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)”
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StackOverFlow
Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
提问by
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.
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。
But, the app crashes on launch with an error
但是，该应用程序在启动时崩溃并出现错误
Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
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)”
Code
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
日志猫
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.access0(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
回答by CommonsWare
LayoutInflater li = LayoutInflater.from(getBaseContext());
LayoutInflater li = LayoutInflater.from(getBaseContext());
First, you do not need getBaseContext(), nor do you need LayoutInflater.from(). Use getLayoutInflater().
首先，你不需要getBaseContext()，也不需要LayoutInflater.from()。使用getLayoutInflater().
Second, you cannot call methods on the Activitysuperclass until after super.onCreate(), except in certain situations. Please postpone your initialization of promptsViewuntil after super.onCreate()has been called.
其次，除非在某些情况下，否则在Activityafter 之前不能调用超类上的方法super.onCreate()。请将您的初始化推迟promptsView到super.onCreate()被调用之后。
回答by himanshu jain
View view = inflater.inflate(R.layout.fragment_name, container, false);
view.getContext();
Use view.getContext()wherever you need Application Context in fragments.
view.getContext()在您需要片段中的应用程序上下文的任何地方使用。
