Calling Java varargs method with single null argument?

If I have a vararg Java method foo(Object ...arg) and I call foo(null, null), I have both arg[0] and arg[1] as nulls. But if I call foo(null), arg itself is null. Why is this happening?

How should I call foo such that foo.length == 1 && foo[0] == null is true?

Solution 1:

The issue is that when you use the literal null, Java doesn't know what type it is supposed to be. It could be a null Object, or it could be a null Object array. For a single argument it assumes the latter.

You have two choices. Cast the null explicitly to Object or call the method using a strongly typed variable. See the example below:

public class Temp{
   public static void main(String[] args){
      foo("a", "b", "c");
      foo(null, null);
      foo((Object)null);
      Object bar = null;
      foo(bar);
   }

   private static void foo(Object...args) {
      System.out.println("foo called, args: " + asList(args));
   }
}

Output:

foo called, args: [a, b, c]
foo called, args: [null, null]
foo called, args: [null]
foo called, args: [null]

Solution 2:

You need an explicit cast to Object:

foo((Object) null);

Otherwise the argument is assumed to be the whole array that the varargs represents.

Solution 3:

A Test Case to illustrate this:

The Java code with a vararg-taking method declaration (which happens to be static):

public class JavaReceiver {
    public static String receive(String... x) {
        String res = ((x == null) ? "null" : ("an array of size " + x.length));
        return "received 'x' is " + res;
    }
}

This Java code (a JUnit4 test case) calls the above (we are using the test case not to test anything, just to generate some output):

import org.junit.Test;

public class JavaSender {

    @Test
    public void sendNothing() {
        System.out.println("sendNothing(): " + JavaReceiver.receive());
    }

    @Test
    public void sendNullWithNoCast() {
        System.out.println("sendNullWithNoCast(): " + JavaReceiver.receive(null));
    }

    @Test
    public void sendNullWithCastToString() {
        System.out.println("sendNullWithCastToString(): " + JavaReceiver.receive((String)null));
    }

    @Test
    public void sendNullWithCastToArray() {
        System.out.println("sendNullWithCastToArray(): " + JavaReceiver.receive((String[])null));
    }

    @Test
    public void sendOneValue() {
        System.out.println("sendOneValue(): " + JavaReceiver.receive("a"));
    }

    @Test
    public void sendThreeValues() {
        System.out.println("sendThreeValues(): " + JavaReceiver.receive("a", "b", "c"));
    }

    @Test
    public void sendArray() {
        System.out.println("sendArray(): " + JavaReceiver.receive(new String[]{"a", "b", "c"}));
    }
}

Running this as a JUnit test yields:

sendNothing(): received 'x' is an array of size 0
sendNullWithNoCast(): received 'x' is null
sendNullWithCastToString(): received 'x' is an array of size 1
sendNullWithCastToArray(): received 'x' is null
sendOneValue(): received 'x' is an array of size 1
sendThreeValues(): received 'x' is an array of size 3
sendArray(): received 'x' is an array of size 3

To make this more interesting, let's call the receive() function from Groovy 2.1.2 and see what happens. It turns out that the results are not the same! This may be a bug though.

import org.junit.Test

class GroovySender {

    @Test
    void sendNothing() {
        System.out << "sendNothing(): " << JavaReceiver.receive() << "\n"
    }

    @Test
    void sendNullWithNoCast() {
        System.out << "sendNullWithNoCast(): " << JavaReceiver.receive(null) << "\n"
    }

    @Test
    void sendNullWithCastToString() {
        System.out << "sendNullWithCastToString(): " << JavaReceiver.receive((String)null) << "\n"
    }

    @Test
    void sendNullWithCastToArray() {
        System.out << "sendNullWithCastToArray(): " << JavaReceiver.receive((String[])null) << "\n"
    }

    @Test
    void sendOneValue() {
        System.out << "sendOneValue(): " + JavaReceiver.receive("a") << "\n"
    }

    @Test
    void sendThreeValues() {
        System.out << "sendThreeValues(): " + JavaReceiver.receive("a", "b", "c") << "\n"
    }

    @Test
    void sendArray() {
        System.out << "sendArray(): " + JavaReceiver.receive( ["a", "b", "c"] as String[] ) << "\n"
    }

}

Running this as a JUnit test yields the following, with the difference to Java highlighted in bold.

sendNothing(): received 'x' is an array of size 0
sendNullWithNoCast(): received 'x' is null
sendNullWithCastToString(): received 'x' is null
sendNullWithCastToArray(): received 'x' is null
sendOneValue(): received 'x' is an array of size 1
sendThreeValues(): received 'x' is an array of size 3
sendArray(): received 'x' is an array of size 3