Creating ArrayList of a Class defined in a variable

Question

Suppose I have something like this

Class klazz = Integer.class;

And I'm interested in doing something like this

List<(klazz Class)> something = new ArrayList<>();

something.add(new Integer(..))
something.add(new Integer(..))
.. etc

I probably need to use reflection, yet however, I'm unsure about how to apply it.

Is it possible to do without reflection? If it's not, how would you find it suitable to implement?

Thanks in advance

Type parameters help you at compile time only. At run time, they disappear. So setting the type parameter at run time, based on different values of klazz doesn't really fit how generics work. I'd suggest just using a List<Object> and having some logic to ensure that only objects of type klazz go into it. — Dawood ibn Kareem
– Dawood ibn Kareem, Commented May 23, 2018 at 0:11
please, if possible add it as a response. I'll check it out later and assign at as response if there's nothing useful yet — Fernando Martín Besteiro
– Fernando Martín Besteiro, Commented May 23, 2018 at 0:14
List<Class> supposed to add only Class elements, new Integer(..) will not fit into this. — J-Alex
– J-Alex, Commented May 23, 2018 at 0:28

J-Alex · Accepted Answer · 2018-05-23 00:37:11Z

2

I cannot completely undertand the requirement, but this may help:

public class Main {
    public static void main(String[] args) {
        ContainerWrapper<Integer> containerWrapper = new ContainerWrapper<>(Integer.class);
        containerWrapper.add(1);
        System.out.println(containerWrapper.containerType());
        System.out.println(containerWrapper.isInteger());
    }
}

class ContainerWrapper<T> {
    // Store type information here
    private Class<T> clazz;
    private List<T> list = new ArrayList<>();

    public ContainerWrapper(Class<T> clazz) {
        this.clazz = clazz;
    }

    public void add(T element) {
        list.add(element);
    }

    public Class<T> containerType() {
        return clazz;
    }

    public boolean isInteger() {
        return clazz.isAssignableFrom(Integer.class);
    }
}

You can check T type using clazz.isAssignableFrom(*.class)

Output:

class java.lang.Integer
true

edited May 23, 2018 at 0:37

answered May 23, 2018 at 0:32

J-Alex

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3 Comments

Fernando Martín Besteiro Over a year ago

You have Integer hardcoded here: ContainerWrapper<Integer>, is there any way this could be dynamically set by a Class variable? If not then I'd have to stick to use no generics at all, but it's not considered good practice.

J-Alex Over a year ago

Only like ContainerWrapper<Object>, as was proposed in the first comment.

Mạnh Quyết Nguyễn Over a year ago

It's nothing different than List<T>.

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