Understanding Java Varargs Behavior with Null and No Arguments

What is the behavior of Java's varargs when passing null as an argument or when no arguments are passed?

boolean testNull(String... string) {
    if(string == null) {
        return true;
    } else {
        System.out.println(string.getClass());
        return false;
    }
}

boolean callTestNull(String s) {
    return testNull(s);
}

In Java, varargs (variable-length arguments) allow passing a variable number of arguments to a method. However, the behavior feels counterintuitive when dealing with null and no arguments. Here, we address the nuances of how varargs interpret these cases.

// Example demonstrating varargs with null and empty
@Test
public void test_cases() {
    assertTrue(instance.testNull(null)); // Expect true for null
    assertFalse(instance.testNull()); // Expect false for empty array
    assertFalse(instance.callTestNull(null)); // Expect false, interpreted as empty array
}

Causes

• When a method with varargs is called with no arguments, Java assigns an empty array instead of null.
• Passing null can be ambiguous; it depends on how it's passed: as a direct argument to the varargs method or through another method.

Solutions

• Understand the distinction between calling the varargs method directly with null and through another method that accepts a single parameter.
• To check for null explicitly, ensure your varargs method correctly interprets null through direct method signatures.

Mistake: Confusing null with an empty array when calling a varargs method indirectly.

Solution: Use a direct varargs method call to clarify behavior.

Mistake: Assuming that passing null yields consistent results across all methods.

Solution: Be aware of method signatures and how arguments are being passed.