Android应用程序中的java.lang.NullPointerException

Question

I am fairly new to java and I have been trying to learn, so I made a webview app for my website that I have published and a few people have reported an error as listed below:

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)

No I know enough to know that java.lang.NullPointerException means that a variable is empty but being used as not empty. And after looking back at my code, it should never be empty and I cannot reproduce it. So here is my code, can anyone figure out what is wrong? And please remember that I am new to Java, so please use as much detail as possible!

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);  
    }  
}

Answer 1

It's probably due to Uri.parse(url).getHost() returning null . Check this before you call equals() .

Answer 2

Line in question if (Uri.parse(url).getHost().equals("mafiastreetwars.com")) {

Looks like your url is null, or the parse result is null but that is less likely. Check to see if things are null before using them!

Answer 3

I suggest you to do it a little bit more nullsave:

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

that solves your technical problem but maybe not youre business case.