Java abstract class function generic Type

Question

I created abstract class for my ArrayList holders

public abstract class ArrayHolder {
    Gson g = new Gson();
}

used like

public class MyHolder extends ArrayHolder {
    ArrayList<MyType> myList = new ArrayList<MyType>();
}

I would like to create abstract function to fill this ArrayList from String and Type argument

like this (String data is JSON in String) : This function should be rewritten

public ArrayList<T> arrayType(String data, T type){
    return g.fromJson(data, new TypeToken<ArrayList<type>(){}.getType());
}

to use it in subclass

public void fillData(String data){
    myList = arrayType(data, MyType.class);
}

How can I do this properly?

Thanks

So if I understand this right: you have a superclass C and you want it to fill an ArrayList in a subclass D through and abstract method in C? — Ryan Amos, Dec 08 '11 at 20:28
@RyanAmos right. But not fill directly, just return ArrayList of right type — Marek Sebera, Dec 08 '11 at 20:29
ah, well just use T. new ArrayList. You don't need the T type parameter. — Ryan Amos, Dec 08 '11 at 20:30

score 3 · Accepted Answer · edited Dec 08 '11 at 21:01

3

Try this...

public abstract class ArrayHolder {
    Gson g = new Gson();

    public <T> ArrayList<T> arrayType(String data){
        return g.fromJson(data, TypeToken.get(new ArrayList<T>().getClass()));
    }
}

use

public class MyCollection extends ArrayHolder {
    private ArrayList<MyType> collection;

    public void setData(String data){
        collection = arrayType(data);
    }
}

edited Dec 08 '11 at 21:01

Marek Sebera

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answered Dec 08 '11 at 20:30

John B

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Both codes throw an error `method getType() is undeclared for type ArrayList` – Marek Sebera Dec 08 '11 at 20:39
I see, please answer below on @RyanAmos answer, why is TypeToken via inner class bad? – Marek Sebera Dec 08 '11 at 20:44
NP, notice that you don't need to pass the type as a parameter to your method. I forgot to remove it. – John B Dec 08 '11 at 20:54
You got it. That's exactly How I needed it :) – Marek Sebera Dec 08 '11 at 20:57
Added example use to your answer. – Marek Sebera Dec 08 '11 at 21:02

Stanislav Levental · Answer 2 · 2011-12-08T20:43:38.990

2

public <T> ArrayList<T> arrayType(String data){
    return g.fromJson(data, new TypeToken<ArrayList<T>>(){}.getType());
}

edited Dec 08 '11 at 20:43

answered Dec 08 '11 at 20:31

Stanislav Levental

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Your answer is closest to solution, I changed type to ArrayList from List and added missing Bracket. – Marek Sebera Dec 08 '11 at 20:42

score 2 · Answer 3 · edited Dec 08 '11 at 20:50

2

Do it like this:

public <T> ArrayList<T> arrayType(String data){
     return g.fromJson(data, new TypeToken<ArrayList<T>>(){}.getType());
}

I haven't dealt with generic typed methods, but from what I've seen, this is right.

edited Dec 08 '11 at 20:50

Matt Fenwick

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answered Dec 08 '11 at 20:31

Ryan Amos

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`TypeToken` should not be created using an anonymous inner class but by using `TypeToken.get(...)` – John B Dec 08 '11 at 20:34
@JohnB what is bad on creating TypeToken via inner class? – Marek Sebera Dec 08 '11 at 20:41
You don't know what non-public abstract methods Google needs in the implementation. They made the class abstract and hid the actual implementation class as a data-hiding mechanism. That is why they provide the `get` method to retrieve a concrete instance. – John B Dec 08 '11 at 20:47

Java abstract class function generic Type

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