Question

suppose i have the following code

E[] arrayVar = (E[])new Object[1];// It causes a compiler warning 

the question is, in what situation when the code will cause a run time error,

Answer 1

That code will only cause a runtime error if you try to assign the result to the reified type of E:

createArray(); // no exception
String[] arr = createArray(); // causes ClassCastException

public static <E> E[] createArray() {
    return (E[]) new Object[1];
}

The safe way to create generic arrays is with the use of Array.newInstance:

public static <E> E[] createArray(Class<E> component) {
    return (E[]) Array.newInstance(component, 1);
}

This method will not cause a ClassCastException.