Bill the Lizard shows us a nice trick to avoid this NPE when used with string literals : call the method on the string literal. It's possible because it's also an object, and cannot be null.

But, as much as this trick code seems sweet, but it just feels like a red herring on the long run, First it only works for comparing strings and even only when comparing a string with a string literal.

To Equality ...

It is what he said at the end that is much more interesting :

The lesson to be learned here is that if you have to resort to "clever" tricks to get something done, with just a little bit of lateral thinking you can probably find a cleaner, simpler way.

Let's take this lesson even further.

I had a Perl & C++ background before coming to the Java world, and I am sometimes puzzled that Java makes things quite unnecessary complicated for the programmer from time to time.

My little suggestion : create a quite simple helper static function :

static boolean isEqual(String a, String b) {
   if (a == b) return true;

   // a & b cannot be null at the same time
   if (a == null || b == null) return false;

   // Now none can be null
   return a.equals(b);
}

Now, the NPE is avoided for any kind of string, with any order. We could also have a smarter equality comparison function by first converting to a string, and comparing the resulting string instead of comparing the original numbers.

static boolean isEqualSmart(Object a, Object b) {
   if (a == b) return true;

   // a & b cannot be null at the same time
   if (a == null || b == null) return false;

   // Now none can be null
   return a.toString().equals(b.toString());
}

With the wonders of Java's function overloading, you can even write specialized functions that convert even more smartly their arguments. And then be able to compare a string with a number (even a primitive type).

... and Beyond

We began about testing equality, but for comparison such as <= or >, you have to decode the output of the quite ugly compareTo() method. The helper static function trick comes also handy here.

First, we design a complete API that enable alphanumeric, and numeric comparison : the so-called smart comparison. The API should take any Object, or even primitive types, and convert it at will to enable hybrid comparison. This will greatly enhance the comparisons abilities of Java.

class OpAlpha {

   // Alphanumeric comparison
   public static boolean lt(Object a, Object b) { 
      /* convert to string and compare */ 
   }
   public static boolean gt(Object a, Object b) { 
      /* convert to string and compare */ 
   }

   // ... rest of the implementation ...
}


class OpNum {

   // numeric comparison
   public static boolean lt(Object a, Object b) { 
      /* convert to numeric and compare */ 
   }

   // ... rest of the implementation ...
}

class Op {
   // smart comparison, depends on the 2nd argument type
   public static boolean lt(Object a, int b) {
      /* convert to int and compare */
   }
   public static boolean lt(Object a, String b) {
      /* convert to string and compare */
   }
   public static boolean lt(Object a, Number b) {
      /* convert to Number and compare */
   }

   // ... rest of the implementation ...
}

The usage is like this :

String a = "124";
assertTrue( Op.eq(a, 124) );
assertFalse( Op.lt(a, 1) );
assertTrue( OpAlpha.lt(a, 1) );

We can see that all the tedious part of handling the conversions is done under the hood. Choosing the comparison to is just a matter of choosing the right class. A little more work could even be done to take the Op classes to implement Comparator in a complete OO way in addition of the static helpers.

Update (01/06/2009)

It seems that equality in Java is quite hazardous, and that you have to be extra careful not to set a comparison-land-mine off by accident.