Java Strings: compareTo() vs. equals()

Question

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?

Answer 1

There are certain things which you need to keep in mind while overriding compareTo in Java e.g. Compareto must be consistent with equals and subtraction should not be used for comparing integer fields as they can overflow. check Things to remember while overriding Comparator in Java for details.

Answer 2

equals() can be more efficient then compareTo().

A very important difference between compareTo and equals:

"myString".compareTo(null);  //Throws java.lang.NullPointerException
"myString".equals(null);     //Returns false

equals() checks if two objects are the same or not and returns a boolean.

compareTo() (from interface Comparable) returns an integer. It checks which of the two objects is "less than", "equal to" or "greater than" the other. Not all objects can be logically ordered, so a compareTo() method doesn't always make sense.

Note that equals() doesn't define the ordering between objects, which compareTo() does.

Now I advise you to review the source code of both methods to conclude that equals is preferable over compareTo that involves some Math calculations.

Answer 3

equals() should be the method of choice in the case of the OP.

Looking at the implementation of equals() and compareTo() in java.lang.String on grepcode, we can easily see that equals is better if we are just concerned with the equality of two Strings:

equals():

1012  public boolean equals(Object anObject) {
1013      if (this == anObject) {
1014          return true;
1015      }
1016      if (anObject instanceof String) {
1017          String anotherString = (String)anObject;
1018          int n = count;
1019          if (n == anotherString.count) {
1020              char v1[] = value;
1021              char v2[] = anotherString.value;
1022              int i = offset;
1023              int j = anotherString.offset;
1024              while (n-- != 0) {
1025                  if (v1[i++] != v2[j++])
1026                      return false;
1027              }
1028              return true;
1029          }
1030      }
1031      return false;
1032  }

and compareTo():

1174  public int compareTo(String anotherString) {
1175      int len1 = count;
1176      int len2 = anotherString.count;
1177      int n = Math.min(len1, len2);
1178      char v1[] = value;
1179      char v2[] = anotherString.value;
1180      int i = offset;
1181      int j = anotherString.offset;
1183      if (i == j) {
1184          int k = i;
1185          int lim = n + i;
1186          while (k < lim) {
1187              char c1 = v1[k];
1188              char c2 = v2[k];
1189              if (c1 != c2) {
1190                  return c1 - c2;
1191              }
1192              k++;
1193          }
1194      } else {
1195          while (n-- != 0) {
1196              char c1 = v1[i++];
1197              char c2 = v2[j++];
1198              if (c1 != c2) {
1199                  return c1 - c2;
1200              }
1201          }
1202      }
1203      return len1 - len2;
1204  }

When one of the strings is a prefix of another, the performance of compareTo() is worse as it still needs to determine the lexicographical ordering while equals() won't worry any more and return false immediately.

In my opinion, we should use these two as they were intended:

• equals() to check for equality, and
• compareTo() to find the lexical ordering.

Answer 4

Here one thing is important while using compareTo() over equals() that compareTo works for the classes that implements 'Comparable' interface otherwise it will throw a NullPointerException. String classes implements Comparable interface while StringBuffer does not hence you can use "foo".compareTo("doo") in String object but not in StringBuffer Object.

Answer 5

Equals can be more efficient then compareTo.

If the length of the character sequences in String doesn't match there is no way the Strings are equal so rejection can be much faster.

Moreover if it is same object (identity equality rather then logical equality), it will also be more efficient.

If they also implemented hashCode caching it could be even faster to reject non-equals in case their hashCode's doesn't match.

Answer 6

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.