API Contents - SCJP Sun Certified Programmer for Java Platform SE6 - page 275

state that you need to be able to “discuss differences between the String , StringBuilder ,
and StringBuffer classes.” The main differences between these classes are as follows:
String String represents an immutable sequence of characters. The string literals in your
code are of type String .
StringBuilder StringBuilder represents a mutable sequence of characters. String-
Builder is like String except the individual characters can be modified and the length can
change. The methods in this class are not synchronized, so do not use an instance of this
class in a situation where multiple threads can access the instance.
StringBuffer StringBuffer represents a thread-safe, mutable sequence of characters. The
methods in this class are exactly identical to the methods of StringBuilder . The only dif-
ference between the two classes is that the methods in StringBuffer are synchronized.
The simplest class to use is String , but it tends to be ineffi cient when working with
character sequences that need to be changed often because a String object is immutable.
Use StringBuilder and StringBuffer in situations where you need to manipulate strings,
because they contain methods like append , insert , and concat that change the string with-
out creating a new object each time.
This section discusses each of these classes in detail, starting with the String class.
The String Class
The String class represents an immutable array of characters. String literals are automati-
cally instantiated into String objects. For example, ”Hello” in the following statement is a
String object:
String s = “Hello”;
You can also instantiate a String dynamically using one of the String constructors:
char [] abc = {'a', 'b', 'c'};
String s2 = new String(abc);
The JVM stores string literals in a special memory called the string pool. Because
String objects are immutable, instances in the string pool can be shared. For example, the
following two String references point to the same instance, and the output is ”true“ :
String one = “today”;
String two = “today”;
if(one == two)
System.out.println(“true”);
else
System.out.println(“false”);