Java String类 源码注释阅读

直接打开String 源码文件（jdk 1.8.0_112）：

1. 首先来看引入的包：

package java.lang;

import java.io.ObjectStreamField;
import java.io.UnsupportedEncodingException;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Formatter;
import java.util.Locale;
import java.util.Objects;
import java.util.StringJoiner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.regex.PatternSyntaxException;

由引入的包可以大致看到，这个类引用了io包的ObjectStreamField; 关于这个类，我们打开，看描述

package java.io;

import java.lang.reflect.Field;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
import sun.reflect.misc.ReflectUtil;

/**
 * A description of a Serializable field from a Serializable class.  An array
 * of ObjectStreamFields is used to declare the Serializable fields of a class.
 *
 * @author      Mike Warres
 * @author      Roger Riggs
 * @see ObjectStreamClass
 * @since 1.2
 */
public class ObjectStreamField
    implements Comparable<Object>

根据释义，了解到这个类是从可以序列化类序列化字段的描述。再回头 来看String， 剩下引入的是util包一些内容。

2. 来看String类的注释

The {@code String} class represents character strings. All
 * string literals in Java programs, such as {@code "abc"}, are
 * implemented as instances of this class.

这个类就是代表了字符串。Java程序中例如”abc”字符串都是这个类的实例。

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>

字符串是常量，它们的创建后值可以改变。字符串缓冲 可以支持可变字符串。因为字符串对象不可变，所以可以共享。
例如：
String str = “abc”;
相当于： char data[] = {‘a’, ‘b’, ‘c’};
String str = new String(data);
字符串还可以这样用：
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 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.

这个类包含了检查序列单个字符的方法、比较字符串、检索字符串、字符串截取、复制字符串为大写或小写。这个类映射基于Unicode编码的Character类。

The Java language provides special support for the string
 * concatenation operator ( + ), 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>.

Java语言提供各种支持对字符串连接操作 或把其他对象转化成字符串。字符串连接操作通过StringBuilder 或StringBuffer类及其方法实现。转化对象可以通过各个对象自己的toString方法实现。

p> Unless otherwise noted, passing a <tt>null</tt> argument to a constructor
 * or method in this class will cause a {@link NullPointerException} to be
 * thrown.
 *
 * <p>A {@code String} represents a string in the UTF-16 format
 * in which <em>supplementary characters</em> are represented by <em>surrogate
 * pairs</em> (see the section <a href="Character.html#unicode">Unicode
 * Character Representations</a> in the {@code Character} class for
 * more information).
 * Index values refer to {@code char} code units, so a supplementary
 * character uses two positions in a {@code String}.
 * <p>The {@code String} class provides methods for dealing with
 * Unicode code points (i.e., characters), in addition to those for
 * dealing with Unicode code units (i.e., {@code char} values).
 * 

接下来 看看这个类伟大的作者们：

 @author  Lee Boynton
 * @author  Arthur van Hoff
 * @author  Martin Buchholz
 * @author  Ulf Zibis

 * @since   JDK1.0