java.lang.Object; cannot be cast to [Ljava.lang.Integer;at com.java.Test.Test.main(Test.java:16)

Question

public class Test {

    public static void main(String[] args) {
        System.out.println(new CountingGenerator.String(12).next());
        List<Integer> list=new ArrayList<Integer>();
        list.add(new Integer(1));
        list.add(new Integer(2));

        Integer[] c = {1,3,3};
        //throw an exception:
        c = (Integer[]) list.toArray();
    }
}

I wonder why this happened ? Integer is a subclass of Object,so it should be Ok instead! please answer me deeply! I want know why? what's the principle ?

List<Integer> list=new ArrayList<Integer>();why "list.toArray() " is an Object array???? — topCoder
– topCoder, Commented Mar 28, 2015 at 8:58
you might want to take a look at this link stackoverflow.com/questions/1115230/… — Toni
– Toni, Commented Mar 28, 2015 at 9:03
There is no automatic recursive cast to cast Arrays of type A to type B. You have to do that manually or by using dedicated methods like 'Arrays.Copyof()'. — Tom
– Tom, Commented Mar 28, 2015 at 9:08
”you can't treat a list of Integer IS-A list of Object “ thanks tom!thanks toni! — topCoder
– topCoder, Commented Mar 28, 2015 at 9:23

singhakash · Accepted Answer · 2015-03-28 09:43:59Z

4

Change the line

 c=(Integer[]) list.toArray();

to

c= list.toArray(c); // add c as parameter

In your list.toArray(); returns Object[] and JVM doesn't know how to blindly downcast Object[] to Integer[].

public Object[] toArray()      //return Object type array
public <T> T[] toArray(T[] a) //Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array

Java Docs

edited Mar 28, 2015 at 9:43

answered Mar 28, 2015 at 9:05

singhakash

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topCoder Over a year ago

you can't treat a list of Integer IS-A list of Object!

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