为什么Java泛型不允许对泛型类型进行类型转换？

Question

public class Main {

    public static <T> void foo(T[] bar) {
        double d = (double) bar[0]; // Error : incompatible types
    }

    public static void main(String[] args) {
        int[] int_buf = new int[8];
        foo(int_buf);
    }
}

The issue is indicated in the code.

Why does Java generics not allow type conversion on generic types?

Answer 1

The problem is deeper than this. Even without the line you highlighted, this program is broken:

public class Main {
    public static <T> void foo(T[] bar) {
        // do nothing
    }

    public static void main(String[] args) {
        int[] int_buf = new int[8];
        foo(int_buf);   <-- problem is here
   }
}

Java generics only support reference types as type parameters; the generic method foo() could be rewritten as follows:

<T extends Object> void foo(T[] bar) { ... }

And here's your problem: there's no T that extends Object such that an int[] is a T[] .

Secondarily, the reason the conversion also fails is that we know nothing about T , so we don't know that there exists a conversion from T to double .

Answer 2

This is because you are not specifiying what the generic type T is. So by default it will think T is an object type, not a number. It's not possible to cast an object to a double, this makes no sense.

If you change to <T extends Number> this should work just fine. Although, you might need to have an Integer array rather than a int array

Answer 3

The Java compiler erases all type parameters in generic code. A direct consequence of this is that you cannot verify which parameterized type for a generic is currently is use. If you test, instanceof will not work either. Since, runtime does not keep track of a type parameters in java, there is no difference between HashSet<Integer> and HashSet<Double> . You can do a ..instanceof HashSet<?> at the most to check if it is an instance of a HashSet.

If something is not clear in what I have written, please refer to the docs .