尝试在空对象引用上调用虚拟方法“java.lang.Object android.content.Context.getSystemService(java.lang.String)" [英] Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference

问题描述

我正在尝试检查我的应用程序是否是第一次启动.如果是，则要求用户输入.然后，检查 Wi-Fi 是否已连接.如果连接了 Wi-Fi，我会使用用户提供的输入来加载 WebView.

I am trying to check if my application is launched for the first time. If yes, the user is asked for an input. Then, check if Wi-Fi is connected. If Wi-Fi is connected, I use the input provides by the user to load a WebView.

但是，应用程序在启动时崩溃并出现错误

But, the app crashes on launch with an error

尝试在空对象引用上调用虚拟方法java.lang.Object android.content.Context.getSystemService(java.lang.String)"

Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference

代码

public class MainActivity extends Activity {


ProgressBar progressbar;
WebView webView;
LayoutInflater li = LayoutInflater.from(getBaseContext());
View promptsView = li.inflate(R.layout.propmt, null);
String URL;

protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    this.getWindow().requestFeature(Window.FEATURE_PROGRESS);
    setContentView(R.layout.activity_main);

    Boolean isFirstRun = getSharedPreferences("PREFERENCE", MODE_PRIVATE)
            .getBoolean("isFirstRun", true);

    if (isFirstRun) {

        otherWork(promptsView);

    }


       getSharedPreferences("PREFERENCE", MODE_PRIVATE).edit()
                .putBoolean("isFirstRun", false).commit();





    ConnectivityManager connManager = (ConnectivityManager) getSystemService(CONNECTIVITY_SERVICE);
    NetworkInfo mWifi = connManager.getNetworkInfo(ConnectivityManager.TYPE_WIFI);

    if (mWifi.isConnected()) {


    getWindow().setFeatureInt( Window.FEATURE_PROGRESS, Window.PROGRESS_VISIBILITY_ON);

    webView = (WebView) findViewById(R.id.webView);
    webView.getSettings().setBuiltInZoomControls(true);
    webView.getSettings().setDisplayZoomControls(false);
    webView.getSettings().setRenderPriority(RenderPriority.HIGH);
    webView.getSettings().setCacheMode(WebSettings.LOAD_NO_CACHE);
    if (Build.VERSION.SDK_INT >= 19) {
        webView.setLayerType(View.LAYER_TYPE_HARDWARE, null);
    }       
    else {
        webView.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
    }
    webView.loadUrl(URL);
    webView.setWebViewClient(new WebViewClient());
    WebSettings webSettings = webView.getSettings();
    webSettings.setJavaScriptEnabled(true);



       webView.setWebViewClient(new MyWebViewClient());

       progressbar=(ProgressBar)findViewById(R.id.progressBar); 

       webView.setWebChromeClient(new WebChromeClient() {

       // this will be called on page loading progress

       @Override

       public void onProgressChanged(WebView view, int newProgress) {

               super.onProgressChanged(view, newProgress);


               progressbar.setProgress(newProgress);
               //loadingTitle.setProgress(newProgress);
               // hide the progress bar if the loading is complete

               if (newProgress == 100) {
               progressbar.setVisibility(View.GONE);

               } else{
               progressbar.setVisibility(View.VISIBLE);

               }

               }

            });

       } else {

           AlertDialog.Builder AD = new AlertDialog.Builder(this);
            AD.setIcon(R.drawable.ic_launcher);
            AD.setTitle("Not Connected To Wi-Fi/WLAN");
            AD.setMessage("You need an Active Wi-Fi/WLAN on Alliance Broadband Network");
            AD.setPositiveButton(android.R.string.ok, new OnClickListener() {

                @Override
                public void onClick(DialogInterface arg0, int arg1) {
                    finish();

                }
            });
            AD.show();

       }


} 

public void otherWork(View promptsView) {
    AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(
            this);

    // set prompts.xml to alertdialog builder
    alertDialogBuilder.setView(promptsView);

    final EditText userInput = (EditText) promptsView
            .findViewById(R.id.editTextDialogUserInput);

    // set dialog message
    alertDialogBuilder
    .setCancelable(false)
    .setPositiveButton("OK",
            new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog,int id) {
            // get user input and set it to result
            // edit text
            //result.setText(userInput.getText());
            SharedPreferences pos;
            String fileName = "file";

            pos = getSharedPreferences(fileName, 0);
            SharedPreferences.Editor editor = pos.edit();
            editor.putString("pwd", userInput.getText().toString());      
            editor.commit();

            pos = getSharedPreferences(fileName, 0);
            String data = pos.getString("pwd", "");


            URL = data.toString();

        }
    })
    .setNegativeButton("Cancel",
            new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog,int id) {
            dialog.cancel();
        }
    });

    // create alert dialog
    AlertDialog alertDialog = alertDialogBuilder.create();

    // show it
    alertDialog.show();
}


       @Override
       public boolean onKeyDown(int keyCode, KeyEvent event) {

       if(keyCode == KeyEvent.KEYCODE_BACK){
           finish();
         }
       return super.onKeyDown(keyCode, event);
       }

       private class MyWebViewClient extends WebViewClient {


     @Override
     public boolean shouldOverrideUrlLoading(WebView view, String url) {

     view.loadUrl(url);
     return true;
     }
     }<br>

LogCat

05-24 01:11:19.279: E/AndroidRuntime(3719): FATAL EXCEPTION: main
05-24 01:11:19.279: E/AndroidRuntime(3719): Process: com.mavenmaverick.ipconnect, PID: 3719
05-24 01:11:19.279: E/AndroidRuntime(3719): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.mavenmaverick.ipconnect/com.mavenmaverick.ipconnect.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.app.ActivityThread.access$800(ActivityThread.java:144)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.os.Looper.loop(Looper.java:135)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.app.ActivityThread.main(ActivityThread.java:5221)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at java.lang.reflect.Method.invoke(Native Method)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at java.lang.reflect.Method.invoke(Method.java:372)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
05-24 01:11:19.279: E/AndroidRuntime(3719): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.view.LayoutInflater.from(LayoutInflater.java:219)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at com.mavenmaverick.ipconnect.MainActivity.<init>(MainActivity.java:47)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at java.lang.reflect.Constructor.newInstance(Native Method)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at java.lang.Class.newInstance(Class.java:1572)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.app.Instrumentation.newActivity(Instrumentation.java:1065)
05-24 01:11:19.279: E/AndroidRuntime(3719):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199)
05-24 01:11:19.279: E/AndroidRuntime(3719):     ... 10 more

推荐答案

LayoutInflater li = LayoutInflater.from(getBaseContext());

LayoutInflater li = LayoutInflater.from(getBaseContext());

首先，您不需要 getBaseContext()，也不需要 LayoutInflater.from().使用 getLayoutInflater().

First, you do not need getBaseContext(), nor do you need LayoutInflater.from(). Use getLayoutInflater().

第二，在super.onCreate() 之后，您不能调用Activity 超类上的方法，除非在某些情况下.请将 promptsView 的初始化推迟到 super.onCreate() 被调用之后.

Second, you cannot call methods on the Activity superclass until after super.onCreate(), except in certain situations. Please postpone your initialization of promptsView until after super.onCreate() has been called.

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