通用并在java中强制转换

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 .

Answer 2

to expand LastStar007's Answer

The problem with that statement is that java cannot determine the type of object E until runtime. also unfortunately, java does not provide a simple way to determine the type of a specified object (getClass() does not work for this).

your best bet (if it is numerical data) would be to check against the maximum of each mnemonic or for character data, check against the storage size per a character. yes it sounds tedious, but java has no typeinfo, like C++ which would make things a lot easier

Answer 3

It will never cause a run-time error because it will always cause a compile-time error. An Object cannot be down-cast to an E , and therefore an Object[] cannot be down-cast to an E[] . Beyond that, you can't create an array of type E because the class has no idea what the type of E is at compile-time.