单个泛型参数同时接受两种不同的类型

Question

In the following scenario I have a method which accepts 2 arrays of Type E. It was my understanding that this E parameter means that both the arrays can be any type, but they must be the same (as I derived from this question). I've tested this with two arrays, one Integer and one Double, but I'm not receiving any error. The output I receive is '14' which is the combined size of both arrays without any error thrown.

Could someone shed some light as to why this works?

public static <E> void showCombinedLength(E[] array1, E[] array2){
    System.out.println(array1.length + array2.length);
}

public static void main(String[] args) {
    Integer[] integerArray = {1, 2, 3, 4, 5, 6, 7};
    Double[] doubleArray = {1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7 };
    PrintArray.showCombinedLength(integerArray, doubleArray);
}

Output:

14

Answer 1

When you define a single generic type in the method definition then you can only use the method with two arrays of the same type.

However, in java, array types are covariant, so both Integer[] and Double[] are subclasses of Object[] (they are also subclasses of Number[]). So your code will always compile, and there is no need to specify multiple type parameters.

Answer 2

Your arrays are both arrays of objets. Call it with

PrintArray.<Integer>showCombinedLength(integerArray, doubleArray);

and the compiler will refuse to compile.

Answer 3

Generics were introduced to Java in version 1.5. Due to the backward compability Java byte code doesn't know that some method is generic and another not. This is why after compilation your method really is:

public static void showCombinedLength(Object[] array1, Object[] array2)

All generic types are really changed to Object . So both Integer[] and Double[] are really arrays of objects which have length method. This is why this code works.