Mini Java Puzzler

I was working on a project and needed to get a Long value from a String. I know the Long class (like the other wrapper classes for primitives) has static methods that do just the thing , so I started typing in my IDE and trusted that I’d find one that worked. As it turns out there are several methods that do similar conversions. Here’s a list of the candidates:

 Long.decode(String nm)
 Long.getLong(String nm)
 Long.parseLong(String nm)
 Long.valueOf(String nm)

So, given that each of these takes a String and returns a Long, what does the following code return? (Notice that the getLongValue() method returns a primitive long:

    public long getLongValue() {
      Long value = Long.getLong("23");
      return value.longValue();
  }

Here’s the list of possible answers:

 A) 23L
 B) 0L (zero)
 C) It depends.

Before you continue reading, what do you think the right answer is… OK, that’s enough thinking. Let’s take a look at the possible answers and see what seems likely.

Now it seems reasonable that Long’s getLong() method should return the value of the String as a Long, so answer ‘A’ looks pretty good. Then again, if it were that easy why would I bother posting this. Answer ‘B’ doesn’t seem very reasonable - unless getLong() isn’t really converting the value of the String. So that leaves answer ‘C’ as the primary suspect. If you’re familiar with Josh Bloch’s Java Puzzler talks from JavaOne, you’ll probably go with ‘C’ because it is kind of vague and that makes it a safe bet.

And you’d be right. The correct answer is ‘C’.

It turns out that Long.getLong() doesn’t try to convert the value of the passed in String directly. It looks for a System property with that name, retrieves the value associated with it, and converts that. So what the getLongValue() method returns depends on the value of the System property “23″. So, what if there is no System property “23″? In that case, the method throws a null pointer exception because Long.getLong() will return a null. Ouch!

I found this the hard way. I needed to convert a String and picked a likely method from the list provided by my IDE. I was greatly surprised when my unit test failed, so I looked up the JavaDoc on it and realized my mistake.

There are a few morals to this story:

   Unit tests are your friends.
 Use the JavaDoc, and make sure you write some
    for people using your code.
 Hurrying can cause unexpected delays; but those
    can become fodder for a blog post. 

Until next time, I leave you with the traditional Java Puzzler farewell,
“Don’t code like my brother.”