Creating a Generic Array

Question

Im having a big problem with this code: I need to create my own Merge Method without using java´s Merge Method.

public static <T extends Comparable> T[] merge(T[] a, T[] b){
    T[] c = (T[]) new Object[a.length + b.length];  

    /*
     *
     * More code
     *
     *
    */
    return c;
}

The problem is that in runtime I have this error: java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable; I have already looked for this problem, and the solution is using Reflection.

The problem i have it in this line: T[ ] c = (T[ ]) new Object[a.length + b.length];

The problem is that I dont understand how to use it, have already try using reflection in many different ways but i can´t solve this.

I really appreciate any answer and tell me where should I change my code.

How about `Arrays.copyOf`? http://stackoverflow.com/a/8220245/303939 — Ming-Tang, Mar 24 '15 at 02:27

score 4 · Answer 1 · answered Mar 24 '15 at 02:31

You could create an array of the same type as the input arrays, using reflection - something like this:

if(a.getClass() != b.getClass()) // or .equals if you prefer. It doesn't matter for Class objects
    throw new IllegalArgumentException("Arrays don't have the same type");

T[] c = Array.newInstance(a.getClass().getComponentType(), a.length + b.length);

Note that it is impossible to use something like T.class. Note also that this won't work as expected for cases like:

Comparable[] result = <Comparable>merge(new Integer[] {1, 2, 3}, new Integer[] {4, 5, 6});

as the result will be an Integer[] instead of a Comparable[]. However, it is the closest you can do without passing Comparable.class to the function, or pre-allocating the result array before calling it.

Or you could use Arrays.copyOf as suggested by Ming-Tang, and then fill the array with null (otherwise it starts with a copy of a):

if(a.getClass() != b.getClass()) // or .equals
    throw new IllegalArgumentException("Arrays don't have the same type");

T[] c = Arrays.copyOf(a, a.length + b.length);
Arrays.fill(c, null);

This has the same issues.

Because of the bizarre and unnatural behaviour of arrays, you should choose the "greatest common superclass". (Code for which is left as an exercise for the interested reader.) When I say "should", you should follow @immibis' advice and use `List`. — Tom Hawtin - tackline, Mar 24 '15 at 03:03
@TomHawtin-tackline Note that I wrote both answers, because either one could be best for the OP's application. (Although one would certainly hope, for his/her sanity, that the List one works) — user253751, Mar 24 '15 at 03:09
Heh, didn't realise it was the same person. Reference arrays were badly messed up (not that `System.arraycopy` is a nice way of dealing with arrays). — Tom Hawtin - tackline, Mar 24 '15 at 03:14
you need to cast the result of `Array.newInstance()` to `T[]` — newacct, Mar 24 '15 at 19:24

score 2 · Answer 2 · answered Mar 24 '15 at 02:33

You could just use List instead.

public static <T extends Comparable> List<T> merge(List<T> a, List<T> b){
    List<T> c = new ArrayList<T>(a.size() + b.size());

    /*
     *
     * More code
     *
     */
    return c;
}

This might not be an option if some other part of your code forces you to use arrays in merge.

Creating a Generic Array

2 Answers2