Scan for wireless networks in Android WiFi

This class provides the primary API for managing all aspects of Wi-Fi connectivity. Get an instance of this class by calling Context.getSystemService(Context.WIFI_SERVICE). It deals with several categories of items:

• The list of configured networks. The list can be viewed and updated, and attributes of individual entries can be modified.
• The currently active Wi-Fi network, if any. Connectivity can be established or torn down, and dynamic information about the state of the network can be queried.
• Results of access point scans, containing enough information to make decisions about what access point to connect to.
• It defines the names of various Intent actions that are broadcast upon any sort of change in Wi-Fi state.

This is the API to use when performing Wi-Fi specific operations. To perform operations that pertain to network connectivity at an abstract level, use ConnectivityManager. Scan for wireless networks in the current area:

package com.android.wifitester;

import java.util.List;
import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.net.wifi.ScanResult;
import android.net.wifi.WifiManager;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

public class WifiTester extends Activity {
  TextView mainText;
  WifiManager mainWifi;
  WifiReceiver receiverWifi;
  List<ScanResult> wifiList;
  StringBuilder sb = new StringBuilder();
  
  public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.main);
   mainText = (TextView) findViewById(R.id.mainText);
   mainWifi = (WifiManager) getSystemService(Context.WIFI_SERVICE);
   receiverWifi = new WifiReceiver();
   registerReceiver(receiverWifi, new IntentFilter(
      WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
   mainWifi.startScan();
   mainText.setText("\nStarting Scan...\n");
  }

  public boolean onCreateOptionsMenu(Menu menu) {
    menu.add(0, 0, 0, "Refresh");
    return super.onCreateOptionsMenu(menu);
  }

  public boolean onMenuItemSelected(int featureId, MenuItem item) {
    mainWifi.startScan();
    mainText.setText("Starting Scan");
    return super.onMenuItemSelected(featureId, item);
  }

  protected void onPause() {
    unregisterReceiver(receiverWifi);
    super.onPause();
  }

  protected void onResume() {
    registerReceiver(receiverWifi, new IntentFilter(
       WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
    super.onResume();
  }
  
  class WifiReceiver extends BroadcastReceiver {
    public void onReceive(Context c, Intent intent) {
    sb = new StringBuilder();
    wifiList = mainWifi.getScanResults();
    for(int i = 0; i < wifiList.size(); i++){
      sb.append(new Integer(i+1).toString() + ".");
      sb.append((wifiList.get(i)).toString());
      sb.append("\n");
    }
    mainText.setText(sb);
    }
  }
}

this exammple has been provided by sissi from androidsnippets.
More details resources :
1.http://groups.google.com/group/android-developers/browse_thread/thread/0f722d5f90cfae69
2.http://developer.android.com/reference/android/net/wifi/WifiManager.html
3.http://developer.android.com/reference/android/net/wifi/package-summary.html
4.http://stackoverflow.com/questions/2981751/android-scan-for-wifi-networks
5.http://stackoverflow.com/questions/3640981/android-scan-for-wifi-network