java.lang.ClassCastException using generics with mergeSort

Question

I got the following exception in my code:

java.lang.ClassCastException: [Ljava.lang.Comparable; cannot be cast to [Ljava.lang.Integer; at the following method:

public static void comprobar() {
    Integer a1[] = crearAleatorioInteger(100000);
    Integer a2[] = Arrays.copyOf(a1, a1.length);
    Ordenacion.quickSort(a1);
    a2 = Ordenacion.mergeSort2(a2); //Here is where the exception is thrown
    if(Ordenacion.sonIguales(a1,a2)) System.out.println("Bien");
    else System.out.println("Mal");
}

I guess the mistake must be in the method mergeSort2() which is in the class Ordenacion, it is the following:

public static <T extends Comparable<T>> T[] mergeSort2(T[] v) {
    return mergeSort2(v, 0, v.length - 1);
}
private static <T extends Comparable<T>> T[] mergeSort2(T[] v, int i, int f){
    if (f - i <= 1) {
        T[] res = (T[]) new Comparable[f-i+1];
        if(f==i) res[0] = v[i];
        else if (v[i].compareTo(v[f]) > 0){
            res[0] = v[f];
            res[1] = v[i];
        }
        else{
            res[0] = v[f];
            res[1] = v[i];
        }
        return res;
    }
    else{
        int m = (i + f) / 2;
        return merge2(mergeSort2(v, i, m),mergeSort2(v, m + 1, f));
    }
}
private static <T extends Comparable<T>> T[] merge2(T[] v1, T[] v2){
    int a = 0, b = 0, k = 0;
    T[] res = (T[]) new Comparable[v1.length+v2.length];
    while(a < v1.length && b < v2.length){
        if (v1[a].compareTo(v2[b]) < 0) res[k++] = v1[a++];
        else res[k++] = v2[b++];
    }
    while (a < v1.length) res[k++] = v1[a++];
    while (b < v2.length) res[k++] = v2[b++];
    return res;
}

Thank you for any help you may offer

BretC · Accepted Answer · 2015-04-14 16:21:24Z

2

You are creating the wrong array type and casting it to a "T[]" here...

T[] res = (T[]) new Comparable[f-i+1];

You need to create an array of type T which is not that straightforward.

As there is no way to find out the actual type of a generic type variable at runtime, you might have to do something like this...

public static <T extends Comparable<T>> T[] mergeSort2(T[] v, Class<T> type) {
    return mergeSort2(v, 0, v.length - 1, type);
}

private static <T extends Comparable<T>> T[] mergeSort2(T[] v, int i, int f, Class<T> type){
    if (f - i <= 1) {
        T[] res = (T[]) Array.newInstance(type, f-i+1);
        ...

UPDATE:

As @pbabcdefp pointed out, you don;t need to pass around the "Class" parameter. You can use this instead...

        T[] res = (T[]) Array.newInstance(v.getClass().getComponentType(), f-i+1);

This only works for arrays, if you had a List, you won't be able to get the type...

edited Apr 14, 2015 at 16:21

answered Apr 14, 2015 at 16:10

BretC

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5 Comments

Paul Boddington Over a year ago

In this case you don't need to pass a Class<T> as you can use v.getClass().getComponentType().

BretC Over a year ago

@pbabcdefp - true, I forgot that this works for Arrays (and I even answered a question like this earlier!!)

Josemafuen Over a year ago

It now says "cannot find symbol - method newInstance(java.lang.Class<capture#1 of ?>, int)

BretC Over a year ago

@JOSEMAFUEN - You are importing the "right" Array right? java.lang.reflect.Array

Josemafuen Over a year ago

Thank u so much, now it works. Though it doesn't give me the right answer at leasts there are no exceptions hahaha

user4668606 · Accepted Answer · 2015-04-14 16:07:37Z

0

You create an array of Comparable here:

T[] res = (T[]) new Comparable[f-i+1];

and try to cast it to Integer[]. Which is obviously wrong, since Comparable is no subclass of Integer and thus the cast cannot be performed. You'll have to create a new array of type T.

answered Apr 14, 2015 at 16:07

user4668606

4 Comments

Pshemo Over a year ago

"You'll have to create a new array of type T" and how can we do that?

Paul Boddington Over a year ago

That is not the line that throws the exception.

Pshemo Over a year ago

What @pbabcdefp means is that at runtime generics will be erased so T[] res = (T[]) new Comparable[f-i+1]; where <T extends Comparable<T>> is equivalent of Comparable[] res = (Comparable[]) new Comparable[f-i+1]; which means that it can't throw this exception. Problem OP is having is here a2 = Ordenacion.mergeSort2(a2); where we are trying to assign Comparable[] to Integer[]. Solution to this problem is in fact creation of array of T type, but new T[] is not valid (because of type erasure) so it would be good to add in your answer proper solution of this problem.

user4668606 Over a year ago

@pbabcdefp this is where the exception is thrown. op just didn't post the full stacktrace but the least important part of it. @Pshemo `(T[]) Arrays.newInstance(Class<T> , int)' could create a 'T[]' downside: you need to provide the class of 'T'.

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