help with Hadoop custom Writable types implementation

[z3r0c001]

I'm trying to implement Writable interface. but not sure how to
serialize/write/read data from nested objects in

public class StorageClass implements Writable{

public String xStr;
public String yStr;

public List<SubStorage> sStor

//omitted ctors


@override
public void write(DataOutput out) throws IOException{
    out.writeChars(xStr);
    out.WriteChars(yStr);

    //WHAT SHOULD I DO FOR List<SubStorage>

}

@override
public void readFields(DataInput in) throws IOException{
    xStr = in.readLine();
    yStr = in.readLine();

    //WHAT SHOULD I DO FOR List<SubStorage>
}

}

public class SubStorage{
    public String x;
    public String y;
}