Generic list conversion to an array

Question

Assuming that I have the following class

public class A <T>{

    private T [] datas;
    // more code here ...
}

And I desire to take advantage of the constructor to initialize the array. Suppose that I have the following constructor

public A(T element){....}

Java does not allow me to use something like

datas = new T[10]

And it will complain that I cannot create a generic array of T But I can still use a work around like:

 @SuppressWarnings("unchecked")
    public A(T element){

       List<T> datasList = new ArrayList<T>();
       datasList.add(element);
       datas =(T[]) datasList.toArray();
    }

I have a warning from the compiler that's why I had to add the @SuppressWarnings, but my point is related to the following comment from the toArray method documentation (Please take a look at the picture)

It talks about the returned array being safe. So does that means it is safe to use this method? If not why? And what would be a better way to do such an initialisation in a constructor? I would like to also consider the case of a variable list of T elements in an overloaded constructor like

public A(T... elements){....}.

You don't have to use a list but could try (T[])Array.newInstance(element.getClass(), 10). This will still generate the warning due to the cast but you can safely ignore that as long as element.getClass() is Class<T> (which it should be in most cases - unless T is defined by something else and element would be a subclass of the defined T). — Thomas
– Thomas, Commented Nov 8, 2016 at 12:20

Thomas · Accepted Answer · 2016-11-08 12:34:06Z

2

You can create an instance of a generic array using the following:

public A(T element){
  int length = 10;
  datas  = (T[])Array.newInstance(element.getClass(), length);
}

However, there's a problem if element would be a subclass of T, e.g. if you'd call it like this:

A<Number> numberA = new A<>( Integer.valueOf(1) );

Here T would be Number but the class of element would be Integer. To mitigate that you could pass a vararg array of type T, e.g. like this:

//firstElement only exists to force the caller to provide at least one element
//if you don't want this then just use the varargs array
A(T firstElement, T... furtherElements){      
  int length = 10;
  Class<?> elementClass = furtherElements.getClass().getComponentType();      
  datas  = (T[])Array.newInstance( elementClass, length);
}

Since varargs always result in an array (even of length 0) you'll get an array of type T and can get the component type of that.

So in the case above numberA.datas would be a Number[] array and not an Integer[] array.

answered Nov 8, 2016 at 12:34

Thomas

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3 Comments

Jeroen van Dijk-Jun Over a year ago

Good point about the subclass, but in that case I would suggest adding "Class< T > clazz" instead of the T firstElement

Thomas Over a year ago

@JeroenvanDijk yes that would be the safest way but would require a different api (which might be hard to change).

Jeroen van Dijk-Jun Over a year ago

True (again) :)

Amir · Accepted Answer · 2016-11-08 14:36:55Z

0

You can pass generics, but you can't call new T (or new T[ ]). Keep in mind that generics are gone after compilation, so it actually only helps when writing the code. Knowing it's gone during runtime, it's also obvious that new T( ) can't be called as generic, T is removed in runtime.

It's safe to do, because you create that list in full control, accepting only objects of your generic type.

A nicer way (imho) is to create a static method as it is purely input-->output. You have to declare your generics before the method return type:

public < T > T[ ] toArray(T... objects) { ... }

edited Nov 8, 2016 at 14:36

Amir

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answered Nov 8, 2016 at 12:26

Jeroen van Dijk-Jun

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