String

TheStringclassrepresents(代表,表示)character strings. All string literals(字面值,文字) in Java programs, such as"abc", are implemented asinstances(实例)of this class.

Strings areconstant(常量,常数); their values cannot be changed after they are created. String buffers support(支持，维持)mutable(易变的,可变的)strings. Because String objects areimmutable(不变的,不可变的)they can beshared(共享). For example:

String str = "abc";

is equivalent to:

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

Here are some more examples of how strings can be used:

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

The classStringincludes methods for examiningindividualcharacters of the sequence(序列的单个字符), forcomparing(比较)strings, for searching strings, for extracting(提取) substrings(子字符串), and for creating acopy of a string(字符串的副本)with(并将)all characterstranslated<wbr><strong><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">to(翻译)</span></strong>uppercase or to lowercase.<strong><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">Case</span><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">mapping(大小写)</span></strong>is based on the Unicode<strong><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">Standard version(标准版本)</span></strong>specified by the<a title="class in java.lang" href="http://blog.sina.com.cn/s/blog_a3bf7bd00100xg9j.html" style="text-decoration:none; color:rgb(65,100,111)"><code>Character</code></a>class.</wbr>

The Java languageprovides(提供,规定) special(特别的,特殊) support(支持)for the stringconcatenation(串联,连接)operator(操作员,运算符)(<wbr>+<wbr>), and for<strong><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">conversion(转换)</span></strong>of other objects to strings. String concatenation is implemented<strong><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">through(通过,穿过)</span></strong>the<code>StringBuilder</code>(or<code>StringBuffer</code>) class and its<code>append</code>method. String conversions are implemented through the method<code>toString</code>,<strong><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">defined by<code>Object(是由Oeject定义)</code></span></strong>and inherited(继承) by all classes in Java. For additional(附加,额外的,有关) information on string concatenation and conversion,<strong><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">see(查阅)</span></strong>Gosling, Joy, and Steele,<em>The Java Language Specification</em>.</wbr></wbr>

Unless(除非,如果不)otherwise(另外的,其他方面的)noted(说明,注意),passing(经过,传递)anullargument to a constructor(构造函数)or method in this class will cause aNullPointerExceptionto be thrown(抛出).

AStringrepresents a string in the UTF-16 format in whichsupplementary(补充,增补)charactersare represented bysurrogate(代理)pairs(see the sectionUnicode Character Representations(表现形式)in theCharacterclass for more information). Index valuesrefer to(指的是,参考)<wbr><code>char</code>code<strong><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">units(单位,单元)</span></strong>, so a supplementary character<strong><span style="color:#ED1C24; word-wrap:normal; word-break:normal; line-height:21px">uses two positions(占用两个位置)</span></strong>in a<code>String</code>.</wbr>

TheStringclass provides methods fordealing with(处理)Unicode code points (i.e., characters), in addition to those for dealing with Unicode code units (i.e.,charvalues).