coding style - What is the proper way to use a .equals method in Java?

Question

I was talking with my CompSci professor and he suggested that all String .equals methods be written as:

"Hello World".equals(fooString);

rather than:

fooString.equals("Hello World");

Both of these lines compile, but I was wondering what are the benefits of the first way? I've always done it the latter way. Is this wrong? What is common/conventional?

Answer 1

The first way guarantees NullPointerException is never thrown, while the second one may risk getting NullPointerException if the fooString happens to be null.

However, in the end, it all boils down to the requirement. If the requirement specifies that the null case should not happen at all, or should be treated differently, then writing in the 2nd way (fooString.equals()) helps you detect the implementation flaw. If you can treat null as just another case, then the 1st way is fine.