一旦UTF-8编码，如何截断java字符串以适合给定的字节数？

Z时代
2024-01-10
分类：问答

我如何截断一个Java，String以便我知道一旦它以UTF-8编码，它将适合给定数量的字节存储？

回答：

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("a\u0080b", 0, 0);
    test("a\u0080b", 1, 1);
    test("a\u0080b", 2, 1);
    test("a\u0080b", 3, 3);
    test("a\u0080b", 4, 4);
    test("a\u0080b", 5, 4);
    test("a\u0800b", 0, 0);
    test("a\u0800b", 1, 1);
    test("a\u0800b", 2, 1);
    test("a\u0800b", 3, 1);
    test("a\u0800b", 4, 4);
    test("a\u0800b", 5, 5);
    test("a\u0800b", 6, 5);
    // surrogate pairs
    test("\uD834\uDD1E", 0, 0);
    test("\uD834\uDD1E", 1, 0);
    test("\uD834\uDD1E", 2, 0);
    test("\uD834\uDD1E", 3, 0);
    test("\uD834\uDD1E", 4, 4);
    test("\uD834\uDD1E", 5, 4);
}