Question

I am writing a Java implementation program in Hadoop MapReduce Framework. And I am writing aclass called CombinePatternReduce.class. In order to debug the reducer in Eclipse, I write a main() function as following:

@SuppressWarnings("unchecked")
public static void main(String[] args) throws IOException, InterruptedException{
    Text key = new Text("key2:::key1:::_ performs better than _");
    IntWritable count5 = new IntWritable(5);
    IntWritable count3 = new IntWritable(3);
    IntWritable count8 = new IntWritable(8);
    List<IntWritable> values = new ArrayList<IntWritable>();
    values.add(count5);
    values.add(count3);
    values.add(count8);
    CombinePatternReduce reducer = new CombinePatternReduce();
    Context dcontext = new DebugTools.DebugReducerContext<Text, IntWritable, KeyPairWritableComparable, WrapperDoubleOrPatternWithWeightWritable>(reducer, key, count3); // here is the problem
    reducer.reduce(key, values, dcontext);      
}

The DebugTools.DebugReducerContext is a class that I write to make the debugging process easier to perform, and it is as following:

public static class DebugReducerContext<KIN, VIN, KOUT, VOUT> extends Reducer<KIN, VIN, KOUT, VOUT>.Context {
    DebugTools dtools = new DebugTools();
    DataOutput out = dtools.new DebugDataOutputStream(System.out);

    public DebugReducerContext(Reducer<KIN, VIN, KOUT, VOUT> reducer, Class<KIN> keyClass, Class<VIN> valueClass) throws IOException, InterruptedException{
        reducer.super(new Configuration(), new TaskAttemptID(), new DebugRawKeyValueIterator(), null, null, 
                null, null, null, null, keyClass, valueClass);
    }

    @Override
    public void write(Object key, Object value) throws IOException, InterruptedException {
        writeKeyValue(key, value, out);
    }

    @Override
    public void setStatus(String status) {
        System.err.println(status);
    }
}

The problem is in the first part of code, namely main(). When I write

Context dcontext = new DebugTools.DebugReducerContext<Text, IntWritable, KeyPairWritableComparable, WrapperDoubleOrPatternWithWeightWritable>(reducer, key, count3);

There is an error that

The constructor DebugTools.DebugReducerContext<Text,IntWritable,KeyPairWritableComparable,WrapperDoubleOrPatternWithWeightWritable>(CombinePatternReduce, Text, IntWritable) is undefined.

When I write

Context dcontext = new DebugTools.DebugReducerContext<Text, IntWritable, KeyPairWritableComparable, WrapperDoubleOrPatternWithWeightWritable>(reducer, key, values);

There is an error that

The constructor DebugTools.DebugReducerContext<Text,IntWritable,KeyPairWritableComparable,WrapperDoubleOrPatternWithWeightWritable>(CombinePatternReduce, Text, List<IntWritable>) is undefined.

Since the documentation of Reducer.Context is

public Reducer.Context(Configuration conf,
                       TaskAttemptID taskid,
                       RawKeyValueIterator input,
                       Counter inputKeyCounter,
                       Counter inputValueCounter,
                       RecordWriter<KEYOUT,VALUEOUT> output,
                       OutputCommitter committer,
                       StatusReporter reporter,
                       RawComparator<KEYIN> comparator,
                       Class<KEYIN> keyClass,
                       Class<VALUEIN> valueClass)
                throws IOException,
                       InterruptedException

I need to pass in a Class<KEYIN> keyClass and Class<VALUEIN> valueClass. So how can I write the main() function (especially the sentence with error) to debug the reducer class?

Answer 1

It’s very clear that the class constructor takes 3 parameters. An instance of a reducer, a class for the key, and a class for the value.

Instead of actually passing the key and value. You need to supply it with link to the class

Context dcontext = new DebugTools.DebugReducerContext<Text, IntWritable, KeyPairWritableComparable, WrapperDoubleOrPatternWithWeightWritable>(reducer, Text.class, IntWritable.class);

Essentially this is reaffirming the type of values the context should be able to handle for reducing.