doSomething(List<? extends ApiDataModel> list)

Well, you could replace <? extends Object> with simply <?>, because they are synonymous.
Yes but to understand it I think is better my explicit version.
Of course @AndyTurner, but I want reforce the extend idea (i'll update to clarify)

bvrakvs · Accepted Answer · 2019-07-19 09:21:33Z

How about just doing something simple like;

public static void myMethod(List<MyClass> list) { doSomething(new ArrayList<>(list)); }

Unless you are removing or adding things to the list, you can use this. Otherwise since you will be instantiating a new list, any changes to it will be lost and not reflected to your list, but changes on the individual elements of MyClass objects will be.

JMSilla · Accepted Answer · 2019-07-19 08:30:57Z

You can create a List from List in this way:

public void myMethod(List<MyClass> list){ List<Object> objectList = new ArrayList<>(); objectList.addAll(list) api.doSomething(objectList); }

Or simply use a non-generic cast (this will generate a warning):

public void myMethod(List<MyClass> list){ api.doSomething((List)list); }

"Or simply use a non-generic cast (this will generate a warning):" You should point out the danger that the warning is highlighting: that doSomething is allowed to add instances of any subclass of Object to the parameter list, which could then break the type safety of other things using list.

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Passing generic list to supertype without explicit casting

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