CharSequence接口的实现类

package jevie.demo;public class CharSequenceDemo implements CharSequence {    private String s;//该类中的私有成员    public CharSequenceDemo(String s) {    //构造方法        this.s = s;    }    //If the string is backwards, the end is the beginning!    private int fromEnd(int i) {        return s.length() - 1 - i;    }    public char charAt(int i) {        if ((i < 0) || (i >= s.length())) {            throw new StringIndexOutOfBoundsException(i);        }        return s.charAt(fromEnd(i));    }    public int length() {        return s.length();    }    public CharSequence subSequence(int start, int end) {        if (start < 0) {            throw new StringIndexOutOfBoundsException(start);        }        if (end > s.length()) {            throw new StringIndexOutOfBoundsException(end);        }        if (start > end) {            throw new StringIndexOutOfBoundsException(start - end);        }        StringBuilder sub =             new StringBuilder(s.subSequence(fromEnd(end), fromEnd(start)));        return sub.reverse();    }    public String toString() {        StringBuilder s = new StringBuilder(this.s);        return s.reverse().toString();    }    //Random int from 0 to max. As random() generates values between 0 and 0.9999    private static int random(int max) {        return (int) Math.round(Math.random() * (max+1));    }    public static void main(String[] args) {        CharSequenceDemo s =            new CharSequenceDemo("Write a class that implements the CharSequence interface found in the java.lang package.");        //exercise charAt() and length()        for (int i = 0; i < s.length(); i++) {            System.out.print(s.charAt(i));        }                System.out.println("");        //exercise subSequence() and length();        int start = random(s.length() - 1);        int end = random(s.length() - 1 - start) + start;        System.out.println(s.subSequence(start, end));        //exercise toString();        System.out.println(s);    }}