Java Strings: compareTo() vs. equals()

Question

When testing for equality of strings 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 Strings) to me.

He did not know why he was taught to use compareTo() instead of equals() for strings, 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?

Answer 1

It appears that both methods pretty much do the same thing, but the compareTo() method takes in a String, not an Object, and adds some extra functionality on top of the normal equals() method. If all you care about is equality, then the equals() method is the best choice, simply because it makes more sense to the next programmer that takes a look at your code. The time difference between the two different functions shouldn't matter unless you're looping over some huge amount of items. The compareTo() is really useful when you need to know the order of Strings in a collection or when you need to know the difference in length between strings that start with the same sequence of characters.

source: http://java.sun.com/javase/6/docs/api/java/lang/String.html

Answer 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.

Answer 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.

Answer 4

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.

Answer 5

compareTo has do do more work if the strings have different lengths. equals can just return false, while compareTo must always examine enough characters to find the sorting order.

Answer 6

compareTo not only applies to Strings but also any other object coz compareTo takes a generic argument T. String is one fo the classes that has implemented the compareTo method by implementing the Comparable interface.(compareTo() is a method fo the comparable Interface). so any class is free to implement the Comparable interface.

But compareTo gives the ordering of objects, used typically in sorting objects in ascending or decsending order while equals will only talk about the equality and say whether they are equal or not.