/**

     * Returns a hash code for this string. The hash code for a

     * {@code String} object is computed as

     * <blockquote><pre>

     * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

     * </pre></blockquote>

     * using {@code int} arithmetic, where {@code s[i]} is the

     * <i>i</i>th character of the string, {@code n} is the length of

     * the string, and {@code ^} indicates exponentiation.

     * (The hash value of the empty string is zero.)

     *

     * @return  a hash code value for this object.

     */
　　　　private int hash;
　　　　private char[] value;

    public int hashCode() {

        int h = hash;

        if (h == 0 && value.length > 0) {

            char val[] = value;

            for (int i = 0; i < value.length; i++) {

                h = 31 * h + val[i];

            }

            hash = h;

        }

        return h;

    }