另外，StringBuffer是线程安全的，而StringBuilder不是。

因此，在实时情况下，当不同的线程正在访问它时，StringBuilder可能会有一个不确定的结果。

----------------------------------------------------------------------------------
                  String                    StringBuffer         StringBuilder
----------------------------------------------------------------------------------                 
Storage Area | Constant String Pool         Heap                   Heap 
Modifiable   |  No (immutable)              Yes( mutable )         Yes( mutable )
Thread Safe  |      Yes                     Yes                     No
 Performance |     Fast                 Very slow                  Fast
----------------------------------------------------------------------------------

In java, String is immutable. Being immutable we mean that once a String is created, we can not change its value. StringBuffer is mutable. Once a StringBuffer object is created, we just append the content to the value of object instead of creating a new object. StringBuilder is similar to StringBuffer but it is not thread-safe. Methods of StingBuilder are not synchronized but in comparison to other Strings, the Stringbuilder runs fastest. You can learn difference between String, StringBuilder and StringBuffer by implementing them.

字符串

String类表示字符串。Java程序中的所有字符串字面值，例如“abc”，都被实现为该类的实例。

字符串对象是不可变的，一旦创建就不能改变。(字符串是常量)

If a String is created using constructor or method then those strings will be stored in Heap Memory as well as SringConstantPool. But before saving in pool it invokes intern() method to check object availability with same content in pool using equals method. If String-copy is available in the Pool then returns the reference. Otherwise, String object is added to the pool and returns the reference. The Java language provides special support for the string concatenation operator (+), and for conversion of other objects to strings. String concatenation is implemented through the StringBuilder(or StringBuffer) class and its append method. String heapSCP = new String("Yash"); heapSCP.concat("."); heapSCP = heapSCP + "M"; heapSCP = heapSCP + 777; // For Example: String Source Code public String concat(String str) { int otherLen = str.length(); if (otherLen == 0) { return this; } int len = value.length; char buf[] = Arrays.copyOf(value, len + otherLen); str.getChars(buf, len); return new String(buf, true); } String literals are stored in StringConstantPool. String onlyPool = "Yash";

StringBuilder和StringBuffer是可变的字符序列。这意味着可以改变这些对象的值。StringBuffer具有与StringBuilder相同的方法，但是StringBuffer中的每个方法都是同步的，因此它是线程安全的。

StringBuffer and StringBuilder data can only be created using new operator. So, they get stored in Heap memory. Instances of StringBuilder are not safe for use by multiple threads. If such synchronization is required then it is recommended that StringBuffer be used. StringBuffer threadSafe = new StringBuffer("Yash"); threadSafe.append(".M"); threadSafe.toString(); StringBuilder nonSync = new StringBuilder("Yash"); nonSync.append(".M"); nonSync.toString(); StringBuffer and StringBuilder are having a Special methods like., replace(int start, int end, String str) and reverse(). NOTE: StringBuffer and SringBuilder are mutable as they provides the implementation of Appendable Interface.

什么时候用哪一个。

If a you are not going to change the value every time then its better to Use String Class. As part of Generics if you want to Sort Comparable<T> or compare a values then go for String Class. //ClassCastException: java.lang.StringBuffer cannot be cast to java.lang.Comparable Set<StringBuffer> set = new TreeSet<StringBuffer>(); set.add( threadSafe ); System.out.println("Set : "+ set); If you are going to modify the value every time the go for StringBuilder which is faster than StringBuffer. If multiple threads are modifying the value the go for StringBuffer.

就我个人而言，我认为StringBuffer在现实世界中没有任何用处。什么时候我想通过操纵字符序列在多个线程之间进行通信?这听起来一点用都没有，但也许我还没有看到光明:)

注意，如果您使用的是Java 5或更新版本，您应该使用StringBuilder而不是StringBuffer。来自API文档:

从JDK 5发行版开始，这个类已经被一个为单个线程使用而设计的等效类StringBuilder所补充。通常应该优先使用StringBuilder类，因为它支持所有相同的操作，但速度更快，因为它不执行同步。

在实践中，您几乎永远不会同时从多个线程中使用它，因此StringBuffer所做的同步几乎总是不必要的开销。