17.6: String Comparison

To compare two strings, it may be tempting to use the == and != operators.

String name1 = "Alan Turing";
String name2 = "Ada Lovelace";
if (name1 == name2) {    // wrong!
    System.out.println("The names are the same.");
}

This code compiles and runs, and most of the time it gets the answer right. But it is not correct, and sometimes it gets the answer wrong. The problem is that the == operator checks whether the two variables refer to the same object (by comparing the references). If you give it two different strings that contain the same letters, it yields false.

The right way to compare strings is with the equals method, like this:

if (name1.equals(name2)) {
    System.out.println("The names are the same.");
}

This example invokes equals on name1 and passes name2 as an argument. The equals method returns true if the strings contain the same characters; otherwise it returns false.

If the strings differ, we can use compareTo to see which comes first in alphabetical order:

int diff = name1.compareTo(name2);
if (diff == 0) {
    System.out.println("The names are the same.");
} else if (diff < 0) {
    System.out.println("name1 comes before name2.");
} else if (diff > 0) {
    System.out.println("name2 comes before name1.");
}

The return value from compareTo is the difference between the first characters in the strings that differ. If the strings are equal, their difference is zero. If the first string (the one on which the method is invoked) comes first in the alphabet, the difference is negative. Otherwise, the difference is positive.

In the preceding code, compareTo returns positive 8, because the second letter of "Ada" comes before the second letter of "Alan" by 8 letters.

Both equals and compareTo are case-sensitive. The uppercase letters come before the lowercase letters, so "Ada" comes before "ada".