Creating (boxed) primitive instance when the class is known

问题

I need a method that returns an instance of the supplied class type. Let's assume that the supplied types are limited to such that an "empty" instance of them can be created. For instance, supplying String.class would return an empty String, supplying an Integer.class would return an Integer whose initial value is zero, and so on. But how do I create (boxed) primitive types on the fly? Like this?

public Object newInstance(Class<?> type) {
    if (!type.isPrimitive()) {
        return type.newInstance(); // plus appropriate exception handling
    } else {
        // Now what?
        if (type.equals(Integer.class) || type.equals(int.class)) {
            return new Integer(0);
        }
        if (type.equals(Long.class) // etc.... 
    }
}

Is the only solution to iterate through all the possible primitive types, or is there a more straightforward solution? Note that both

int.class.newInstance()

and

Integer.class.newInstance()

throw an InstantiationException (because they don't have nullary constructors).

回答1:

I suspect the simplest way is to have a map:

private final static Map<Class<?>, Object> defaultValues = 
    new HashMap<Class<?>, Object>();
static
{
    defaultValues.put(String.class, "");
    defaultValues.put(Integer.class, 0);
    defaultValues.put(int.class, 0);
    defaultValues.put(Long.class, 0L);
    defaultValues.put(long.class, 0L);
    defaultValues.put(Character.class, '\0');
    defaultValues.put(char.class, '\0');
    // etc
}

Fortunately all these types are immutable, so it's okay to return a reference to the same object on each call for the same type.

来源：https://stackoverflow.com/questions/1728650/creating-boxed-primitive-instance-when-the-class-is-known