Java Language Arrays Array Covariance
Example
Object arrays are covariant, which means that just as Integer is a subclass of Number, Integer[] is a subclass of Number[]. This may seem intuitive, but can result in surprising behavior:
Integer[] integerArray = {1, 2, 3};
Number[] numberArray = integerArray; // valid
Number firstElement = numberArray[0]; // valid
numberArray[0] = 4L; // throws ArrayStoreException at runtime
Although Integer[] is a subclass of Number[], it can only hold Integers, and trying to assign a Long element throws a runtime exception.
Note that this behavior is unique to arrays, and can be avoided by using a generic List instead:
List<Integer> integerList = Arrays.asList(1, 2, 3);
//List<Number> numberList = integerList; // compile error
List<? extends Number> numberList = integerList;
Number firstElement = numberList.get(0);
//numberList.set(0, 4L); // compile error
It's not necessary for all of the array elements to share the same type, as long as they are a subclass of the array's type:
interface I {}
class A implements I {}
class B implements I {}
class C implements I {}
I[] array10 = new I[] { new A(), new B(), new C() }; // Create an array with new
// operator and array initializer.
I[] array11 = { new A(), new B(), new C() }; // Shortcut syntax with array
// initializer.
I[] array12 = new I[3]; // { null, null, null }
I[] array13 = new A[] { new A(), new A() }; // Works because A implements I.
Object[] array14 = new Object[] { "Hello, World!", 3.14159, 42 }; // Create an array with
// new operator and array initializer.
Object[] array15 = { new A(), 64, "My String" }; // Shortcut syntax
// with array initializer.
