String equality in Java

I have seen both of these when checking the equality of two Java String's:

// Method A
String string1;
// ...
if("MyString".equals(string1)) {
    // ...
}

and

// Method B
String string1;
// ...
if(string1.equals("MyString")) {
    // ...
}

My question is: which one is better and mor开发者_运维问答e widely used?

If you are sure that string1 can never be null then option 2 is readable and preferred. Otherwise option 1. Intention of option 1 is to avoid potential null pointer.

Method A won't throw a null pointer exception. There is no better of the two. It depends on whether on not you want it to throw a npe (and you might want that in your overall design).

Method B will fail with NullPointerException on null string1, whereas Method A will never throw this. Some authorities mandate this "defensive" programming. They have influenced me to do it, though it still does not come naturally!

It's also possible to write

if (string1 != null && string1.equals("MyString")) ...

though tools such as FindBugs flags this as a possible error, assuming that you should have made sure that string1 was already non-null. (Can you rely on the order of evaluation?).

So there are different schools of thought.

Method a does not throw NullPointerException and hence very convenient. It is widely used also.

Exceptions are for exceptional processing and have more overhead than checking for error conditions and handling them with regular logic. If you have been programming for a decade a NPE is a cringe matter which usually indicates careless code. Avoid them by using "constant".equals(variable) and people who read your code and use it will be happier.

The second one is more widely used. Neither is better.

It's the same idea as

if (1 == x)

but without a specific reason. but for a different reason. (Null pointer as noted by others).