Arrays and Strings - Beginning Java - page 167

NOTE YoulearnmoreaboutthisinChapter5,whenyoulookathowtodefineyour
own classes. For the moment, just note that you don't necessarily need to pass any
arguments to a method because some methods don't require any arguments. On the
other hand, several arguments can be required. It all depends on how the method
was defined in the class.
The equals() method requires one argument that you put between the parentheses. This must be the
String object that is to be compared with the original object. As you saw earlier, the method returns
true if the string passed to it ( string3 in the example) is identical to the string pointed to by the String
object that owns the method; in this case, string1 . As you also saw in the previous section, you could
just as well call the equals() method for the object string3 , and pass string1 as the argument to com-
pare the two strings. In this case, the expression to call the method would be
string3.equals(string1)
and you would get exactly the same result.
The statements in the program code after outputting the values of string3 and string1 are:
if(string1.equals(string3)) { // Now test for
equality
System.out.println("string1.equals(string3) is true." +
" so strings are equal.");
} else {
System.out.println("string1.equals(string3) is false." +
" so strings are not equal.");
}
The output from this shows that calling the equals() method for string1 with string3 as the argument
returns true . After the if statement you make string3 reference a new string. You then compare the
values of string1 and string3 once more, and, of course, the result of the comparison is now false .
Finally, you compare string1 with string3 using the equalsIgnoreCase() method. Here the result is
true because the strings differ only in the case of the first three characters.
String Interning
Having convinced you of the necessity for using the equals method for comparing strings, I can now reveal
that there is a way to make comparing strings with the == operator effective. The mechanism to make this
possible is called string interning . String interning ensures that no two String objects encapsulate the same
string, so all String objects encapsulate unique strings. This means that if two String variables reference
strings that are identical, the references must be identical, too. To put it another way, if two String variables
contain references that are not equal, they must refer to strings that are different. So how do you arrange that
all String objects encapsulate unique strings? You just call the intern() method for every new String
object that you create. To show how this works, I can amend a bit of an earlier example:
String string1 = "Too many ";
String string2 = "cooks";
String string3 = "Too many cooks";