/*
         NSData不可变data
         */
        //获取data
        NSData * data = [NSData dataWithContentsOfFile:@"/Users/song/Desktop/测试/abc.txt"];
        NSLog(@"data的长度为：%ld",data.length);
        
        //把加载到data中的文件写入到指定位置
        BOOL isWriteSuccess = [data writeToFile:@"/Users/song/Desktop/测试/abcnew.txt" atomically:YES];
        if (isWriteSuccess) {
            NSLog(@"写入成功");
        }else{
            NSLog(@"写入失败");
        }
        
        /*
         NSMutableData可变的data
         */
        NSMutableData * mutableData = [[NSMutableData alloc] init];
        NSString * str1 = @"www.";
        NSData * data1 = [str1 dataUsingEncoding:NSUTF8StringEncoding];
        [mutableData appendData:data1];
        NSString * str2 = @"baidu.";
        NSData * data2 = [str2 dataUsingEncoding:NSUTF8StringEncoding];
        [mutableData appendData:data2];
        NSString * str3 = @"com";
        NSData * data3 = [str3 dataUsingEncoding:NSUTF8StringEncoding];
        [mutableData appendData:data3];
        BOOL isWriteMuTableSuccess = [mutableData writeToFile:@"/Users/song/Desktop/测试/mutableabc.txt" atomically:YES];
        if (isWriteMuTableSuccess) {
            NSLog(@"写入成功");
        }else{
            NSLog(@"写入失败");
        }