Downcasting from Object to Integer Runtime Error: java.lang.ClassCastException

Run time exception-- java.lang.ClassCastingException...

Integer intArr[] = new Integer[arrList.size()];
ArrayList <Integer> arrList =new ArrayList();
intArr=(Integer[])arrList.toArray(); // returns Object class which is downcaste to Integer;

I understand down-casting is not sa开发者_Python百科fe but why is this happening? I also tried to converting ArrayList to String to Integer to int, but I get the same error.

Try to do this

intArr = arrList.toArray(new Integer[arrList.size()]);

What you get is a typed Integer Array and not a Object array.

First of all, this doesn't bind the ArrayList to type Integer.

ArrayList <Integer> arrList =new ArrayList();

Instead, this is what happens, arrList is assigned to an ArrayList of raw type, but that isn't a problem.

The problem lies in,

intArr=(Integer[])arrList.toArray();

since arrList is a raw-type (due to the assignment, it gets assigned as new ArrayList<Object>() by the compiler), you're effectively getting an Object[] instead.

Try assigning arrList to new ArrayList<Integer>() and do this:

intArr = arrList.toArray(new Integer[arrList.size()]);

The problem here is that you are trying to convert an array of objects to an array of integers. Array is an object in itself and Integer[] is not a sub-class of ArrayList, nor vice versa. What you have to do in your case is cast individual items, something like this:

Integer intArr[] = new Integer[arrList.size()];
for(int i=0; i<intArr.length; i++)
{
    intArr[i] = (Integer)arrList.get(i);
}

Naturally, you may get ClassCastException if individual elements in the array list are not of type Integer.

toArray(T[] a) takes a paramter: "a - the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new array of the same runt"