StringBuilder 与 StringBuffer 区别

/**
 * 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.
 *
 * <p>The principal operations on a <code>StringBuilder</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 builder. The
 * <code>append</code> method always adds these characters at the end
 * of the builder; the <code>insert</code> method adds the characters at
 * a specified point.
 * <p>
 * For example, if <code>z</code> refers to a string builder object
 * whose current contents are "<code>start</code>", then
 * the method call <code>z.append("le")</code> would cause the string
 * builder to contain "<code>startle</code>", whereas
 * <code>z.insert(4, "le")</code> would alter the string builder to
 * contain "<code>starlet</code>".
 * <p>
 * In general, if sb refers to an instance of a <code>StringBuilder</code>,
 * then <code>sb.append(x)</code> has the same effect as
 * <code>sb.insert(sb.length(),&nbsp;x)</code>.
 *
 * Every string builder has a capacity. As long as the length of the
 * character sequence contained in the string builder does not exceed
 * the capacity, it is not necessary to allocate a new internal
 * buffer. If the internal buffer overflows, it is automatically made larger.
 *
 * <p>Instances of <code>StringBuilder</code> are not safe for
 * use by multiple threads. If such synchronization is required then it is
 * recommended that {@link java.lang.StringBuffer} be used.