JavaSE进阶

/**

 * The {@code String} class represents character strings. All

 * string literals in Java programs, such as {@code "abc"}, are

 * implemented as instances of this class.

 * <p>

 * 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. For example:

 * <blockquote><pre>

 *     String str = "abc";

 * </pre></blockquote><p>

 * is equivalent to:

 * <blockquote><pre>

 *     char data[] = {'a', 'b', 'c'};

 *     String str = new String(data);

 * </pre></blockquote><p>

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

 * <blockquote><pre>

 *     System.out.println("abc");

 *     String cde = "cde";

 *     System.out.println("abc" + cde);

 *     String c = "abc".substring(2,3);

 *     String d = cde.substring(1, 2);

 * </pre></blockquote>

 * <p>

 * The class {@code String} includes methods for examining

 * individual characters of the sequence, for comparing strings, for

 * searching strings, for extracting substrings, and for creating a

 * copy of a string with all characters translated to uppercase or to

 * lowercase. Case mapping is based on the Unicode Standard version

 * specified by the {@link java.lang.Character Character} class.

 * <p>

 * The Java language provides special support for the string

 * concatenation operator (&nbsp;+&nbsp;), and for conversion of

 * other objects to strings. String concatenation is implemented

 * through the {@code StringBuilder}(or {@code StringBuffer})

 * class and its {@code append} method.

 * String conversions are implemented through the method

 * {@code toString}, defined by {@code Object} and

 * inherited by all classes in Java. For additional information on

 * string concatenation and conversion, see Gosling, Joy, and Steele,

 * <i>The Java Language Specification</i>.

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。

public final class String

    implements java.io.Serializable, Comparable<String>, CharSequence {

    /** The value is used for character storage. */

    private final char value[];

    /** Cache the hash code for the string */

    private int hash; // Default to 0