Android Amazon DynamoDb error in basic action

I am trying to build a simple android app that can interact with Amazon DynamoDB. I have download the latest Android Studio and also AWS mobile SDK. What I am trying to do now is simply insert a row into my table "customer". However when I run the app I encounter the following error. Would really appreciate if you guys can help me take a look and sorry if I make some stupid mistakes.(I have follow the doc to set up identification pool, create new policy, put stuff in AsyncTask..etc)

Thanks,

An

Error

07-21 23:31:59.781    4207-4234/com.example.an.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: AsyncTask #1
Process: com.example.an.myapplication, PID: 4207
java.lang.RuntimeException: An error occured while executing doInBackground()
        at android.os.AsyncTask$3.done(AsyncTask.java:304)
        at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
        at java.util.concurrent.FutureTask.run(FutureTask.java:242)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
        at java.lang.Thread.run(Thread.java:818)
 Caused by: com.amazonaws.mobileconnectors.dynamodbv2.dynamodbmapper.DynamoDBMappingException: No interface com.amazonaws.mobileconnectors.dynamodbv2.dynamodbmapper.DynamoDBHashKey annotation found in class class com.example.an.myapplication.customers
        at com.amazonaws.mobileconnectors.dynamodbv2.dynamodbmapper.DynamoDBMapper.needAutoGenerateAssignableKey(DynamoDBMapper.java:720)
        at com.amazonaws.mobileconnectors.dynamodbv2.dynamodbmapper.DynamoDBMapper.save(DynamoDBMapper.java:780)
        at com.amazonaws.mobileconnectors.dynamodbv2.dynamodbmapper.DynamoDBMapper.save(DynamoDBMapper.java:688)
        at com.example.an.myapplication.MainActivity$db.doInBackground(MainActivity.java:67)
        at com.example.an.myapplication.MainActivity$db.doInBackground(MainActivity.java:56)
        at android.os.AsyncTask$2.call(AsyncTask.java:292)
        at java.util.concurrent.FutureTask.run(FutureTask.java:237)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
        at java.lang.Thread.run(Thread.java:818)

MainActivity.java

package com.example.an.myapplication;
import android.content.Context;
import android.os.AsyncTask;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import com.amazonaws.auth.CognitoCachingCredentialsProvider;
import com.amazonaws.regions.Regions;
import com.amazonaws.services.dynamodbv2.*;
import com.amazonaws.mobileconnectors.dynamodbv2.dynamodbmapper.*;



public class MainActivity extends ActionBarActivity {
private CognitoCachingCredentialsProvider credentialsProvider;
private static final String TAG = MainActivity.class.getSimpleName();


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    credentialsProvider = new CognitoCachingCredentialsProvider(
            this.getApplicationContext(),    /* get the context for the current activity */
            "us-east-1:ef8bfc69-88b7-4714-ae0a-223f08a20fe5", // Identity Pool ID
            Regions.US_EAST_1 // Region
    );
    new db().execute("");
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

private class db extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... params) {

        AmazonDynamoDBClient ddbClient = new AmazonDynamoDBClient(credentialsProvider);
        DynamoDBMapper mapper = new DynamoDBMapper(ddbClient);
        customers customer = new customers();
        customer.setCustomerID("GG");
        customer.setBalance(5);
        mapper.save(customer);
        return "Executed";
    }

    @Override
    protected void onPostExecute(String result) {}

    @Override
    protected void onPreExecute() {}

    @Override
    protected void onProgressUpdate(Void... values) {}
}
}

customer.java

package com.example.an.myapplication;
import com.amazonaws.mobileconnectors.dynamodbv2.dynamodbmapper.*;


@DynamoDBTable(tableName = "customer")
public class customers {
private String id;
private int balance;

@DynamoDBIndexHashKey(attributeName = "CustomerID")
public String getCustomerID() {
    return id;
}

public void setCustomerID(String id) {
    this.id = id;
}

@DynamoDBAttribute(attributeName = "Balance")
public int getBalance() {
    return balance;
}

public void setBalance(int balance) {
    this.balance = balance;
}
}