String、StringBuffer与StringBuilder的区别

String、StringBuffer与StringBuilder的区别

官方文档：

• String

public final class String extends Object 
implements Serializable, Comparable, CharSequence 
String 类代表字符串。Java 程序中的所有字符串字面值（如 “abc” ）都作为此类的实例实现。

字符串是常量；它们的值在创建之后不能更改。字符串缓冲区支持可变的字符串。因为 String 对象是不可变的，所以可以共享。例如：

 String str = "abc"  

等效于：

 char data[] = {'a', 'b', 'c'}; String str = new String(data);

下面给出了一些如何使用字符串的更多示例：

 System.out.println("abc"); String cde = "cde"; System.out.println("abc" + cde); String c = "abc".substring(2,3); String d = cde.substring(1, 2);

String 类包括的方法可用于检查序列的单个字符、比较字符串、搜索字符串、提取子字符串、创建字符串副本并将所有字符全部转换为大写或小写。大小写映射基于 Character 类指定的 Unicode 标准版。

Java 语言提供对字符串串联符号（”+”）以及将其他对象转换为字符串的特殊支持。字符串串联是通过 StringBuilder（或 StringBuffer）类及其 append 方法实现的。字符串转换是通过 toString 方法实现的，该方法由 Object 类定义，并可被 Java 中的所有类继承。有关字符串串联和转换的更多信息，请参阅 Gosling、Joy 和 Steele 合著的 The Java Language Specification。

除非另行说明，否则将 null 参数传递给此类中的构造方法或方法将抛出 NullPointerException。

String 表示一个 UTF-16 格式的字符串，其中的增补字符 由代理项对 表示（有关详细信息，请参阅 Character 类中的 Unicode 字符表示形式）。索引值是指 char 代码单元，因此增补字符在 String 中占用两个位置。

String 类提供处理 Unicode 代码点（即字符）和 Unicode 代码单元（即 char 值）的方法。

• StringBuffer

public final class StringBuffer extends Object 
implements Serializable, CharSequence

线程安全的可变字符序列。一个类似于 String 的字符串缓冲区，但不能修改。虽然在任意时间点上它都包含某种特定的字符序列，但通过某些方法调用可以改变该序列的长度和内容。

可将字符串缓冲区安全地用于多个线程。可以在必要时对这些方法进行同步，因此任意特定实例上的所有操作就好像是以串行顺序发生的，该顺序与所涉及的每个线程进行的方法调用顺序一致。

StringBuffer 上的主要操作是 append 和 insert 方法，可重载这些方法，以接受任意类型的数据。每个方法都能有效地将给定的数据转换成字符串，然后将该字符串的字符追加或插入到字符串缓冲区中。append 方法始终将这些字符添加到缓冲区的末端；而 insert 方法则在指定的点添加字符。

例如，如果 z 引用一个当前内容为 “start” 的字符串缓冲区对象，则此方法调用 z.append(“le”) 会使字符串缓冲区包含 “startle”，而 z.insert(4, “le”) 将更改字符串缓冲区，使之包含 “starlet”。

通常，如果 sb 引用 StringBuilder 的一个实例，则 sb.append(x) 和 sb.insert(sb.length(), x) 具有相同的效果。

当发生与源序列有关的操作（如源序列中的追加或插入操作）时，该类只在执行此操作的字符串缓冲区上而不是在源上实现同步。

每个字符串缓冲区都有一定的容量。只要字符串缓冲区所包含的字符序列的长度没有超出此容量，就无需分配新的内部缓冲区数组。如果内部缓冲区溢出，则此容量自动增大。从 JDK 5 开始，为该类补充了一个单个线程使用的等价类，即 StringBuilder。与该类相比，通常应该优先使用 StringBuilder 类，因为它支持所有相同的操作，但由于它不执行同步，所以速度更快。

• StringBuilder

public final class StringBuilder extends Object 
implements Serializable, CharSequence

一个可变的字符序列。此类提供一个与 StringBuffer 兼容的 API，但不保证同步。该类被设计用作 StringBuffer 的一个简易替换，用在字符串缓冲区被单个线程使用的时候（这种情况很普遍）。如果可能，建议优先采用该类，因为在大多数实现中，它比 StringBuffer 要快。

在 StringBuilder 上的主要操作是 append 和 insert 方法，可重载这些方法，以接受任意类型的数据。每个方法都能有效地将给定的数据转换成字符串，然后将该字符串的字符追加或插入到字符串生成器中。append 方法始终将这些字符添加到生成器的末端；而 insert 方法则在指定的点添加字符。

将 StringBuilder 的实例用于多个线程是不安全的。如果需要这样的同步，则建议使用 StringBuffer。

1 .上String、StringBuffer与StringBuilder的原代码中的部分代码

<code class="hljs axapta has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//String </span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">String</span> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">java</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">io</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Serializable</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Comparable</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">String</span>>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CharSequence</span> {</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> value[]; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * Allocates a new {@code String} so that it represents the sequence of * characters currently contained in the character array argument. The * contents of the character array are copied; subsequent modification of * the character array does not affect the newly created string. * * @param value * The initial value of the string */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String(String original) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.value = Arrays.copyOf(value, value.length); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 把传入构造函数original字符串切分成字符数组并赋给value[]; </span> } } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//StringBuffer </span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">StringBuffer</span> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">AbstractStringBuilder</span> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">java</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">io</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Serializable</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CharSequence</span> {</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * Constructs a string buffer initialized to the contents of the * specified string. The initial capacity of the string buffer is * <code>16</code> plus the length of the string argument. * * @param str the initial contents of the buffer. * @exception NullPointerException if <code>str</code> is <code>null</code> */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> StringBuffer(String <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>.length() + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//继承父类的构造器，并创建一个大小为str.length()+16的value[]数组 </span> append(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//将str切分成字符序列并加入到value[]中 </span> } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> synchronized StringBuffer append(String <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span>.append(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>; } } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//StringBuilder</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">StringBuilder</span> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">AbstractStringBuilder</span> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">java</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">io</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Serializable</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CharSequence</span> {</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * Constructs a string builder initialized to the contents of the * specified string. The initial capacity of the string builder is * <code>16</code> plus the length of the string argument. * * @param str the initial contents of the buffer. * @throws NullPointerException if <code>str</code> is <code>null</code> */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> StringBuilder(String <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>.length() + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>); append(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>); } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> StringBuilder append(String <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span>.append(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>; } } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//AbstractStringBuilder</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">abstract</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">AbstractStringBuilder</span> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Appendable</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CharSequence</span> {</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * The value is used for character storage. */</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>[] value; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/** * Creates an AbstractStringBuilder of the specified capacity. */</span> AbstractStringBuilder(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> capacity) { value = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span>[capacity]; } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> AbstractStringBuilder append(String <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span> == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"null"</span>; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> len = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>.length(); ensureCapacityInternal(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">count</span> + len); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">str</span>.getChars(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, len, value, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">count</span>); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">count</span> += len; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>; } }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; 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padding: 0px 5px;">88</li><li style="box-sizing: border-box; padding: 0px 5px;">89</li><li style="box-sizing: border-box; padding: 0px 5px;">90</li><li style="box-sizing: border-box; padding: 0px 5px;">91</li><li style="box-sizing: border-box; padding: 0px 5px;">92</li><li style="box-sizing: border-box; padding: 0px 5px;">93</li><li style="box-sizing: border-box; padding: 0px 5px;">94</li><li style="box-sizing: border-box; padding: 0px 5px;">95</li><li style="box-sizing: border-box; padding: 0px 5px;">96</li><li style="box-sizing: border-box; padding: 0px 5px;">97</li><li style="box-sizing: border-box; padding: 0px 5px;">98</li><li style="box-sizing: border-box; padding: 0px 5px;">99</li><li style="box-sizing: border-box; padding: 0px 5px;">100</li><li style="box-sizing: border-box; padding: 0px 5px;">101</li><li style="box-sizing: border-box; padding: 0px 5px;">102</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li><li style="box-sizing: border-box; padding: 0px 5px;">66</li><li style="box-sizing: border-box; padding: 0px 5px;">67</li><li style="box-sizing: border-box; padding: 0px 5px;">68</li><li style="box-sizing: border-box; padding: 0px 5px;">69</li><li style="box-sizing: border-box; padding: 0px 5px;">70</li><li style="box-sizing: border-box; padding: 0px 5px;">71</li><li style="box-sizing: border-box; padding: 0px 5px;">72</li><li style="box-sizing: border-box; padding: 0px 5px;">73</li><li style="box-sizing: border-box; padding: 0px 5px;">74</li><li style="box-sizing: border-box; padding: 0px 5px;">75</li><li style="box-sizing: border-box; padding: 0px 5px;">76</li><li style="box-sizing: border-box; padding: 0px 5px;">77</li><li style="box-sizing: border-box; padding: 0px 5px;">78</li><li style="box-sizing: border-box; padding: 0px 5px;">79</li><li style="box-sizing: border-box; padding: 0px 5px;">80</li><li style="box-sizing: border-box; padding: 0px 5px;">81</li><li style="box-sizing: border-box; padding: 0px 5px;">82</li><li style="box-sizing: border-box; padding: 0px 5px;">83</li><li style="box-sizing: border-box; padding: 0px 5px;">84</li><li style="box-sizing: border-box; padding: 0px 5px;">85</li><li style="box-sizing: border-box; padding: 0px 5px;">86</li><li style="box-sizing: border-box; padding: 0px 5px;">87</li><li style="box-sizing: border-box; padding: 0px 5px;">88</li><li style="box-sizing: border-box; padding: 0px 5px;">89</li><li style="box-sizing: border-box; padding: 0px 5px;">90</li><li style="box-sizing: border-box; padding: 0px 5px;">91</li><li style="box-sizing: border-box; padding: 0px 5px;">92</li><li style="box-sizing: border-box; padding: 0px 5px;">93</li><li style="box-sizing: border-box; padding: 0px 5px;">94</li><li style="box-sizing: border-box; padding: 0px 5px;">95</li><li style="box-sizing: border-box; padding: 0px 5px;">96</li><li style="box-sizing: border-box; padding: 0px 5px;">97</li><li style="box-sizing: border-box; padding: 0px 5px;">98</li><li style="box-sizing: border-box; padding: 0px 5px;">99</li><li style="box-sizing: border-box; padding: 0px 5px;">100</li><li style="box-sizing: border-box; padding: 0px 5px;">101</li><li style="box-sizing: border-box; padding: 0px 5px;">102</li></ul>

2 .我们可以看出在String中构造器传入的字符串对象被切分成字符序列，存放在其私有final的类变量value[]中。 
new String(“java”)使得value[]={‘j’,’a’,’v’,’a’}，之后这个String对象中的value[]再也不能改变了，只能读，所以是线程安全的。

StringBuffer和StringBuilder都继承了AbstractStringBuilder 这个抽象类，在他们的构造函数中传入字符串，会调用父类AbstractStringBuilder中对应的构造函数和append函数，父类构造函数新建一个长度为（字符串长度+末尾附加16个空元素）的数组然后通过父类append函数把字符串转换成字符数组value[]中（这里的char[] value 没有final，StringBuffer和StringBuilder中的字符串是可变的）。以后通过append()方法将新字符串加入value[]末尾。这样也就改变了value[]的内容和大小了。

StringBuffer对方法加了同步锁或者对调用的方法加了同步锁，所以是线程安全的。 
而StringBuilder不是线程安全的。

①

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span> s = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ja"</span> +<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"va"</span> </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

②

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span> s1=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ja"</span>; StringBuffer sb=<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> StringBuffer(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"va"</span>); sb.append(s1);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

③

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span> s1=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ja"</span>; <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span> s2 = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"va"</span>; <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span> s = s1 +s2</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

在编译阶段就能够确定的字符串常量，完全没有必要创建String或StringBuffer对象。直接使用字符串常量的”+”连接操作效率最高。 
时间上 ①<③

StringBuffer对象的append效率要高于两个String对象的”+”连接操作。 
时间上 ②<③

一般来说 执行时间上从快到慢 StringBuilder、StringBuffer、String

非多线程操作字符串缓冲区建议使用 StringBuilder。