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();
}