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();
}