public final class StringBuffer extends  implements , 

A thread-safe,  mutable可变 sequence序列 of characters. A string buffer is like a  , but can be  modified修改. At any point in time it contains some  particular特定 sequence of characters, but the length and  content内容 of the sequence can be changed through  certain某些 method  calls调用.

String buffers are safe for use by multiple threads多线程. The methods are synchronized同步where necessary必要 so that all the operations操作 on any particular特定 instance实例 behaveas if they occur发生 in some serial order串行顺序 that is consistent一致 with the order of the method calls made by each of the individual threads每个线程 involved涉及.

The principal主要 operations操作 on a StringBuffer are the append and insert methods, which areoverloaded重载 so as to accept接受 data of any type. Each（方法能） effectively 有效converts转换 a given datum数据 to a string and then appends or inserts the characters of that string to the string buffer字符缓冲区. The append method always adds these（字符） characters at the end of the buffer; the insert method adds the characters at a specified指定 point.

For example, if z refers to引用 a string buffer object whose current contents当前内容 are "start", then the method call调用 z.append("le") would cause the string buffer to contain "startle", whereas z.insert(4, "le") would alter the string buffer to contain "starlet".

In general通常, if sb refers to an instance实例 of a StringBuffer, then sb.append(x) has the same effect效果 as sb.insert(sb.length(), x).

Whenever an operation操作 occurs发生 involving有关 a source sequence源序列 (such as appending or inserting from a source sequence) this class synchronizes only on the string bufferperforming the operation执行此操作, not on the source（同步）.

Every string buffer has a capacity容量. As long as the length of the character sequence contained in the string buffer does not exceed 超过the capacity, it is not necessary to allocate分配 a newinternal内部 buffer array. If the internal buffer overflows超过溢出, it is automatically自动made larger. As of release开始 JDK 5, this class has been supplemented补充 with an equivalent等价 class designed for专为 use by a single thread, . The StringBuilder class should generally通常 be used in preference to优先this one, as it supports all of the sameoperations操作 but it is faster, as it performs执行 no synchronization.