generate array list of dynamic type in java

Question

what I have is a super class

class Geometry{
}

and two classes that extends it:

class ShapeOne extends Geometry{}

class ShapeTwo extends Geometry{}

what I want to achieve is to generate a list(read from the database) of objects of the type ShapeOne or ShapeTwo or any other that is instance of Geometry, but dynamically passing the type as a parameter, for example like:

public ArrayList< Geometry Object > getList(/**passing typeof generated list**/){
  // getting data from Database;
  return new ArrayList<typeof generated list>();
}

so the call would be then like:
 getList(ShapeTwo);

thanks for any help :)

Sergey Kalinichenko · Accepted Answer · 2014-12-04 19:51:53Z

4

You can do it by passing Class<T>, like this:

public <T extends GeometryObject> List<T> getList(Class<T> itemClass) throws Exception {
    List<T> res = new ArrayList<T>();
    for (int i = 0 ; i != 10 ; i++) {
        res.add(itemClass.newInstance());
    }
    return res;
}

Demo.

Note: the above call to newInstance assumes that the class has a parameterless constructor.

edited Dec 4, 2014 at 19:51

answered Dec 4, 2014 at 19:46

Sergey Kalinichenko

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3 Comments

user1908375 Over a year ago

ok, than my <T> class should have a static newInstance(), right?

Sergey Kalinichenko Over a year ago

@user1908375 Class<T> already provides newInstance() method. You do need a parameterless constructor in order for this to work.

mike Over a year ago

...only if the class has a public default (no-args) constructor.

Community · Accepted Answer · 2017-05-23 12:21:27Z

You cannot do that. Due to type erasure, List<Geometry> is not a super class of List<ShapeOne>. This is better explained here: Is List<Dog> a subclass of List<Animal>? Why aren't Java's generics implicitly polymorphic?.

Some alternatives to solve your problem:

Return List<ShapeOne> or List<ShapeTwo> or List<TheExpectedClass> rather than List<Geometry>.
Return <T extends Geometry> List<T> and pass Class<T> clazz as argument. Probably you would need to use an additional parameter to recognize if the data recovered from database belongs to a specific subclass of Geometry.

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