附在帖子上是我的Android代码，用于简单的wifi扫描应用程序。这是基于API 25/26构建的。每当我启动应用程序时它都会崩溃。我试图在模拟器（Lollipop）上运行，Google Pixel在7.1.2上运行。请查看代码并尽可能帮助我。
MainActivity.java
package com.example.fahad.test;
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.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.List;
public class MainActivity extends Activity implements View.OnClickListener{
WifiManager wifi;
ListView lv;
Button buttonScan;
int size = 0;
List<ScanResult> results;
String ITEM_KEY = "key";
ArrayList<String> arraylist = new ArrayList<>();
ArrayAdapter adapter;
/* Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
getActionBar().setTitle("Widhwan Setup Wizard");
setContentView(R.layout.activity_main);
buttonScan = (Button) findViewById(R.id.scan);
buttonScan.setOnClickListener(this);
lv = (ListView)findViewById(R.id.wifilist);
wifi = (WifiManager) getApplicationContext().getSystemService(Context.WIFI_SERVICE);
if (wifi.isWifiEnabled() == false)
{
Toast.makeText(getApplicationContext(), "wifi is disabled..making it enabled", Toast.LENGTH_LONG).show();
wifi.setWifiEnabled(true);
}
this.adapter = new ArrayAdapter<>(this,android.R.layout.simple_list_item_1,arraylist);
lv.setAdapter(this.adapter);
scanWifiNetworks();
}
public void onClick(View view)
{
scanWifiNetworks();
}
private void scanWifiNetworks(){
arraylist.clear();
registerReceiver(wifi_receiver, new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
wifi.startScan();
Log.d("WifScanner", "scanWifiNetworks");
Toast.makeText(this, "Scanning....", Toast.LENGTH_SHORT).show();
}
BroadcastReceiver wifi_receiver= new BroadcastReceiver()
{
@Override
public void onReceive(Context c, Intent intent)
{
Log.d("WifScanner", "onReceive");
results = wifi.getScanResults();
size = results.size();
unregisterReceiver(this);
try
{
while (size >= 0)
{
size--;
arraylist.add(results.get(size).SSID);
adapter.notifyDataSetChanged();
}
}
catch (Exception e)
{
Log.w("WifScanner", "Exception: "+e);
}
}
};
}
activity_main.xml中
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:padding="10dp"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ListView
android:id="@+id/wifilist"
android:layout_width="match_parent"
android:layout_height="312dp"
android:layout_weight="0.97" />
<Button
android:id="@+id/scan"
android:layout_width="match_parent"
android:layout_height="50dp"
android:layout_gravity="bottom"
android:layout_margin="15dp"
android:background="@android:color/holo_green_light"
android:text="Scan Again" />
</LinearLayout>
的AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.fahad.test">
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.CHANGE_WIFI_STATE" />
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity
android:name=".MainActivity"
android:label="@string/app_name"
android:theme="@style/AppTheme.NoActionBar">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
08-08 00:18:04.891 2887-2887/com.example.fahad.test E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.fahad.test, PID: 2887
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fahad.test/com.example.fahad.test.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
at com.example.fahad.test.MainActivity.onCreate(MainActivity.java:40)
at android.app.Activity.performCreate(Activity.java:5937)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
08-08 00:18:04.895 1517-1846/system_process W/ActivityManager: Force finishing activity com.example.fahad.test/.MainActivity
08-08 00:18:04.910 1138-1516/? D/gralloc_ranchu: gralloc_alloc: Creating ashmem region of size 1019904
[ 08-08 00:18:04.914 1517: 1846 D/ ]
HostConnection::get() New Host Connection established 0x9c55ad80, tid 1846
08-08 00:18:04.919 1138-1138/? E/EGL_emulation: tid 1138: eglCreateSyncKHR(1865): error 0x3004 (EGL_BAD_ATTRIBUTE)
08-08 00:18:04.974 1517-2907/system_process D/OpenGLRenderer: Render dirty regions requested: true
08-08 00:18:04.974 1517-1538/system_process D/Atlas: Validating map...
[ 08-08 00:18:05.033 1517: 2907 D/ ]
HostConnection::get() New Host Connection established 0x9d9fd470, tid 2907
08-08 00:18:05.040 1517-2907/system_process I/OpenGLRenderer: Initialized EGL, version 1.4
08-08 00:18:05.040 1517-2907/system_process W/OpenGLRenderer: Failed to choose config with EGL_SWAP_BEHAVIOR_PRESERVED, retrying without...
08-08 00:18:05.053 1517-2907/system_process D/EGL_emulation: eglCreateContext: 0xac7a6520: maj 3 min 1 rcv 4
08-08 00:18:05.060 1517-2907/system_process D/EGL_emulation: eglMakeCurrent: 0xac7a6520: ver 3 1
08-08 00:18:05.061 1517-2907/system_process E/eglCodecCommon: glUtilsParamSize: unknow param 0x000082da
08-08 00:18:05.061 1517-2907/system_process E/eglCodecCommon: glUtilsParamSize: unknow param 0x00008cdf
08-08 00:18:05.061 1517-2907/system_process E/eglCodecCommon: glUtilsParamSize: unknow param 0x00008824
08-08 00:18:05.066 1517-2907/system_process D/OpenGLRenderer: Enabling debug mode 0
08-08 00:18:05.086 1517-2907/system_process D/EGL_emulation: eglMakeCurrent: 0xac7a6520: ver 3 1
08-08 00:18:05.414 2347-2665/com.android.calendar I/GlobalDismissManager: no sender configured
08-08 00:18:05.414 2347-2665/com.android.calendar D/AlertService: Beginning updateAlertNotification
08-08 00:18:05.426 2347-2665/com.android.calendar D/AlertService: No fired or scheduled alerts
08-08 00:18:05.437 1517-1538/system_process W/ActivityManager: Activity pause timeout for ActivityRecord{1923856f u0 com.example.fahad.test/.MainActivity t28 f}
08-08 00:18:05.439 2347-2665/com.android.calendar D/AlertService: Scheduling next alarm with AlarmScheduler. sEventReminderReceived: null
08-08 00:18:05.453 2347-2665/com.android.calendar D/AlarmScheduler: No events found starting within 1 week.
08-08 00:18:05.476 1821-1999/com.android.launcher D/EGL_emulation: eglMakeCurrent: 0xaef0d100: ver 3 1
08-08 00:18:05.568 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.581 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.585 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.625 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.629 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.642 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.646 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.678 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.692 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.697 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.717 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.729 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.735 2100-2211/com.google.android.gms I/Icing: Indexing 683F7A6F22BBD90B1D3ABECD88AADD143975EBF3 from com.google.android.gms
08-08 00:18:05.736 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.747 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.752 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.763 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.770 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.780 1602-2022/com.android.systemui D/EGL_emulation: eglMakeCurrent: 0xaef21100: ver 3 1
08-08 00:18:05.817 2100-2211/com.google.android.gms I/Icing: Indexing done 683F7A6F22BBD90B1D3ABECD88AADD143975EBF3
08-08 00:18:06.050 1821-1999/com.android.launcher W/OpenGLRenderer: Incorrectly called buildLayer on View: ShortcutAndWidgetContainer, destroying layer...
08-08 00:18:06.050 1821-1999/com.android.launcher W/OpenGLRenderer: Incorrectly called buildLayer on View: ShortcutAndWidgetContainer, destroying layer...
08-08 00:18:06.050 1821-1999/com.android.launcher W/OpenGLRenderer: Incorrectly called buildLayer on View: ShortcutAndWidgetContainer, destroying layer...
08-08 00:18:08.007 1517-1532/system_process I/ActivityManager: Killing 1877:com.android.externalstorage/u0a6 (adj 15): empty #17
08-08 00:18:08.008 1517-1532/system_process W/libprocessgroup: failed to open /acct/uid_10006/pid_1877/cgroup.procs: No such file or directory
08-08 00:18:08.013 1517-1625/system_process W/libprocessgroup: failed to open /acct/uid_10006/pid_1877/cgroup.procs: No such file or directory
真的需要你的帮助。谢谢。
答案 0 :(得分：0)
在您的MainActivity中，onCreate()：
getSupportActionBar().setTitle("Widhwan Setup Wizard");
并扩展AppCompatActivity而不是Activity