compareTo() vs. equals()

When testing for equality of String's in Java I have always used equals() because to me this seems to be the most natural method for it. After all, its name already says what it is intended to do. However, a colleague of mine recently told me had been taught to use compareTo() == 0 instead of equals(). This feels unnatural (as compareTo() is meant to provide an ordering and not compare for equality) and even somewhat dangerous (because compareTo() == 0 does not necessarily imply equality in all cases, even though I know it does for String's) to me.

He did not know why he was taught to use compareTo() instead of equals() for String's, and I could also not find any reason why. Is this really a matter of personal taste, or is there any real reason for either method?

Solution 1:

A difference is that "foo".equals((String)null) returns false while "foo".compareTo((String)null) == 0 throws a NullPointerException. So they are not always interchangeable even for Strings.

Solution 2:

The 2 main differences are that:

1. equals will take any Object as a parameter, but compareTo will only take Strings.
2. equals only tells you whether they're equal or not, but compareTo gives information on how the Strings compare lexicographically.

I took a look at the String class code, and the algorithm within compareTo and equals looks basically the same. I believe his opinion was just a matter of taste, and I agree with you -- if all you need to know is the equality of the Strings and not which one comes first lexicographically, then I would use equals.

Solution 3:

When comparing for equality you should use equals(), because it expresses your intent in a clear way.

compareTo() has the additional drawback that it only works on objects that implement the Comparable interface.

This applies in general, not only for Strings.