Conditionals and Loops - Java Software Solutions: Foundations of Program Design - page 228

Comparing Objects
The Unicode relationships among characters make it easy to sort characters and
strings of characters. If you have a list of names, for instance, you can put them
in alphabetical order based on the inherent relationships among characters in the
character set.
However, you should not use the equality or relational operators to compare
String objects. The String class contains a method called equals that returns a
boolean value that is true if the two strings being compared contain exactly the
same characters and is false otherwise. For example:

if (name1.equals(name2))
System.out.println ("The names are the same.");
else
System.out.println ("The names are not the same.");

Assuming that name1 and name2 are String objects, this condition
determines whether the characters they contain are an exact match.
Because both objects were created from the String class, they both
respond to the equals message. Therefore, the condition could have
been written as name2.equals(name1), and the same result would
occur.
It is valid to test the condition (name1 == name2) , but that actually tests to
see whether both reference variables refer to the same String object. For any
object, the == operator tests whether both reference variables are aliases of each
other (whether they contain the same address). That's different from testing to see
whether two different String objects contain the same characters.
Keep in mind that a string literal (such as "Nathan" ) is a convenience and is
actually a shorthand technique for creating a String object. An interesting issue
related to string comparisons is the fact that Java creates a unique object for string
literals only when needed. That is, if the string literal "Hi" is used multiple times
in a method, only one String object is created to represent it. Therefore, the con-
ditions of both if statements in the following code are true:

KEY CONCEPT
The compareTo method can be used
to determine the relative order of
strings.

String str = "software";
if (str == "software")
System.out.println ("References are the same");
if (str.equals("software"))
System.out.println ("Characters are the same");

The first time the string literal "software" is used, a String object is created
to represent it and the reference variable str is set to its address. Each subsequent
time the literal is used, the original object is referenced.