String,StringBuffer,StringBuilder比较

String,StringBuffer,StringBuilder这三个类都实现了同样的接口，java.io.Serializable, Comparable<String>, CharSequence也就是说他们大概的功能都是相同的，但是因为某些微小的变化导致了他们存在一些功能的差异。
首先：String一旦new出来值将不会改变，
      JDK分析
      private final char value[];
      StringBuffer,StringBuilder的值都是可以改变的，
      char value[];
第二：StringBuffer,StringBuilder性能差异
       StringBuffer线程安全的

/**
 * A thread-safe, mutable sequence of characters. 
 * A string buffer is like a {@link String}, but can be modified. At any 
 * point in time it contains some particular sequence of characters, but 
 * the length and content of the sequence can be changed through certain 
 * method calls.
 * <p>
 * String buffers are safe for use by multiple threads. The methods 
 * are synchronized where necessary so that all the operations on any 
 * particular instance behave as if they occur in some serial order 
 * that is consistent with the order of the method calls made by each of 
 * the individual threads involved.
 * <p>
 * The principal operations on a <code>StringBuffer</code> are the 
 * <code>append</code> and <code>insert</code> methods, which are 
 * overloaded so as to accept data of any type. Each effectively 
 * converts a given datum to a string and then appends or inserts the 
 * characters of that string to the string buffer. The 
 * <code>append</code> method always adds these characters at the end 
 * of the buffer; the <code>insert</code> method adds the characters at 
 * a specified point. 
 * <p>   

   而StringBuilder却不是的，

/**
 * A mutable sequence of characters.  This class provides an API compatible
 * with <code>StringBuffer</code>, but with no guarantee of synchronization.
 * This class is designed for use as a drop-in replacement for
 * <code>StringBuffer</code> in places where the string buffer was being
 * used by a single thread (as is generally the case).   Where possible,
 * it is recommended that this class be used in preference to
 * <code>StringBuffer</code> as it will be faster under most implementations.
 * 

综合而言，String的值无法改变，StringBuffer,StringBuilder可以，但是StringBuilder的性能比

StringBuffer更高。