How to add values in reducer class into an arraylist (Hadoop)?

Question

My Mapper output:

1504652886  Geography
8904209587  Science
8904209587  Math
9341024668  English9
9341024668  Science

I am trying to write a reducer class now that will combine the common keys and generate an output as shown below:

1504652886  Geography
8904209587  Science, Math
9341024668  English9, Science

In the reducer class, I tried to make an arraylist that will contain all courses for a particular ID but I am surely doing something wrong. My code is as below:

public static class Reduce extends Reducer<Text, Text, Text, Text> {

    @Override 
    public void reduce(Text key, Iterable<Text> values, Context context)
    throws IOException, InterruptedException {
        ArrayList<String> courses = new ArrayList<String>();
        for(Text x : values)
        {
            courses.add((Text) x);
        }
    }      
}

But I am missing something and getting error:

The method add(String) in the type ArrayList<String> is not applicable for the arguments (Text)

Can anybody please advise how to get the output?

Answer 1

Hadoop's Text class has a toString() method that returns a String representation of this object. So could just replace the following in your code:

for(Text x : values)
{
    courses.add(x.toString());
}

Then, you will need to convert the ArrayList back to Text, however, in order to emit it as a key.

You are also missing the write() method, which actually emits the output.

You could use a StringBuilder instead, which should be faster than using an ArrayList:

public static class Reduce extends Reducer<Text, Text, Text, Text> {

    Text valueToEmit = new Text();

    public void reduce(Text key, Iterable<Text> values, Context context)
    throws IOException, InterruptedException {
        StringBuilder sb = new StringBuilder();
        for(Text x : values)
        {
            sb.append(x.toString()).append(",");
        }

        valueToEmit.set(sb.substring(0,sb.length()-1)); //to remove the last ','
        context.write(key, valueToEmit);
    } 
}