驱动程序代码：

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);
    }
}

减速机代码

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，在驾驶课上我设置了 job.setOutputKeyClass(Text.class) 以及 job.setOutputValueClass(LongWritable.class) ，但在减速机课上我在写一个字符串 context.write(new Text(key1), "ABC"); . 我认为在运行程序时应该有一个错误，因为输出类型不匹配，而且reducer的键应该实现 WritableComparable 价值观应该实现 Writable 接口。奇怪的是，这个程序运行得很好。我不明白为什么没有例外。