Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer

Question

I have written a generic class and below is the constructor of the class. I want to do something like this as written in line

elements = (E[])new Object[siz         


        

Answer 1

Here is the most-minimal code necessary to reproduce your exception.

class Stack {
    protected E[] elements = (E[])new Object[1];
}

class IntStack extends Stack {
    void push(Integer i) {
        // subtly accessing elements as Integer[] which it's not
        elements[0] = i;
    }
}

Java generics are implemented with type erasure so after compilation, this code translates to something like this:

class Stack {
    protected Object[] elements = new Object[1];
}

class IntStack extends Stack {
    void push(Integer i) {
        // throws ClassCastException
        ((Integer[])elements)[0] = i;
    }
}

Clearly a new Object[] is not an Integer[]. Notice how the cast gets moved to somewhere you did not explicitly put it. This is why (E[])new Object[size] was an unchecked cast and displayed a warning.

Instead, you should use Object[] and perform the unchecked cast only when you need to return an element to the outside world.

class Stack {
    private Object[] elements;
    private int size;

    Stack(int len) {
        elements = new Object[len];
    }

    void push(E e) {
        elements[size] = e;
        size++;
    }

    E pop() {
       @SuppressWarnings("unchecked");
       E e = (E)elements[size - 1];
       size--;
       return e;
    }
}