3

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);
    }
}

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，在驱动程序类中我设置了job.setOutputKeyClass(Text.class)and job.setOutputValueClass(LongWritable.class)，但在减速器类中我正在编写一个 string context.write(new Text(key1), "ABC");。我认为运行程序时应该会出错，因为输出类型不匹配，并且reducer的键应该实现WritableComparable，值应该实现Writable接口。奇怪的是，这个程序运行良好。我不明白为什么没有例外。

4

2 回答 2

3

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 作为键，Text 作为值，如果你不定义 outPutFormat 类，它会期望得到可写的默认行为，这是默认的，但如果你提到它，它会隐式地将其转换为给定的类型。;

于 2014-02-23T13:05:05.567 回答
1

尝试这个

//job.setOutputValueClass(LongWritable.class); if you comment this line you get an error
this will for only define the key value pair by defaul it depent on the output format and
it will be text so this is not giving any error
于 2014-02-23T13:05:05.597 回答