CharSequence类

CharSequence是char类型的一个可读序列，它本身是一个接口，CharBuffer、String、StringBuffer、StringBuilder这个四个类

实现了这个接口。此接口对于不同种类的char序列提供统一的只读访问

以下是这个函数的API 它只定义了四个方法

/**
* This interface represents an ordered set of characters and defines the
* methods to probe them.
*/
public interface CharSequence {

/**
* Returns the number of characters in this sequence.
*
* @return the number of characters.
*/
public int length();

/**
* Returns the character at the specified index, with the first character
* having index zero.
*
* @param index
*            the index of the character to return.
* @return the requested character.
* @throws IndexOutOfBoundsException
*             if {@code index < 0} or {@code index} is greater than the
*             length of this sequence.
*/
public char charAt(int index);

/**
* Returns a {@code CharSequence} from the {@code start} index (inclusive)
* to the {@code end} index (exclusive) of this sequence.
*
* @param start
*            the start offset of the sub-sequence. It is inclusive, that
*            is, the index of the first character that is included in the
*            sub-sequence.
* @param end
*            the end offset of the sub-sequence. It is exclusive, that is,
*            the index of the first character after those that are included
*            in the sub-sequence
* @return the requested sub-sequence.
* @throws IndexOutOfBoundsException
*             if {@code start < 0}, {@code end < 0}, {@code start > end},
*             or if {@code start} or {@code end} are greater than the
*             length of this sequence.
*/
public CharSequence subSequence(int start, int end);

/**
* Returns a string with the same characters in the same order as in this
* sequence.
*
* @return a string based on this sequence.
*/
public String toString();
}