How to make a function that returns a generic object?

Question

I am trying to make a function that returns a T/Any object. The only thing is that i am not quite sure how to do it.

 fun readMockData(context: Context, filename: String): Any {
    val json = context.assets.open("$filename.json").bufferedReader().use(BufferedReader::readText)
    return RetrofitSingleton.GSON.fromJson(json, object : TypeToken<Any>() {}.type)
}

This is what i tried at first, but i am getting an error. How am i supposed to make this function properly?

<T> and Any are not the same thing. What exactly are you trying to achieve? — AlexT
– AlexT, Commented Jun 11, 2021 at 9:20
i try to return a generic object that i just can cast to whatever object i need. — Meyben
– Meyben, Commented Jun 11, 2021 at 9:23
i get a object from this json, turns it to the right object and return it normally. But i make the same function that look so similar so many times, so i wondered if i could make something more generic? if that makes sense — Meyben
– Meyben, Commented Jun 11, 2021 at 9:25

broot · Accepted Answer · 2021-06-11 09:34:30Z

2

If I understand your problem correctly, this is what you need:

@OptIn(ExperimentalStdlibApi::class)
inline fun <reified T> readMockData(context: Context, filename: String): T {
    val json = context.assets.open("$filename.json").bufferedReader().use(BufferedReader::readText)
    return RetrofitSingleton.GSON.fromJson(json, typeOf<T>().javaType)
}

Note it has to be inline, because otherwise there is no way to acquire a type. If your function is longer than above, it may be a good idea to split it into an inline reified wrapper and the main function that accepts Type as a parameter.

It uses reflection, so you need to add a dependency to it. Also, typeOf() is experimental, but from my experience it just works (at least on JVM) and it is there for a long time, so I guess it won't change.

edited Jun 11, 2021 at 9:34

answered Jun 11, 2021 at 9:29

broot

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1 Comment

broot Over a year ago

In the meantime I noticed I probably missed .javaType in my original answer. I edited it, but please confirm if you needed to add it, so the answer will be the correct one (for future readers). @Meyben

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