空通用类型的Java通用数组

Question

Consider the Array.newInstance static method as a way of creating a generic type array in Java. What I'm failing to see how to do is create a generic array from a null generic type argument:

/**
* Creates and fills a generic array with the given item
*/
public static <T> T[] create(T item, int length)
{
   T[] result = (T[]) Array.newInstance(item.getClass(), length);

   for(int i = 0; i < length; i++)
      result[i] = item;

   return result;
}

The above works when I call eg create("abc", 10); I'm getting a String[] of length 10 with "abc" in all positions of the array. But how could I make a null string argument return a String array of length 10 and null in all positions?

eg

String nullStr = null;
String[] array = create(nullStr, 10); // boom! NullPointerException

Is there perhaps a way to get the class of "item" without using one of its members (as it's null)?
I know I can just new up an array String[] array = new String[10] , but that's not the point.

Thanks

Answer 1

Maybe this is useful.

public static <T> T[] create(Class<T> clazz, int length)
{
   T[] result = (T[]) Array.newInstance(clazz, length);
   return result;
}

Answer 2

As you point out, you can't call:

create(null, 10)

because your code can't determine the type (it ends up calling null.getClass() => NPE).

You could pass the class separately:

public static <T> T[] create(Class<T> type, int length, T fillValue)
{
   T[] result = (T[]) Array.newInstance(type, length);

   for(int i = 0; i < length; i++)
      result[i] = fillValue;

   return result;
}

// Some optional convenience signatures:

public static <T> T[] create(Class<T> type, int length) {
    return create(type, length, null);
}

public static <T> T[] create(T fillValue, int length) {
    if (fillValue == null) {
        throw new IllegalArgumentException("fillValue cannot be null");
    }
    return create(fillValue.getClass(), length, fillValue);
}

public static void main(String[] args) {
    String[] a = create(String.class, 10, null);
}

Answer 3

Well, why not change the method to take a Class object instead, and just directly pass in String.class ? Java can't get the class type of null because it has no class type! Any object can be null.