Dynamic initialization of ArrayList<anyClassObject>

Question

Normally if we want to initialize a generic non-primitive ArrayList we do this

ArrayList<?> arrayList = new ArrayList<MyClass.class>();

But I want to do something similar to this no matter which class object I pass, i.e

private void getModel(Class responseType){

   //Something similar, because this does not work..                                                       
   ArrayList<?> arrayList = new ArrayList<responseType>();
}

Any Help would be greatly appreciated.

how about this private void getModel(T extends Object){ ArrayList<T> arrayList = new ArrayList<T>(); } — vinoth
– vinoth, Commented Mar 29, 2013 at 5:29
Generics is for compile time type-safety. ArrayList<MyType> is type-erased to ArrayList after compilation. It's not possible to invoke new ArrayList<MyType>() at runtime, you have to call new ArrayList(). — Bhesh Gurung
– Bhesh Gurung, Commented Mar 29, 2013 at 5:31
@Vinoth please elaborate This doesn't work as well ArrayList<? extends responseType> arrayList = (ArrayList<? extends responseType>) new ArrayList<Class>(); — Ahmad Ali Nasir
– Ahmad Ali Nasir, Commented Mar 29, 2013 at 5:36
I already wrote that you cannot initialize a list of some type that you don't you about. Your only option is to use the non generic version ArrayList. — Bhesh Gurung
– Bhesh Gurung, Commented Mar 29, 2013 at 6:17

Festus Tamakloe · Accepted Answer · 2013-03-29 21:07:17Z

17

Try something like this

     private <T> void setModel(Class<T> type) {
      ArrayList<T> arrayList = new ArrayList<T>();
   }

If you want to get the list back then

private <T> ArrayList<T> getModel(Class<T> type) {
      ArrayList<T> arrayList = new ArrayList<T>();
      return arrayList;
   }

EDIT

A FULL EXAMPLE SHOWING HOW TO USE GENERIC TYPE FOR ARRAYLIST

Tester class with main method and the generic Method

public class Tester {

    private <T> ArrayList<T> getModels(Class<T> type) {
        ArrayList<T> arrayList = new ArrayList<T>();
        return arrayList;
    }


    public static void main(String[] args) {
        Data data = new Data(12, "test_12");
        Magic magic = new Magic(123, "test_123");

        Tester t = new Tester();

        ArrayList<Data> datas = (ArrayList<Data>) t.getModels(Data.class);
        datas.add(data);
        for(Data data2 : datas) {
            System.out.println(data2);
        }

        ArrayList<Magic> magics = (ArrayList<Magic>) t.getModels(Magic.class);
        magics.add(magic);
        for(Magic magic2 : magics) {
            System.out.println(magic2);
        }

    }

}

Another possibility to use the same things without parameter since we don't use it inside the method

public class Tester {

    private <T> ArrayList<T> getModel() {
        ArrayList<T> arrayList = new ArrayList<T>();
        return arrayList;
    }


    public static void main(String[] args) {
        Data data = new Data(12, "test_12");
        Magic magic = new Magic(123, "test_123");

        Tester t = new Tester();

        ArrayList<Data> datas =  t.getModel();
        datas.add(data);
        for(Data data2 : datas) {
            System.out.println(data2);
        }

        ArrayList<Magic> magics = t.getModel();
        magics.add(magic);
        for(Magic magic2 : magics) {
            System.out.println(magic2);
        }

    }

}

Model class (Data)

public class Data {

    private Integer id;
    private String name;


    public Data() {
    }


    public Data(Integer id, String name) {
        super();
        this.id = id;
        this.name = name;
    }


    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


    @Override
    public String toString() {
        return "Data [" + (id != null ? "id=" + id + ", " : "") + (name != null ? "name=" + name : "") + "]";
    }

}

Model class (Magic)

public class Magic {

    private Integer id;
    private String name;


    public Magic() {
    }


    public Magic(Integer id, String name) {
        super();
        this.id = id;
        this.name = name;
    }


    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


    @Override
    public String toString() {
        return "Data [" + (id != null ? "id=" + id + ", " : "") + (name != null ? "name=" + name : "") + "]";
    }

}

edited Mar 29, 2013 at 21:07

answered Mar 29, 2013 at 6:01

Festus Tamakloe

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4 Comments

Daniel Pryden Over a year ago

You don't even need the Class<T> parameter, since it's never used. Just leave it off and the type will be inferred from the call site. Of course, then you've just reinvented Guava's Lists.newArrayList() method.

Ahmad Ali Nasir Over a year ago

@Festus where you are casting the class type ? i.e Class<T> type ?

newacct Over a year ago

you are not using type. Why not declare it as private <T> ArrayList<T> getModels()?

newacct Over a year ago

@FestusTamakloe: yeah, it is possible that the compiler will have issues inferring T in some cases if you remove that parameter, but the caller can always explicitly specify it

newacct · Accepted Answer · 2013-03-29 20:49:15Z

2

This works:

private void getModel(){
   ArrayList<?> arrayList = new ArrayList<Object>();
}

I mean, it is unclear what you are trying to do. Generics is purely compile-timem, to perform compile-time type checking. Therefore, if the type parameter is not known at compile time, it would be useless.

answered Mar 29, 2013 at 20:49

newacct

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Comments

Dev Blanked · Accepted Answer · 2013-03-29 06:10:05Z

0

Try using following

public <T> List<T> getList(Class<T> requiredType) {
    return new ArrayList<T>();
}

public void useList() {
    List<Integer> ints = getList(Integer.class);
    List<String> lists = getList(String.class);
}

answered Mar 29, 2013 at 6:10

Dev Blanked

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2 Comments

newacct Over a year ago

well in this case, you are not using requiredType. Why not just declare it as public <T> List<T> getList()?

Dev Blanked Over a year ago

if you have a scenario where you are aware of the class type of the object (may be you are loading some data from a file which has class name info etc) then the above method would be useful. Because otherwise you will have to cast to the correct type with if/else checks. But you are right .. other than in such a scenario we can simply have T or simply need not have a method like this at all

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