How to customize Writable class in Hadoop?

Question

I\'m trying to implement Writable class, but i have no idea on how to implement a writable class if in my class there is nested object, such as list, etc. Could any body help me

Answer 1

Serializing collections is quite simple.

@Override
public void readFields(DataInput in) throws IOException {
    int size = in.readInt();
    list= new ArrayList<Field>(size);
    for(int i = 0; i < size; i++){
        Field f = new Field();
        f.readFields(in);
        list.add(f);
    }
}

@Override
public void write(DataOutput out) throws IOException {
    out.writeInt(list.size());
    for (Field l : list) {
        l.write(out);
    }
}

Field has to implement Writable as well.

Answer 2

This tutorial explains better : http://www.hadoopmaterial.com/2013/10/custom-hadoop-writable-data-type.html