//NSString转NSData
        NSString *string = @"abcd12345";
        NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding];
        NSLog(@"data = %@",data);

        //NSData转NSString
        NSString *string1 = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
        NSLog(@"string1 = %@",string1);

        //NSData转换为UIImage
        NSData *imageData = [NSData dataWithContentsOfFile: imagePath];
        UIImage *image = [UIImage imageWithData: imageData];    

         //UIImage转换为NSData
         NSData *imageData = UIImagePNGRepresentation(image);

        //char * 转 NSData
        const char *string2 = "hello!";
        NSData *data1 = [NSData dataWithBytes:string2 length:strlen(string2)+1];
        NSLog(@"data1 = %@,data1Length = %lu,data1Bytes = %s",data1,(unsigned long)[data1 length],[data1 bytes]);

        //bytes转NSData
        NSData *data2 = [NSData dataWithBytes:[data1 bytes]  length:strlen(string2)+1];
        NSLog(@"data2 = %@,data2Bytes = %s",data2,[data2 bytes]);

        //NSData转Byte
        NSString *string3 = @"Byte";
        NSData *data3 = [string3 dataUsingEncoding:NSUTF8StringEncoding];
        Byte *byte = (Byte *)[data3 bytes];
        NSLog(@"byte = %s",byte);