Generic array creation via own class

Question

To create array of generic, I have seen below recommendation in many websites including this one, too. However, I wonder why we do not use directly the generic class in array creation. What is the reason to push us to not use below array creation method?

What currently is used

public class X<T>  {

    private Y<T> tTypeClass = null;

    public T[] Array() {       
        T[] array = (T[]) Array.newInstance(tTypeClass.getObject().getClass(),4); 
    }
}

Why we do not use

public class X<T>  {

    public T[] arrayCreation() {       
        T[] array = (T[]) Array.newInstance(this.getClass(),4); 
    }
}

David Xu · Accepted Answer · 2014-07-29 09:00:13Z

1

You can't write code like T[] arr = new T[xx] because Java generics are type-erased. The Runtime has no idea what T is during runtime, because the Java compiler replaced all generics with casts at compile time.

Also,

public class X<T>  {

    public T[] arrayCreation() {       
        T[] array = (T[]) Array.newInstance(this.getClass(),4); 
    }
}

will cause a ClassCastException because it creates an array of X[] and tries to cast it to T[].

answered Jul 29, 2014 at 9:00

David Xu

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Comments

sp00m · Accepted Answer · 2014-07-29 09:01:02Z

0

Because the cast is not safe. For example:

String[] s = new X<String>().arrayCreation();

will result in java.lang.ClassCastException.

answered Jul 29, 2014 at 9:01

sp00m

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