You should almost never use == and almost always use equals(). It will only work with == if both strings reference the same object. There's an intern() method on String to return the same reference for a given string value. String literals are implicitly interned. Only if you have a very good reason should you use == for string comparison, and even then you need to be very careful.
The only good reason is performance, and very rarely will it matter. Only optimize once you know for sure that you need to do so. It's otherwise normally not worth the hassle. If you're looking at some open source code, they might have a case whereby the comparison is in a very tight loop or called very frequently and the optimization can help. Or it was just prematurely optimized and maybe seemed safe.
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