Source code analysis of charsequence interface in jdk8

/**


  
  
  


   
   
   
Created by caoxiaohong on 17/11/18 23:18.

*/

  
  
  
import java.lang.*;
/**

  
  
  


   
   
   
A CharSequence is a readable sequence of char values. This

   
   
   
interface provides uniform,read-only access to many different kinds of

   
   
   
char sequences.

   
   
   
A char value represents a character in the Basic

Multilingual Plane (BMP)@H_403_22@ or a surrogate. Refer to <a
href="Character.html#unicode">Unicode Character Representation for details.


CharSequence就是一个可读的字符序列.对于不同类型的字符序列,这一接口都以统一且只读的方式去读取.
一个字符值代表了BMP中的一个字符或者一个代理.(BMP是什么?BMP包含了现代大多数语言的字符集)



 This interface does not refine the general contracts of the {@link

java.lang.Object#equals(java.lang.Object) equals} and {@link
java.lang.Object#hashCode() hashCode} methods.  The result of comparing two
objects that implement CharSequence is therefore,in general,* undefined.  Each object may be implemented by a different class,and there
is no guarantee that each class will be capable of testing its instances
for equality with those of the other.  It is therefore inappropriate to use
arbitrary CharSequence instances as elements in a set or as keys in
a map. 



这一接口,并没有提炼出Object类定义的equals()方法和hashCode()方法的通用规范(但是,像其他的接口,比如Map就有equals()方法和
hashCode()方法,再比如,虽然List接口没有给出这两个方法,但是抽象类Abstract重新定义了这两个方法).因此,对于两个不仅实现了
CharSequence接口的的对象(可能还继承了其他的类),进行比较时,通常,其结果也是未定义的(因为对于继承了Object的类来说,根据具体的实现,* 比较时,是可以有两种选择的,要么比较地址,使用两个等号==,要么比较内容,使用equals.但是这个接口并没有定义equals方法,也没有用到通常我们
做两个对象比较时,用到的equals方法和hashCode方法之间的关系,也就是你重写equals方法时,必须重写hashCode方法,这一点我之前在Object
源码的分析中说过了).每个对象都可以由不同的类来实现,因此,我们无法保证每个类都有能和其他类实例测试等价性的能力.因此,使用任意的
CharSequence实例作为set集合的元素或者map中的key,这种做法都是不合适的(为什么不合适呢?因为CharSequence实例是没有equals方法和
hashCode方法的,所以对应的实例的比较就取决于其继承的类或者其他实现的接口,那么两个被比较的类如果因为继承的类或者实现的接口
(并且对应继承的类或者接口都对equals和hashCode给出了自己的定义)不同,所以一旦被比较,出现什么结果都是不可控的,故不适合.).


@author Mike McCloskey
@since 1.4
@spec JSR-51

*/

  
  
  

public interface CharSequence {

/**

   
   
   


    
    
    
Returns the length of this character sequence.  The length is the number

    
    
    
of 16-bit chars in the sequence.


    
    
    



    
    
    
返回字符序列的长度.

    
    
    
长度是16bit的整数倍.(因为String类采用的是UTF-16编码,一个字符占用2个字节长度)

    
    
    



    
    
    
@return  the number of chars in this sequence

*/

int length();

   
   
   
/**
 * Returns the <code>char</code> value at the specified index.  An index ranges from zero
 * to <tt>length() - 1</tt>.  The first <code>char</code> value of the sequence is at
 * index zero,the next at index one,and so on,as for array
 * indexing. </p>
 *
 * 返回指定索引index位置处的字符.索引index的范围是[0,length()-1].
 *
 * <p>If the <code>char</code> value specified by the index is a
 * <a href="{@docRoot}/java/lang/Character.html#unicode"&gt;surrogate</a>,the surrogate
 * value is returned.
 *
 * 如果指定索引位置处的字符值为代表(字符)(surrogate的出现原因:因为UTF-16采用2个字节存储<a href="https://www.jb51.cc/tag/yige/" target="_blank" class="keywords">一个</a>字符,但是有的字符存储只需要<a href="https://www.jb51.cc/tag/yige/" target="_blank" class="keywords">一个</a>字节,比如英文
 * 字符,那么下<a href="https://www.jb51.cc/tag/yige/" target="_blank" class="keywords">一个</a>字节也不能继续存储其他的字符,而只能存储<a href="https://www.jb51.cc/tag/yige/" target="_blank" class="keywords">一个</a>代表字符,来占用这<a href="https://www.jb51.cc/tag/yige/" target="_blank" class="keywords">一个</a>字节的位置,接下来的<a href="https://www.jb51.cc/tag/yige/" target="_blank" class="keywords">一个</a>字节处才能继续存储下<a href="https://www.jb51.cc/tag/yige/" target="_blank" class="keywords">一个</a>字符),* 那么返回的也会是这个代表(字符)值.
 *
 * @param   index   the index of the <code>char</code> value to be returned
 *
 * @return  the specified <code>char</code> value
 *
 * @throws  <a href="https://www.jb51.cc/tag/indexoutofboundsexception/" target="_blank" class="keywords">indexoutofboundsexception</a>
 *          if the <tt>index</tt> argument is negative or not less than
 *          <tt>length()</tt>
 */
char charAt(int index);

/**
 * Returns a new <code>CharSequence</code> that is a subsequence of this sequence.
 * The subsequence starts with the <code>char</code> value at the specified index and
 * ends with the <code>char</code> value at index <tt>end - 1</tt>.  The length
 * (in <code>char</code>s) of the
 * returned sequence is <tt>end - start</tt>,so if <tt>start == end</tt>
 * then an empty sequence is returned. </p>
 *
 * 返回<a href="https://www.jb51.cc/tag/yige/" target="_blank" class="keywords">一个</a>新的字符序列,这个序列是原字符序列的子序列.
 * 子序列的<a href="https://www.jb51.cc/tag/jiequ/" target="_blank" class="keywords">截取</a>开始位置为:原序列中start的位置;
 *        <a href="https://www.jb51.cc/tag/jiequ/" target="_blank" class="keywords">截取</a>结束位置为:原序列中(end-1)的位置.
 * 因此子字符序列的长度为(end-start)
 * 所以,如果传入参数start=end,则返回子序列为空序列.
 *
 * @param   start   the start index,inclusive
 * @param   end     the end index,exclusive
 *
 * @return  the specified subsequence
 *
 * @throws  <a href="https://www.jb51.cc/tag/indexoutofboundsexception/" target="_blank" class="keywords">indexoutofboundsexception</a>
 *          if <tt>start</tt> or <tt>end</tt> are negative,*          if <tt>end</tt> is greater than <tt>length()</tt>,*          or if <tt>start</tt> is greater than <tt>end</tt>
 */
CharSequence subSequence(int start,int end);

/**
 * Returns a string containing the characters in this sequence in the same
 * order as this sequence.  The length of the string will be the length of
 * this sequence. </p>
 *
 * 返回字符序列的字符串形式,字符串中字符的顺序和字符序列保持一致.字符串的长度和字符序列一致.
 *
 * @return  a string consisting of exactly this sequence of characters
 */
public java.lang.String toString();



     }