package 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) or a surrogate. Refer to Unicode Character Representation for details.
*
*
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.
*
* @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.
*
* @return the number of chars in this sequence
*/
int length();
/**
* Returns the char value at the specified index. An index ranges from zero
* to length() - 1. The first char value of the sequence is at
* index zero, the next at index one, and so on, as for array
* indexing.
*
*
If the char value specified by the index is a
* surrogate, the surrogate
* value is returned.
*
* @param index the index of the char value to be returned
*
* @return the specified char value
*
* @throws IndexOutOfBoundsException
* if the index argument is negative or not less than
* length()
*/
char charAt(int index);
/**
* Returns a new CharSequence that is a subsequence of this sequence.
* The subsequence starts with the char value at the specified index and
* ends with the char value at index end - 1. The length
* (in chars) of the
* returned sequence is end - start, so if start == end
* then an empty sequence is returned.
*
* @param start the start index, inclusive
* @param end the end index, exclusive
*
* @return the specified subsequence
*
* @throws IndexOutOfBoundsException
* if start or end are negative,
* if end is greater than length(),
* or if start is greater than end
*/
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.
*
* @return a string consisting of exactly this sequence of characters
*/
public String toString();
}
/**
* String、Stringbuffer、StringBuilder 三个字符相关的类都实现
* 了CharSequence 类，所以说，以后见到CharSequence描述的就是字符串
*/
public class CharSequenceStudy {
public static void main(String[] args) {
CharSequence str = "www.ayit.com";
CharSequence sub = str.subSequence(4,8);
System.out.println(sub);
System.out.println(str.length());
}
}
拓展一下：对于抽象类或者接口来说不可以直接使用new的方式创建对象，但是可以直接给它赋值； CharSequence b = "s"; > 。CharSequence b = "s" 是一个类型强转操作，等于CharSequence b = (CharSequence) new String("s")。
