/**
 * 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. For additional information on string
 * concatenation and conversion, see <i>The Java&trade; Language Specification</i>.
 *
 * <p> Unless otherwise noted, passing a {@code null} 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).
 *
 * <p>Unless otherwise noted, methods for comparing Strings do not take locale
 * into account.  The {@link java.text.Collator} class provides methods for
 * finer-grain, locale-sensitive String comparison.
 *
 * @implNote The implementation of the string concatenation operator is left to
 * the discretion of a Java compiler, as long as the compiler ultimately conforms
 * to <i>The Java&trade; Language Specification</i>. For example, the {@code javac} compiler
 * may implement the operator with {@code StringBuffer}, {@code StringBuilder},
 * or {@code java.lang.invoke.StringConcatFactory} depending on the JDK version. The
 * implementation of string conversion is typically through the method {@code toString},
 * defined by {@code Object} and inherited by all classes in Java.
 *
 * @author  Lee Boynton
 * @author  Arthur van Hoff
 * @author  Martin Buchholz
 * @author  Ulf Zibis
 * @see     java.lang.Object#toString()
 * @see     java.lang.StringBuffer
 * @see     java.lang.StringBuilder
 * @see     java.nio.charset.Charset
 * @since   1.0
 * @jls     15.18.1 String Concatenation Operator +
 */
public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
}

（3）方法上常用文档标记

1. @param：该文档标记后面写方法的参数名，再写参数描述。

   /**
   * @param str 
   * the {@code CharSequence} to check (may be {@code null})
   */
   public static boolean containsWhitespace(@Nullable CharSequence str) {}
   
   

2. @return：该文档标记后面写返回值得描述。

   /**
   * @return {@code true} if the {@code String} is not {@code null}, its
   */
   public static boolean hasText(@Nullable String str){}
   

3. @throws：该文档标记后面写异常的类型和异常的描述，用于描述该方法可能抛出的异常。

   /**
   * @throws IllegalArgumentException when the given source contains invalid encoded sequences
   */
   public static String uriDecode(String source, Charset charset){}
   
   

4. @exception：该标注用于描述方法签名throws对应的异常。

   /**
   * @exception IllegalArgumentException if <code>key</code> is null.
   */
   public static Object get(String key) throws IllegalArgumentException {}
   

5. @see：可用在类与方法上，表示参考的类或方法。

   /**
   * @see java.net.URLDecoder#decode(String, String)
   */
   public static String uriDecode(String source, Charset charset){}
   

以上是方法上常用的文档标注，方法上的文档格式如下：

1. 概要描述：通常用一段话简要的描述该方法的基本内容。
2. 详细描述：通常用几大段话详细描述该方法的功能与相关情况。
3. 文档标注：用于标注该方法的参数、返回值、异常、参略等信息。

以下是String类中charAt方法的示例：

/**
     * Returns the {@code char} value at the
     * specified index. An index ranges from {@code 0} to
     * {@code length() - 1}. The first {@code char} value of the sequence
     * is at index {@code 0}, the next at index {@code 1},
     * and so on, as for array indexing.
     *
     * <p>If the {@code char} value specified by the index is a
     * <a href="Character.html#unicode">surrogate</a>, the surrogate
     * value is returned.
     *
     * @param      index   the index of the {@code char} value.
     * @return     the {@code char} value at the specified index of this string.
     *             The first {@code char} value is at index {@code 0}.
     * @exception  IndexOutOfBoundsException  if the {@code index}
     *             argument is negative or not less than the length of this
     *             string.
     */
    public char charAt(int index) {}

/**
     * The value is used for character storage.
     *
     * @implNote This field is trusted by the VM, and is a subject to
     * constant folding if String instance is constant. Overwriting this
     * field after construction will cause problems.
     *
     * Additionally, it is marked with {@link Stable} to trust the contents
     * of the array. No other facility in JDK provides this functionality (yet).
     * {@link Stable} is safe here, because value is never null.
     */
    private final byte[] value;

package demo2;
/**
 * <p>这是一个测试javadoc的类
 * 
 * @author codepeace
 * @version 1.0
 * @since  1.8
 *
 */
public class HelloWorld {
	String name;
	
	/**
	 * 
	 * @param name
	 * @return name 
	 * @throws Exception
	 * {@code name} 
	 */
	public String test(String name)throws Exception{
		return name;
	}
}

package demo2;
/**
 * <p>这是一个测试javadoc的类
 * 
 * @author codepeace
 * @version 1.0
 * @since  1.8
 *
 */
public class Doc {
	String name;
	
	/**
	 * 
	 * @param name
	 * @return name 
	 * @throws Exception
	 * {@code name} 
	 */
	public String test(String name)throws Exception{
		return name;
	}
}