这是一个简单的循环，用于计算UTF-8表示形式的大小，并在超出时截断：
public static String truncateWhenUTF8(String s, int maxBytes) { int b = 0; for (int i = 0; i < s.length(); i++) { char c = s.charAt(i); // ranges from http://en.wikipedia.org/wiki/UTF-8 int skip = 0; int more; if (c <= 0x007f) { more = 1; } else if (c <= 0x07FF) { more = 2; } else if (c <= 0xd7ff) { more = 3; } else if (c <= 0xDFFF) { // surrogate area, consume next char as well more = 4; skip = 1; } else { more = 3; } if (b + more > maxBytes) { return s.substring(0, i); } b += more; i += skip; } return s;}这 确实可以
处理出现在输入字符串中的代理对。Java的UTF-8编码器（正确）将代理对输出为单个4字节序列而不是两个3字节序列，因此
truncateWhenUTF8()将返回最长的截断字符串。如果您在实现中忽略代理对，则截短的字符串可能会短于所需的长度。
我没有对该代码做很多测试，但是这里有一些初步测试：
private static void test(String s, int maxBytes, int expectedBytes) { String result = truncateWhenUTF8(s, maxBytes); byte[] utf8 = result.getBytes(Charset.forName("UTF-8")); if (utf8.length > maxBytes) { System.out.println("BAD: our truncation of " + s + " was too big"); } if (utf8.length != expectedBytes) { System.out.println("BAD: expected " + expectedBytes + " got " + utf8.length); } System.out.println(s + " truncated to " + result);}public static void main(String[] args) { test("abcd", 0, 0); test("abcd", 1, 1); test("abcd", 2, 2); test("abcd", 3, 3); test("abcd", 4, 4); test("abcd", 5, 4); test("au0080b", 0, 0); test("au0080b", 1, 1); test("au0080b", 2, 1); test("au0080b", 3, 3); test("au0080b", 4, 4); test("au0080b", 5, 4); test("au0800b", 0, 0); test("au0800b", 1, 1); test("au0800b", 2, 1); test("au0800b", 3, 1); test("au0800b", 4, 4); test("au0800b", 5, 5); test("au0800b", 6, 5); // surrogate pairs test("uD834uDD1E", 0, 0); test("uD834uDD1E", 1, 0); test("uD834uDD1E", 2, 0); test("uD834uDD1E", 3, 0); test("uD834uDD1E", 4, 4); test("uD834uDD1E", 5, 4);}更新了 修改后的代码示例，现在可以处理代理对。
