在Android应用程序中的java.lang.NullPointerException

我对Java相当陌生，我一直在努力学习，所以我为我的网站制作了一个webview应用程序，并发布了一些错误，如下所示：在Android应用程序中的java.lang.NullPointerException

java.lang.NullPointerException 
at com.alexriggs.android.mafiastreetwars.MafiaStreetWarsActivity$MyWebViewClient.shouldOverrideUrlLoading(MafiaStreetWarsActivity.java:87) 
at android.webkit.CallbackProxy.uiOverrideUrlLoading(CallbackProxy.java:227) 
at android.webkit.CallbackProxy.handleMessage(CallbackProxy.java:334) 
at android.os.Handler.dispatchMessage(Handler.java:99) 
at android.os.Looper.loop(Looper.java:130) 
at android.app.ActivityThread.main(ActivityThread.java:3704) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:507) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:875) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:633) 
at dalvik.system.NativeStart.main(Native Method)

不知道我知道足够的知道java.lang.NullPointerException意味着一个变量是空的，但被用作非空。回顾我的代码后，它永远不应该是空的，我不能重现它。所以这里是我的代码，任何人都可以弄清楚什么是错的？请记住我是Java新手，请尽可能详细地使用它！

package com.alexriggs.android.mafiastreetwars; 

import com.alexriggs.android.mafiastreetwars.R; 

import android.app.Activity; 
import android.app.AlertDialog; 
import android.content.DialogInterface; 
import android.content.Intent; 
import android.net.Uri; 
import android.os.Bundle; 
import android.util.Log; 
import android.view.KeyEvent; 
import android.view.Menu; 
import android.view.MenuInflater; 
import android.view.MenuItem; 
import android.webkit.WebView; 
import android.webkit.WebViewClient; 
import android.widget.Toast; 

public class MafiaStreetWarsActivity extends Activity { 
    /** Called when the activity is first created. */ 
    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     MenuInflater inflater = getMenuInflater(); 
     inflater.inflate(R.menu.menu, menu); 
     return true; 
    } 
    public boolean onOptionsItemSelected(MenuItem item) { 
     switch (item.getItemId()) { 
      case R.id.exit:{ 
       Toast.makeText(this, "Thanks For playing Mafia Street Wars!", Toast.LENGTH_LONG).show(); 
       finish(); 
       break; 
      } 
      case R.id.select_and_copy: 
      { 
       Toast.makeText(getApplicationContext(), "Select Text by dragging over the text you want to copy.", Toast.LENGTH_LONG).show(); 
       selectAndCopyText(myWebView); 
       return true; 
      } 
     } 
     return true; 
    } 
    private void selectAndCopyText(WebView view) { 
     try 
     { 
      KeyEvent shiftPressEvent = new KeyEvent(0, 0, KeyEvent.ACTION_DOWN, 
                KeyEvent.KEYCODE_SHIFT_LEFT, 0, 0); 
      shiftPressEvent.dispatch(view); 
     } 
     catch (Exception e) 
     { 
      Log.e("dd", "Exception in emulateShiftHeld()", e); 
     } 

    } 
    final Activity activity = this; 
    WebView myWebView; 

    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.main); 
     AlertDialog alertDialog = new AlertDialog.Builder(this).create(); 
     alertDialog.setTitle("Remember..."); 
     alertDialog.setMessage("This app is just for mobile access. This game is meant to be played on the computer at MafiaStreetWars.com"); 
     alertDialog.setButton("Thanks for the reminder!", new DialogInterface.OnClickListener() { 
      public void onClick(DialogInterface dialog, int which) { 
       // here you can add functions 
      } 
     }); 
     alertDialog.setIcon(R.drawable.icon); 
     alertDialog.show(); 
     myWebView = (WebView) findViewById(R.id.webview); 
     myWebView.getSettings().setJavaScriptEnabled(true); 
     myWebView.setWebViewClient(new MyWebViewClient()); 
     myWebView.getSettings().setBuiltInZoomControls(true); 
     myWebView.getSettings().setLoadWithOverviewMode(true); 
     myWebView.getSettings().setUseWideViewPort(true); 
     myWebView.getSettings().setPluginsEnabled(true); 
     Toast.makeText(getApplicationContext(), "Page is loading... Please wait!", Toast.LENGTH_LONG).show(); 
     myWebView.loadUrl("http://mafiastreetwars.com/?pk_campaign=Android&pk_kwd=MainApp"); 
    } 
    private class MyWebViewClient extends WebViewClient { 


     public boolean shouldOverrideUrlLoading(WebView view, String url) { 
      if (Uri.parse(url).getHost().equals("mafiastreetwars.com")) { 
       return false; 
      } 
      // Otherwise, give the default behavior (open in browser) 
      Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse(url)); 
      startActivity(intent); 
      return true; 
     } 
     } 
    @Override 
    public boolean onKeyDown(int keyCode, KeyEvent event) { 
     if ((keyCode == KeyEvent.KEYCODE_BACK) && myWebView.canGoBack()) { 
      myWebView.goBack(); 
      return true; 
     } 
     return super.onKeyDown(keyCode, event); 
    } 
}

来源

2013-10-08 alexander7567

请学习使用调试器。它会告诉你你的问题在哪里。 –

@AdamArold就像我说的，我不能重现问题.. – alexander7567

这可能是由于Uri.parse(url).getHost()返回null。请在致电equals()之前检查此信息。

来源

2013-10-08 14:20:40

问题if (Uri.parse(url).getHost().equals("mafiastreetwars.com")) {

行看起来像你的网址为空，或者解析结果为空，但认为不太可能。在使用它们之前检查一下是否为空。

来源

2013-10-08 14:23:39

我建议你做一点点nullsave：

if (url==null) 
    return false; 
Uri lUri = Uri.parse(url); 
if ("mafiastreetwars.com".equals(lUri.getHost())) { 
      return false; 
}

，解决您的技术问题，但也许不是您选择的商业案例。

来源

2013-10-08 14:43:39

在Android应用程序中的java.lang.NullPointerException

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