String, StringBuilder 與StringBuffer的區(qū)別與聯(lián)系

@(JAVA)[java]

1、區(qū)別

（1）String構(gòu)建的對(duì)象不能改變，每次對(duì)String進(jìn)行操作時(shí)，如兩個(gè)String相加，需要新建一個(gè)String對(duì)象，然后容納最終的結(jié)果。
而StringBuilder與StringBuffer構(gòu)建的對(duì)象可以隨時(shí)在修改其內(nèi)容，而無(wú)需生成新的對(duì)象。一般新建一個(gè)對(duì)象是會(huì)生成16個(gè)字節(jié)的空間，之后根據(jù)需要再增加空間。
由于一般新構(gòu)建一個(gè)對(duì)象涉及分配內(nèi)存空間分配、無(wú)引用對(duì)象過(guò)多時(shí)的垃圾回收等，因此，對(duì)于操作頻繁的字符串需使用StringBuilder或StringBuffer。

（2）StringBuilder與StringBuffer二者之間的區(qū)別在于前者的性能更高，但非線程安全的。而StringBuffer是線程安全的。
因此在大部分情況下，建議使用StringBuilder，除非涉及多線程情況。

2、官方文檔中的一些說(shuō)明

（1）String

The String class represents character strings. Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable they can be shared.

（2）StringBuilder

A mutable sequence of characters.This class provides an API compatible with StringBuffer, but with no guarantee of synchronization. 因此，StringBuilder與StringBuffer的 API基本完全一致。

This class is designed for use as a drop-in replacement for StringBuffer 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 StringBuffer as it will be faster under most implementations.

The principal operations on a StringBuilder are the append and insert 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 append method always adds these characters at the end of the builder; the insertmethod adds the characters at a specified point.

For example, if z refers to a string builder object whose current contents are “start”, then the method call z.append(“l(fā)e”) would cause the string builder to contain “startle”, whereas z.insert(4, “l(fā)e”) would alter the string builder to contain “starlet”.

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

Instances of StringBuilder are not safe for use by multiple threads. If such synchronization is required then it is recommended that StringBuffer be used.

（3）StringBuffer

A thread-safe, mutable sequence of characters. A string buffer is like a 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.

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.

The principal operations on a StringBuffer are the append and insert 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 append method always adds these characters at the end of the buffer; the insert method adds the characters at a specified point.

For example, if z refers to a string buffer object whose current contents are “start”, then the method call z.append(“l(fā)e”) would cause the string buffer to contain “startle”, whereas z.insert(4, “l(fā)e”) would alter the string buffer to contain “starlet”.

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

Whenever an operation occurs involving a source sequence (such as appending or inserting from a source sequence) this class synchronizes only on the string buffer performing the operation, not on the source.

Every string buffer has a capacity. As long as the length of the character sequence contained in the string buffer does not exceed the capacity, it is not necessary to allocate a new internal buffer array. If the internal buffer overflows, it is automatically made larger. As of release JDK 5, this class has been supplemented with an equivalent class designed for use by a single thread, StringBuilder. The StringBuilder class should generally be used in preference to this one, as it supports all of the same operations but it is faster, as it performs no synchronization.

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