String.hashcode()

String.java中對hashCode的定義如下：

    /**
     * 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.
     */
    public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            hash = h = isLatin1() ? StringLatin1.hashCode(value)
                                  : StringUTF16.hashCode(value);
        }
        return h;
    }

StringLatin1.hashCode

    public static int hashCode(byte[] value) {
        int h = 0;
        for (byte v : value) {
            h = 31 * h + (v & 0xff);
        }
        return h;
    }

StringUTF16.hashCode

    public static int hashCode(byte[] value) {
        int h = 0;
        int length = value.length >> 1;
        for (int i = 0; i < length; i++) {
            h = 31 * h + getChar(value, i);
        }
        return h;
    }

一般都建議，如果重新定義equals方法，必須重新定義hashCode方法，以便用戶可以插入到散列表中。