尝试在泛型中使用基本类型时出现奇怪的编译时行为

Question

import java.lang.reflect.Array;

public class PrimitiveArrayGeneric {
    static <T> T[] genericArrayNewInstance(Class<T> componentType) {
        return (T[]) Array.newInstance(componentType, 0);
    }

    public static void main(String args[]) {
        int[] intArray;
        Integer[] integerArray;

        intArray = (int[]) Array.newInstance(int.class, 0);
        // Okay!

        integerArray = genericArrayNewInstance(Integer.class);
        // Okay!

        intArray = genericArrayNewInstance(int.class);
        // Compile time error:
           // cannot convert from Integer[] to int[]

        integerArray = genericArrayNewInstance(int.class);
        // Run time error:
           // ClassCastException: [I cannot be cast to [Ljava.lang.Object;
    }    
}

I'm trying to fully understand how generics works in Java. Things get a bit weird for me in the 3rd assignment in the above snippet: the compiler is complaining that Integer[] cannot be converted to int[] . The statement is 100% true, of course, but I'm wondering WHY the compiler is making this complaint.

If you comment that line, and follow the compiler's "suggestion" as in the 4th assignment, the compiler is actually satisfied!!! NOW the code compiles just fine! Which is crazy, of course, since like the run time behavior suggests, int[] cannot be converted to Object[] (which is what T[] is type-erased into at run time).

So my question is: why is the compiler "suggesting" that I assign to Integer[] instead for the 3rd assignment? How does the compiler reason to arrive to that (erroneous!) conclusion?

---

There is a lot of confusion in the two answers so far, so I created another baffling example to illustrate the underlying issue here:

public class PrimitiveClassGeneric {    
    static <T extends Number> T test(Class<T> c) {
        System.out.println(c.getName() + " extends " + c.getSuperclass());
        return (T) null;
    }
    public static void main(String args[]) {
        test(Integer.class);
        // "java.lang.Integer extends class java.lang.Number"

        test(int.class);
        // "int extends null"
    }
}

Am I the only one that thinks it's absolutely crazy that the compiler lets the above code compiles?

It wouldn't be unreasonable to print c.getSuperclass().getName() in the above code, for example, since I specified that T extends Number . Of course now getName() will throw NullPointerException when c == int.class , since c.getSuperclass() == null .

And to me, that's a very good reason to reject the code from compiling in the first place.

---

Perhaps the ultimate craziness:

    int.class.cast(null);

That code compiles AND runs fine.

Answer 1

The type of int.class is Class<Integer> , so genericArrayNewInstance() would be inferred to return a Integer[] . But the function actually creates an int[] , so it would have a class cast exception when it is returned. Basically, the cast to T[] inside the function is not legitimate in this case, because int[] is not a T[] (primitives can't be used in type variables). You cannot handle primitive array types generically; so you either have to have your method just return type Object , or you have to make separate methods for reference types and for primitive types.

Answer 2

A few points:

1. primitives are autoboxed to their object counterparts (wrappers) when needed
2. primitive arrays are objects, so they are not autoboxed.
3. generics cannot use primitives as type parameters

For your examples, here are my assumptions:

in 3 the autoboxing happens to the type parameter , but doesn't happen to the returned array
in 4 the autoboxing happens to the type parameter , but doesn't happen to the method argument , so in fact an int[] is generated, but Integer[] is expected

Autoboxing in the case of type parameter might not be exactly autoboxing , but is something with the same idea.

Update: your 2nd example has nothing wrong. int.class is a Class , so the compiler has no reason to reject it.

Answer 3

I agree with the original poster. This is crazy. Why can't I use primitive with generic? This may not be compiler problem, but it's the language's issue. Simply wrong to skip primitive types from generic.

For this:

intArray = (int[]) Array.newInstance(int.class, 0);

int.class is just a Class object. So it's ok to pass over. "int" is a type, so it's not ok because it's clearly primitive. Not to say that's the "best" way to create the language, just to adhere to the language.

This is so crazy that I can't create a wrapper for memory (array) allocation of primitives using generic. If using objects, that is so bloat for a huge collections that is wasteful. People who created the Java language/machine clearly have a bit limit in their brain. They can do it wrong the 1st time, but fixing it takes a decade, and not doing it right.