This discussion can be useful for following types of question as well

• How an event can be fired by selecting a combo box / Drop Down List box in android? 
• How spinner's activity can be tracked on a change of a item selection?

package com.example.testproject;

import java.util.ArrayList;
import java.util.List;

import android.os.Bundle;
import android.app.Activity;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.Spinner;

/**
 * This is the class for this activity and it implements 
 * the Interface OnItemSelectedListener
 * @author Avijit Pramanik
 *
 */
public class MainActivityActivity extends Activity 
     implements OnItemSelectedListener {
 Spinner spName;
 EditText txtAge;

 List<String> listName=new ArrayList<String>();
 List<String> listAge=new ArrayList<String>();

  /**
  * onCreate method is fired for initializing
  * your activity
  */
 protected void onCreate(Bundle savedInstanceState){
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main_activity);

  spName=(Spinner)findViewById(R.id.spinnerName);
  txtAge=(EditText)findViewById(R.id.editTextAge);

  //This method populate the ArrayList<String> and then 
  //use that ArrayList to ArrayAdapter which eventually 
  //is the source of the Spinner data
  populateSpinner();
  
  //Here Spinner has been set this Class for Listen on any
  //changes by selecting any of its item which eventually 
  //trigger the method onItemSelected.
  spName.setOnItemSelectedListener(this);

  }

  public void populateSpinner(){
  listName.add("Arnold");
  listName.add("Peter");
  listName.add("John");

   ArrayAdapter<String> adapter=new ArrayAdapter<String>(this,android.R.layout.simple_spinner_item,listName);
  adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
  spName.setAdapter(adapter);

  listAge.add("14");
  listAge.add("15");
  listAge.add("16");
 }

  /**
  * This method is fired when something selected on the Spinner.
  */
 public void onItemSelected(AdapterView<?> parentView,View v,int position,long id){
  String strAge=listAge.get(position).toString();
  txtAge.setText(strAge);
 }

  public void onNothingSelected(AdapterView<?> parentView){
  //
 }
}

main_activity.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivityActivity" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

    <Spinner
        android:id="@+id/spinnerName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/textView1"
        android:layout_below="@+id/textView1"
        android:layout_marginTop="36dp" />

    <EditText
        android:id="@+id/editTextAge"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/spinner1"
        android:layout_marginTop="24dp"
        android:ems="10" >

        <requestFocus />
    </EditText>

</RelativeLayout>