Android 安卓：扫描Wifi网络+；可选列表

我正在尝试创建一个应用程序，让你扫描可用的wifi网络，然后在一个可选列表中显示它们。为此，我尝试使用以下代码：

package android.nacho.WifiScan;

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.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.AdapterView.OnItemClickListener;

public class WifiScan extends Activity {
    TextView mainText;
    WifiManager mainWifi;
    WifiReceiver receiverWifi;
    List<ScanResult> wifiList;
    StringBuilder sb = new StringBuilder();
    ListView listView = new ListView(this);
    ArrayAdapter<String> adapter;
    Context context = this;

    static String[] NETWORKS;

    public void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_wifi_scan);
       mainText = (TextView) findViewById(R.id.text);
       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) {


            wifiList = mainWifi.getScanResults();
            NETWORKS = new String[wifiList.size()];
            for(int i = 0; i < wifiList.size(); i++){

                NETWORKS[i]=i+"-"+(wifiList.get(i)).toString();


            }


            System.out.println("debería imprimir: "+NETWORKS[2]);


            adapter = new ArrayAdapter<String>(context, android.R.layout.simple_list_item_multiple_choice, NETWORKS);
            // adapter.setDropDownViewResource(android.R.layout.select_dialog_multichoice);
            listView.setAdapter(adapter);
            listView.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
            listView.setOnItemClickListener(new OnItemClickListener() {

                public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,long arg3) {


                }

            });

            setContentView(listView);

        }
    }
}

包android.nacho.WifiScan；
导入java.util.List；
导入android.app.Activity；
导入android.content.BroadcastReceiver；
导入android.content.Context；
导入android.content.Intent；
导入android.content.IntentFilter；
导入android.net.wifi.ScanResult；
导入android.net.wifi.WifiManager；
导入android.os.Bundle；
导入android.view.Menu；
导入android.view.MenuItem；
导入android.view.view；
导入android.widget.AdapterView；
导入android.widget.ArrayAdapter；
导入android.widget.ListView；
导入android.widget.TextView；
导入android.widget.AdapterView.OnItemClickListener；
公共类WifiScan扩展活动{
文本视图主文本；
WifiManager mainWifi；
无线接收器接收器；
列表wifiList；
StringBuilder sb=新的StringBuilder（）；
ListView ListView=新建ListView（此）；
阵列适配器；
上下文=这个；
静态字符串[]网络；
创建时的公共void（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）；
setContentView（R.layout.activity\u wifi\u scan）；
mainText=（TextView）findViewById（R.id.text）；
mainWifi=（WifiManager）getSystemService（Context.WIFI\u SERVICE）；
receiverWifi=新WifiReceiver（）；
registerReceiver（接收方WiFi，新的意向过滤器（WifiManager.SCAN\u RESULTS\u AVAILABLE\u ACTION））；
mainWifi.startScan（）；
mainText.setText（\\n开始扫描…\\n”）；
}
公共布尔onCreateOptions菜单（菜单）{
添加（0,0,0，“刷新”）；
返回super.onCreateOptions菜单（菜单）；
}
公共布尔值onMenuItemSelected（int-featureId，MenuItem项）{
mainWifi.startScan（）；
setText（“开始扫描”）；
返回super.onMenuItemSelected（featureId，item）；
}
受保护的void onPause（）{
未注册的接收器（接收器）；
super.onPause（）；
}
受保护的void onResume（）{
registerReceiver（接收方WiFi，新的意向过滤器（WifiManager.SCAN\u RESULTS\u AVAILABLE\u ACTION））；
super.onResume（）；
}
类WifiReceiver扩展了BroadcastReceiver{
接收时公共无效（上下文c，意图）{
wifiList=mainWifi.getScanResults（）；
NETWORKS=新字符串[wifiList.size（）]；
对于（int i=0；i

但我得到了这个错误：

06-18 12:36:32.670: W/dalvikvm(15499): threadid=1: thread exiting with
uncaught exception (group=0x40dfb930)

06-18 12:36:32.670: E/AndroidRuntime(15499): FATAL EXCEPTION: main

 06-18 12:36:32.670: E/AndroidRuntime(15499):
java.lang.RuntimeException: Unable to instantiate activity
 ComponentInfo{android.nacho.WifiScan/android.nacho.WifiScan.WifiScan}:
 java.lang.NullPointerException

 06-18 12:36:32.670: E/AndroidRuntime(15499):   at
 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2106)

 06-18 12:36:32.670: E/AndroidRuntime(15499):   at
 android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)

 06-18 12:36:32.670: E/AndroidRuntime(15499):   at
 android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
 06-18 12:36:32.670: D/AndroidRuntime(15499): Shutting down VM 06-18
 12:36:32.670: E/AndroidRuntime(15499):     at
 android.os.Looper.loop(Looper.java:137) 06-18 12:36:32.670:
 E/AndroidRuntime(15499):   at
 android.app.ActivityThread.main(ActivityThread.java:5039) 06-18
 12:36:32.670: E/AndroidRuntime(15499):     at
 java.lang.reflect.Method.invokeNative(Native Method) 06-18
 12:36:32.670: E/AndroidRuntime(15499):     at
 java.lang.reflect.Method.invoke(Method.java:511) 06-18 12:36:32.670:
 E/AndroidRuntime(15499):   at
 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
 06-18 12:36:32.670: E/AndroidRuntime(15499):   at
 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 06-18
 12:36:32.670: E/AndroidRuntime(15499):     at
 dalvik.system.NativeStart.main(Native Method) 06-18 12:36:32.670:
 E/AndroidRuntime(15499): Caused by: java.lang.NullPointerException
 06-18 12:36:32.670: E/AndroidRuntime(15499):   at
 android.content.ContextWrapper.getResources(ContextWrapper.java:89)
 06-18 12:36:32.670: E/AndroidRuntime(15499):   at
 android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:78)
 06-18 12:36:32.670: E/AndroidRuntime(15499):   at
 android.view.View.<init>(View.java:3224) 06-18 12:36:32.670:
 E/AndroidRuntime(15499):   at android.view.View.<init>(View.java:3279)
 06-18 12:36:32.670: E/AndroidRuntime(15499):   at
 android.view.ViewGroup.<init>(ViewGroup.java:431) 06-18 12:36:32.670:
 E/AndroidRuntime(15499):   at
 android.widget.AdapterView.<init>(AdapterView.java:235) 06-18
 12:36:32.670: E/AndroidRuntime(15499):     at
 android.widget.AbsListView.<init>(AbsListView.java:766) 06-18
 12:36:32.670: E/AndroidRuntime(15499):     at
 android.widget.ListView.<init>(ListView.java:143) 06-18 12:36:32.670:
 E/AndroidRuntime(15499):   at
 android.widget.ListView.<init>(ListView.java:139) 06-18 12:36:32.670:
 E/AndroidRuntime(15499):   at
 android.widget.ListView.<init>(ListView.java:135) 06-18 12:36:32.670:
 E/AndroidRuntime(15499):   at
 android.nacho.WifiScan.WifiScan.<init>(WifiScan.java:27) 06-18
 12:36:32.670: E/AndroidRuntime(15499):     at
 java.lang.Class.newInstanceImpl(Native Method) 06-18 12:36:32.670:
 E/AndroidRuntime(15499):   at
 java.lang.Class.newInstance(Class.java:1319) 06-18 12:36:32.670:
 E/AndroidRuntime(15499):   at
 android.app.Instrumentation.newActivity(Instrumentation.java:1054)
 06-18 12:36:32.670: E/AndroidRuntime(15499):   at
 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097)
 06-18 12:36:32.670: E/AndroidRuntime(15499):   ... 11 more

06-18 12:36:32.670:W/dalvikvm（15499）：threadid=1:线程以
未捕获异常（组=0x40dfb930）
06-18 12:36:32.670:E/AndroidRuntime（15499）：致命异常：main
06-18 12:36:32.670:E/AndroidRuntime（15499）：
java.lang.RuntimeException:无法实例化活动
ComponentInfo{android.nacho.WifiScan/android.nacho.WifiScan.WifiScan}：
java.lang.NullPointerException
06-18 12:36:32.670:E/AndroidRuntime（15499）：在
android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2106）
06-18 12:36:32.670:E/AndroidRuntime（15499）：在
android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2230）
06-18 12:36:32.670:E/AndroidRuntime（15499）：在
android.app.ActivityThread$H.handleMessage（ActivityThread.java:1234）
06-18 12:36:32.670:D/AndroidRuntime（15499）：关闭VM 06-18
12:36:32.670:E/AndroidRuntime（15499）：在
android.os.Looper.loop（Looper.java:137）06-18 12:36:32.670:
E/AndroidRuntime（15499）：在
android.app.ActivityThread.main（ActivityThread.java:5039）06-18
12:36:32.670:E/AndroidRuntime（15499）：在
java.lang.reflect.Method.Invokenactive（本机方法）06-18
12:36:32.670:E/AndroidRuntime（15499）：在
java.lang.reflect.Method.invoke（Method.java:511）06-18 12:36:32.670:
E/AndroidRuntime（15499）：在
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run（ZygoteInit.java:793）
06-18 12:36:32.670:E/AndroidRuntime（15499）：在
com.android.internal.os.ZygoteInit.main（ZygoteInit.java:560）06-18
12:36:32.670:E/AndroidRuntime（15499）：在
dalvik.system.NativeStart.main（本机方法）06-18 12:36:32.670：
E/AndroidRuntime（15499）：由以下原因引起：java.lang.NullPointerException
06-18 12:36:32.670:E/AndroidRuntime（15499）：在
android.content.ContextWrapper.getResources（ContextWrapper.java:89）
06-18 12:36:32.670:E/AndroidRuntime（15499）：在
android.view.ContextThemeWrapper.getResources（ContextThemeWrapper.java:78）
06-18 12:36:32.670:E/AndroidRuntime（15499）：在
android.view.view.（view.java:3224）06-1812:36:32.670:
E/AndroidRuntime（15499）：位于android.view.view.（view.java:3279）
06-18 12:36:32.670:E/AndroidRuntime（15499）：在
android.view.ViewGroup.（ViewGroup.java:431）06-18 12:36:32.670:
E/AndroidRuntime（15499）：在
android.widget.AdapterView.（AdapterView.java:235）06-18
12:36:32.670:E/AndroidRuntime（15499）：在
android.widget.AbsListView.（AbsListView.java:766）06-18
12:36:32.670:E/AndroidRuntime（15499）：在
android.widget.ListView.（ListView.java:143）06-18 12:36:32.670:
E/AndroidRuntime（15499）：在
android.widget.ListView.（ListView.java:139）06-18 12:36:32.670:
E/AndroidRuntime（15499）：在
android.widget.ListView.（ListView.java:135）06-18 12:36:32.670:
E/AndroidRunti
public class MainActivity extends Activity {


    WifiManager mainWifi;
    WifiReceiver receiverWifi;

    StringBuilder sb = new StringBuilder();

    private final Handler handler = new Handler();


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


        mainWifi = (WifiManager) getSystemService(Context.WIFI_SERVICE);

        receiverWifi = new WifiReceiver();
        registerReceiver(receiverWifi, new IntentFilter(
                WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
        if (mainWifi.isWifiEnabled() == false) {
            mainWifi.setWifiEnabled(true);
        }


        doInback();
    }

    public void doInback() {
        handler.postDelayed(new Runnable() {

            @Override
            public void run() {
                // TODO Auto-generated method stub
                mainWifi = (WifiManager) getSystemService(Context.WIFI_SERVICE);

                if (receiveWifi == null)
                    receiverWifi = new WifiReceiver();
                registerReceiver(receiverWifi, new IntentFilter(
                        WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
                mainWifi.startScan();
                doInback();
            }
        }, 1000);

    }

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

    public boolean onMenuItemSelected(int featureId, MenuItem item) {
        mainWifi.startScan();

        return super.onMenuItemSelected(featureId, item);
    }


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

    @Override
    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) {

            ArrayList<String> connections = new ArrayList<String>();
            ArrayList<Float> Signal_Strenth = new ArrayList<Float>();

            sb = new StringBuilder();
            List<ScanResult> wifiList;
            wifiList = mainWifi.getScanResults();
            for (int i = 0; i < wifiList.size(); i++) {

                connections.add(wifiList.get(i).SSID);
            }


        }
    }


}

if (receiveWifi==null) {
   receiveWifi = new WifiReceiver();
}