Java record-44-Object array trap

Java record-44-Object array trap
Below is a very simple program, but there are unexpected traps:

    public static void main(String[] args) {        ArrayList list = new ArrayList();        list.add(new Integer(1));        list.add(new Integer(2));         //      Integer[] ins = (Integer[])list.toArray();                Object[] ins = list.toArray();        for(int i=0;i<ins.length;i++){            System.out.println(ins[i]);        }    }

 

The above program can be executed, no problem. However, the annotated line of code throws the following exception during runtime:

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer;            at my.arraylist.ArrayListTest.main(ArrayListTest.java:16)

 

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

 

List. the toArray () method returns an array of Object [], and the array of Object [] is forcibly converted to an Integer [] array. The literal understanding is correct and there is no problem during compilation, however, an exception is thrown during running. Why? The preceding example shows that the Object [] array cannot be forcibly converted to an Integer [] array. The Integer Object inherits the Object, but the Integer [] array does not inherit the Object [] array. The literal understanding does not seem to work. For a deeper understanding, the Object [] array can not only store Integer objects, but also objects such as strings. If an Object [] array stores Integer objects, string objects, and Boolean objects; if the Object [] array is forcibly converted to an Integer [] array, the string and Boolean objects cannot be converted to Integer objects. Next article: http://www.bkjia.com/kf/201510/447439.html