Java中十进制字符串转十六进制的性能解读

String twoChars = decimalStr.substring(i, i + 2);
int decimalValue = Integer.parseInt(twoChars);

int value = (decimalStr.charAt(i) - '0') * 10 + 
           (decimalStr.charAt(i + 1) - '0');

StringBuilder hexBuilder = new StringBuilder();

StringBuilder hexBuilder = new StringBuilder(decimalStr.length() / 2);

String hexPart = String.format("%02X", decimalValue);

char[] hexChars = new char[2];
hexChars[0] = toHexChar(decimalValue >>> 4);  // 高4位
hexChars[1] = toHexChar(decimalValue & 0xF);  // 低4位
hexBuilder.append(hexChars);

// 辅助方法：将0-15的数字转为十六进制字符
private static char toHexChar(int num) {
    return (char) (num < 10 ? num + '0' : num - 10 + 'A');
}

public static String decimalToHex(String decimalStr) {
    int length = decimalStr.length();
    if (length == 0) return "";
    
    char[] decimalChars = decimalStr.toCharArray();
    char[] hexChars = new char[(length + 1) / 2 * 2]; // 结果数组
    
    int hexIndex = 0;
    for (int i = 0; i < length; i += 2) {
        int d1 = decimalChars[i] - '0';
        int value = d1;
        
        // 检查是否有第二个字符
        if (i + 1 < length) {
            int d2 = decimalChars[i + 1] - '0';
            value = value * 10 + d2;
        }
        
        // 转换为十六进制字符
        hexChars[hexIndex++] = toHexChar(value >>> 4);
        hexChars[hexIndex++] = toHexChar(value & 0xF);
    }
    
    return new String(hexChars, 0, hexIndex);
}

// 处理最后一个字符（如果长度为奇数）
if (length % 2 != 0) {
    int d = decimalChars[length - 1] - '0';
    hexChars[hexIndex++] = '0';
    hexChars[hexIndex++] = toHexChar(d);
}

decimalStr = decimalStr.replaceAll("[^0-9]", "");

// 手动过滤非数字字符
int filteredLength = 0;
for (int i = 0; i < length; i++) {
    char c = decimalStr.charAt(i);
    if (c >= '0' && c <= '9') {
        decimalChars[filteredLength++] = c;
    }
}

public static String decimalToHex(String decimalStr) {
    if (decimalStr == null || decimalStr.isEmpty()) {
        return "";
    }
    
    char[] decimalChars = decimalStr.toCharArray();
    int length = decimalChars.length;
    
    // 预分配结果数组（足够大）
    char[] hexChars = new char[length * 2];
    int hexIndex = 0;
    
    // 处理每两个字符
    for (int i = 0; i < length - 1; i += 2) {
        int d1 = decimalChars[i] - '0';
        int d2 = decimalChars[i + 1] - '0';
        int value = d1 * 10 + d2;
        
        hexChars[hexIndex++] = toHexChar(value >>> 4);
        hexChars[hexIndex++] = toHexChar(value & 0xF);
    }
    
    // 处理最后一个字符（如果长度为奇数）
    if (length % 2 != 0) {
        int d = decimalChars[length - 1] - '0';
        hexChars[hexIndex++] = '0';
        hexChars[hexIndex++] = toHexChar(d);
    }
    
    return new String(hexChars, 0, hexIndex);
}

private static char toHexChar(int num) {
    return (char) (num < 10 ? num + '0' : num - 10 + 'A');
}