Using WiFi API

package com.example;

import java.util.List;

import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.IntentFilter;
import android.net.wifi.WifiConfiguration;
import android.net.wifi.WifiInfo;
import android.net.wifi.WifiManager;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

public class WiFiDemo extends Activity implements OnClickListener {
	private static final String TAG = "WiFiDemo";
	WifiManager wifi;
	BroadcastReceiver receiver;

	TextView textStatus;
	Button buttonScan;

	/** Called when the activity is first created. */
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);

		// Setup UI
		textStatus = (TextView) findViewById(R.id.textStatus);
		buttonScan = (Button) findViewById(R.id.buttonScan);
		buttonScan.setOnClickListener(this);

		// Setup WiFi
		wifi = (WifiManager) getSystemService(Context.WIFI_SERVICE);

		// Get WiFi status
		WifiInfo info = wifi.getConnectionInfo();
		textStatus.append("\n\nWiFi Status: " + info.toString());

		// List available networks
		List<WifiConfiguration> configs = wifi.getConfiguredNetworks();
		for (WifiConfiguration config : configs) {
			textStatus.append("\n\n" + config.toString());
		}

		// Register Broadcast Receiver
		if (receiver == null)
			receiver = new WiFiScanReceiver(this);

		registerReceiver(receiver, new IntentFilter(
				WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
		Log.d(TAG, "onCreate()");
	}

	@Override
	public void onStop() {
		unregisterReceiver(receiver);
	}

	public void onClick(View view) {
		Toast.makeText(this, "On Click Clicked. Toast to that!!!",
				Toast.LENGTH_LONG).show();

		if (view.getId() == R.id.buttonScan) {
			Log.d(TAG, "onClick() wifi.startScan()");
			wifi.startScan();
		}
	}

}

package com.example;

import java.util.List;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.net.wifi.ScanResult;
import android.net.wifi.WifiManager;
import android.util.Log;
import android.widget.Toast;

public class WiFiScanReceiver extends BroadcastReceiver {
  private static final String TAG = "WiFiScanReceiver";
  WiFiDemo wifiDemo;

  public WiFiScanReceiver(WiFiDemo wifiDemo) {
    super();
    this.wifiDemo = wifiDemo;
  }

  @Override
  public void onReceive(Context c, Intent intent) {
    List<ScanResult> results = wifiDemo.wifi.getScanResults();
    ScanResult bestSignal = null;
    for (ScanResult result : results) {
      if (bestSignal == null
          || WifiManager.compareSignalLevel(bestSignal.level, result.level) < 0)
        bestSignal = result;
    }

    String message = String.format("%s networks found. %s is the strongest.",
        results.size(), bestSignal.SSID);
    Toast.makeText(wifiDemo, message, Toast.LENGTH_LONG).show();

    Log.d(TAG, "onReceive() message: " + message);
  }

}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical" android:layout_width="fill_parent"
  android:layout_height="fill_parent">

  <Button android:layout_width="wrap_content"
    android:layout_height="wrap_content" android:id="@+id/buttonScan"
    android:text="Scan"></Button>
  <ScrollView android:id="@+id/ScrollView01"
    android:layout_width="wrap_content" android:layout_height="wrap_content">
    <TextView android:layout_width="fill_parent"
      android:layout_height="wrap_content" android:id="@+id/textStatus"
      android:text="WiFiDemo" />
  </ScrollView>

</LinearLayout>

<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
  <uses-permission android:name="android.permission.CHANGE_WIFI_STATE" />