How to set mapOutputValueClass as Enum in hadoop

Question

My mapper output value is a type of an Enum, and I set the mapOutputValueClass as the Enum.class, but always get
Type mismatch in value from map expected StatisticTypes, recieved StatisticTypes$2

It seems like the it received a nested class instead of a subclass, is there anything wrong with my Enum class?

Job set up:

job.setMapOutputValueClass(StatisticTypes.class);

Output:

for(StatisticTypes statistic : dimensionCountMap.get(key)) {
    context.write(key, statistic);
}

Enum:

public enum StatisticTypes implements Writable{
MAX {
    @Override
    public boolean aggregate(long v, LongWritable userId) {
        if (v > value) {
            value = v;
            this.userId = userId;
            return true;
        }
        return false;
    }

    @Override
    public void write(DataOutput out) throws IOException {
        ObjectWritable objWritable = new ObjectWritable(this);
        objWritable.write(out);
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        ObjectWritable objWritable = new ObjectWritable();
        objWritable.readFields(in);
    }
};
public LongWritable userId;
public long value;
public abstract boolean aggregate(long v, LongWritable userId);
public long getValue() {
    return value;
};
public LongWritable getUserId() {
    return userId;
}
@Override
public void write(DataOutput out) throws IOException {
    ObjectWritable objWritable = new ObjectWritable(this);
    objWritable.write(out);
}

@Override
public void readFields(DataInput in) throws IOException {
    ObjectWritable objWritable = new ObjectWritable();
    objWritable.readFields(in);
}

}