API Contents - SCJP Sun Certified Programmer for Java Platform SE6 - page 278

The empty String on line 10 is instantiated, and then line 12 appends an ”a” . However,
because the String object is immutable, a new String object is assigned to alpha and the
”” object becomes eligible for garbage collection. The next time through the loop, alpha
is assigned a new String object ”ab” and the ”a” object becomes eligible for garbage col-
lection. The next iteration assigns alpha to ”abc” and the ”ab” object becomes eligible for
garbage collection, and so on.
This sequence of events continues, and after 26 iterations through the loop, a total of 27
objects are instantiated, 26 of which are immediately eligible for garbage collection. A bet-
ter technique in this situation would be to use the StringBuffer or StringBuilder classes,
which I discuss in detail in the next section.
The StringBuilder and StringBuffer Classes
The java.lang package contains two classes for representing strings as a mutable sequence
of characters: StringBuilder and StringBuffer . The StringBuffer class has been around
since the fi rst version of Java, while StringBuilder was added in J2SE 5.0. The two classes
have the exact same method signatures and constructor parameters. The only difference
between them is that StringBuffer is thread-safe and StringBuilder is not. If you are
working with arrays of characters in a multithreaded application, use the StringBuffer
class. Otherwise, if threads are not an issue for your particular situation, use the String-
Builder class.
Let's look at an example. The following for loop is similar to the example in the previ-
ous section, except it uses a single StringBuilder object instead of 27 String objects:
15. StringBuilder alpha = new StringBuilder(26);
16. for(char current = 'a'; current <= 'z'; current++) {
17. alpha.append(current);
18. }
19. System.out.println(alpha);
The StringBuilder class contains a constructor that takes in an int to represent the ini-
tial capacity. On line 15, a new StringBuilder object is instantiated with an initial capac-
ity of 26. This does not mean only 26 characters can appear in alpha . The capacity merely
specifi es an initial buffer. If the capacity of a StringBuilder object is reached, the length
of the StringBuilder object is automatically increased. We can improve performance if we
give a StringBuilder a capacity, but it does not affect the behavior of the object.
The call to append on line 17 increases the size of the StringBuilder object by one
each time through the for loop and appends the value of current to the end of alpha .
The StringBuilder object grows as needed, but there is only one instance in memory, as
Figure 4.2 shows.