NSData转16进制字符串收到NSNull,尝试以下方式都是空值:

[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
[NSString stringWithUTF8String:[data bytes]];

以上获得的新字符就会为nil，这时我们应该这样转:

//十六进制的string为转换data
+ (NSData *) stringToHexData:(NSString *)hexStr
{
    int len = [hexStr length] / 2;    // Target length
    unsigned char *buf = malloc(len);
    unsigned char *whole_byte = buf;
    char byte_chars[3] = {'\0','\0','\0'};
    
    int i;
    for (i=0; i < [hexStr length] / 2; i++) {
        byte_chars[0] = [hexStr characterAtIndex:i*2];
        byte_chars[1] = [hexStr characterAtIndex:i*2+1];
        *whole_byte = strtol(byte_chars, NULL, 16);
        whole_byte++;
    }
    
    NSData *data = [NSData dataWithBytes:buf length:len];
    free( buf );
    return data;
}


//data转换为十六进制的string
+ (NSString *)hexStringFromData:(NSData *)myD{
    
    Byte *bytes = (Byte *)[myD bytes];
    //下面是Byte 转换为16进制。
    NSString *hexStr=@"";
    for(int i=0;i<[myD length];i++)
        
    {
        NSString *newHexStr = [NSString stringWithFormat:@"%x",bytes[i]&0xff];///16进制数
        
        if([newHexStr length]==1)
            
            hexStr = [NSString stringWithFormat:@"%@0%@",hexStr,newHexStr];
        
        else
            
            hexStr = [NSString stringWithFormat:@"%@%@",hexStr,newHexStr];
    }
    NSLog(@"hex = %@",hexStr);
    
    return hexStr;
}