Question

I use java 7, and I create a varargs method

public class JavaApplicationTest {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        addBundleMessage("b", Integer.SIZE, "key", (Object) null);
    }

    public static void addBundleMessage(String bundleName, Integer severity, String key, Object... params) {
        if (params == null)
            System.out.println("params Is null");
        else
            System.out.println("Params not null");
    }
}

If I don’t cast the object, the IDEs Netbeans or Eclipse complains but it compiles :

non-varargs call of varags method with inexact argument type for last
parameter

When No cast : it display params Is null

When I cast null to (Object) it display Params not null

When I cast null to (Object[]) it display params Is null

Is it normal behavior ?

Answer 1

Yes, that is “normal” behaviour.

The confusion comes from varargs being bolted on to the language later and being implemented using an array as the “real parameter” to hold the variable number of parameters.

public static void addBundleMessage(String bundleName, Integer severity,
   String key, Object... params) {

is almost the same as

public static void addBundleMessage(String bundleName, Integer severity,
   String key, Object[] params) {

and under the hoods, the params are passed wrapped as an Object[].

However, you still have the option to pass in that array yourself.

If you use (Object[]) null, you get a parameter (that is null).

If you use (Object) null, it uses the varargs method, so it passes in an array to hold the parameters (of which there is only one which is null).

For “normal use” of a varargs method, params will never be null. The “worst” you can get is an empty array (if there are no arguments at all).