首先看文档,其实也是用 String.indexOf 来实现的

     /**
     * Returns true if and only if this string contains the specified
     * sequence of char values.
     *
     * @param s the sequence to search for
     * @return true if this string contains {@code s}, false otherwise
     * @since 1.5
     */
    public boolean contains(CharSequence s) {
        return indexOf(s.toString()) >= 0;
    } 

而 String.indexOf 最后长这样

    @IntrinsicCandidate
    public static int indexOf(byte[] value, int valueCount, byte[] str, int strCount, int fromIndex) {
        byte first = str[0];
        int max = (valueCount - strCount);
        for (int i = fromIndex; i <= max; i++) {
            // Look for first character.
            if (value[i] != first) {
                while (++i <= max && value[i] != first);
            }
            // Found first character, now look at the rest of value
            if (i <= max) {
                int j = i + 1;
                int end = j + strCount - 1;
                for (int k = 1; j < end && value[j] == str[k]; j++, k++);
                if (j == end) {
                    // Found whole string.
                    return i;
                }
            }
        }
        return -1;
    }

可以看见就是笨办法...从第一个字母开始对比,不行就下一位,一个一个试哈哈