Android应用意外崩溃

Question

I am new to Android developement and had recently started on a project to get WiFi SSID , BSSID and MAC address. However, the app crashed unexpectedly immediately after opening it.

The error log in Android Studio says that:

FATAL EXCEPTION: main
Process: com.example.android.wifi, PID: 3875

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.wifi/com.example.android.wifi}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2993)
at
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3248)
at ...
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
at
android.app.Activity.findViewById(Activity.java:2277)
at
com.example.android.wifi.MainActivity.<init>(MainActivity.java:16)
at ...

My java file can be found at http://www.yikjin.ga/MainActivity.java

There were no syntax errors recorded by Android Studio.

Thanks in advance!

Answer 1

From your error is seems you are trying to get the View inside the constructor of your class or you are calling findViewById() outside.

com.example.android.wifi.MainActivity.<init>(MainActivity.java:16)

Move those code to onCreate() method after calling setContentView()

Update:

Change

TextView viewWifiSSID = (TextView) findViewById(R.id.network);
TextView viewWifiBSSID = (TextView) findViewById(R.id.bssid);
TextView viewWifiMAC = (TextView) findViewById(R.id.mac);  

to

TextView viewWifiSSID;
TextView viewWifiBSSID;
TextView viewWifiMAC;

and add the following to onCreate() after setContentView()

viewWifiSSID = (TextView) findViewById(R.id.network);
viewWifiBSSID = (TextView) findViewById(R.id.bssid);
viewWifiMAC = (TextView) findViewById(R.id.mac);

Answer 2

you are referring object before activity is created ... onCreate() is called when your activity is created so refer object inside onCreate() ...

TextView viewWifiSSID, viewWifiBSSID, viewWifiMAC;

protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);

  viewWifiSSID = (TextView) findViewById(R.id.network);
  viewWifiBSSID = (TextView) findViewById(R.id.bssid);
  viewWifiMAC = (TextView) findViewById(R.id.mac);

  getWifi();
}