此算法的来自网上，我做了修改与封装，拿出来以方便使用。

/**
* 拼音首字母的工具
*
* @author leizhimin 2009-7-4 16:41:28
*/
public class PinyinToolkit {
        private PinyinToolkit() {
        }

        //字母Z使用了两个标签，这里有２７个值
        //i, u, v都不做声母, 跟随前面的字母
        private static char[] chartable = {
                        '啊', '芭', '擦', '搭', '蛾', '发', '噶', '哈', '哈',
                        '击', '喀', '垃', '妈', '拿', '哦', '啪', '期', '然',
                        '撒', '塌', '塌', '塌', '挖', '昔', '压', '匝', '座'
        };
        //首字母表
        private static char[] initialtable = {
                        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
                        'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r',
                        's', 't', 'u', 'v', 'w', 'x', 'y', 'z'
        };

        private static int[] table = new int[27];

        //初始化
        static {
                for (int i = 0; i < 27; ++i) {
                        table[i] = gbValue(chartable[i]);
                }
        }

        /**
         * 根据一个包含汉字的字符串返回一个汉字拼音首字母的字符串
         *
         * @param SourceStr 源字符串
         * @return 拼音首字母的字符串
         */
        public static String cn2Pinyin(String SourceStr) {
                String Result = "";
                int StrLength = SourceStr.length();
                int i;
                try {
                        for (i = 0; i < StrLength; i++) {
                                Result += Char2Initial(SourceStr.charAt(i));
                        }
                } catch (Exception e) {
                        Result = "";
                }
                return Result;
        }

        //------------------------private方法区------------------------
        /**
         * 输入字符,得到他的声母,英文字母返回对应的大写字母,其他非简体汉字返回 '0'
         *
         * @param ch 字符
         * @return 拼音首字母
         */
        private static char Char2Initial(char ch) {
                if (ch >= 'a' && ch <= 'z')
                        return (char) (ch - 'a' + 'A');
                if (ch >= 'A' && ch <= 'Z')
                        return ch;
                int gb = gbValue(ch);
                if (gb < table[0])
                        return ch;
                int i;
                for (i = 0; i < 26; ++i) {
                        if (match(i, gb))
                                break;
                }
                if (i >= 26)
                        return ch;
                else
                        return initialtable[i];
        }


        private static boolean match(int i, int gb) {
                if (gb < table[i])
                        return false;
                int j = i + 1;
                //字母Z使用了两个标签
                while (j < 26 && (table[j] == table[i]))
                        ++j;
                if (j == 26)
                        return gb <= table[j];
                else
                        return gb < table[j];

        }

        /**
         * 取出汉字的编码
         * cn 汉字
         */
        private static int gbValue(char ch) {
                String str = new String();
                str += ch;
                try {
                        byte[] bytes = str.getBytes("GB2312");
                        if (bytes.length < 2)
                                return 0;
                        return (bytes[0] << 8 & 0xff00) + (bytes[1] & 0xff);
                } catch (Exception e) {
                        return 0;
                }
        }

        public static void main(String[] args) throws Exception {
                System.out.println(cn2Pinyin("熔岩"));
                System.out.println(cn2Pinyin(""));
                System.out.println(cn2Pinyin("熔岩abc123"));
        }
}

运行结果：

ry

ryABC123

Process finished with exit code 0

特别说明：本工具支持的中文文字有限，如果您有更好的算法，也请奉献出来，或与我交流。

当然，如果要获取完整的拼音，则需要借助拼音库来实现了。