java - Driver 中的 job.setOutputKeyClass 和 setOutputValueClass 与 reducer 的 context.write 方法不匹配，程序仍然运行正常。怎么办？

Question

驱动代码:

public class WcDriver {

    public static void main(String[] args) throws IOException,
        InterruptedException, ClassNotFoundException {
        Configuration conf = new Configuration();

        Job job = new Job(conf, "WcDriver");
        job.setJarByClass(WcDriver.class);

        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(LongWritable.class);

        job.setInputFormatClass(TextInputFormat.class);
        job.setOutputFormatClass(TextOutputFormat.class);

        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        job.setMapperClass(WcMapper.class);
        job.setReducerClass(WcReducer.class);

        job.waitForCompletion(true);
    }
}

reducer 代码

public class WcReducer extends Reducer<Text, LongWritable, Text,String>
{   
    @Override
    public void reduce(Text key, Iterable<LongWritable> values, Context context) throws IOException, InterruptedException {
        String key1 = null;
        int total = 0;
        for (LongWritable value : values) {
            total += value.get();
            key1= key.toString();
        }
        context.write(new Text(key1), "ABC");
    }
}

在这里，在驱动程序类中我设置了 job.setOutputKeyClass(Text.class) 和 job.setOutputValueClass(LongWritable.class)，但在 reducer 类中我正在编写字符串 context.write(new Text(key1), "ABC");。我认为运行程序时应该会出现错误，因为输出类型不匹配，而且 reducer 的键应该实现 WritableComparable 并且值应该实现 Writable 接口(interface)。奇怪的是，这个程序运行良好。我不明白为什么没有异常(exception)。

Answer 1

尝试这样做:

 //  job.setOutputFormatClass(TextOutputFormat.class); 
// comment this line, and you'll sure get exception of casting.

这是因为，TextOutputFormat 假定 LongWritable 为键，Text 为值，如果您不定义 outPutFormat 类，它将期望获得可写的默认行为，这是默认情况，但如果您提及它, 它会将其隐式转换为给定类型。;