我已经使用WebView创建了android应用
功能正常，但是当我添加私有void createDialog（）来显示alertdialog时，则会出现此错误：
NullPointerException：尝试调用虚拟方法'void android.app.Dialog.show（）'
关于Logcat中的空对象引用
这是我的片段代码：
public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener {
public Dialog mDialog;
public Button mDialogyes, mDialogno;
ProgressBar progressBar;
WebView webView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
progressBar = (ProgressBar) findViewById(R.id.progressBar);
webView=(WebView) findViewById(R.id.webView);
webView.setVisibility(View.INVISIBLE);
webView.loadUrl("http://m.jazz.com.pk/smart/wifi_3g.html");
WebSettings webSettings=webView.getSettings();
webSettings.setJavaScriptEnabled(true);
webView.setWebChromeClient(new WebChromeClient());
webView.setWebViewClient(new WebViewClient() {
@Override
public void onPageStarted(WebView view, String url, Bitmap favicon) {
super.onPageStarted(view, url, favicon);
setProgressBarVisibility(View.VISIBLE);
}
@Override
public void onPageFinished(WebView view, String url) {
super.onPageFinished(view, url);
setProgressBarVisibility(View.GONE);
webView.setVisibility(View.VISIBLE);
}
@Override
public void onReceivedError(WebView view, WebResourceRequest request, WebResourceError error) {
super.onReceivedError(view, request, error);
setProgressBarVisibility(View.GONE);
}
});
webView.loadUrl("http://m.jazz.com.pk/smart/wifi_3g.html");
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
}
});
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawer.addDrawerListener(toggle);
toggle.syncState();
NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
navigationView.setItemIconTintList(null);
navigationView.setNavigationItemSelectedListener(this);
ConnectivityManager cm = (ConnectivityManager) getApplicationContext().getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo activenetnetwork = cm.getActiveNetworkInfo();
if(activenetnetwork != null)
{
if(activenetnetwork.getType() == ConnectivityManager.TYPE_WIFI)
{
//Toast.makeText(getApplicationContext(),activenetnetwork.getTypeName(),Toast.LENGTH_LONG).show();
}else if(activenetnetwork.getType() == ConnectivityManager.TYPE_MOBILE)
{
//Toast.makeText(getApplicationContext(),activenetnetwork.getTypeName(),Toast.LENGTH_LONG).show();
}
}else {
ActionBar actionBar = getSupportActionBar();
actionBar.setTitle("Web page not available");
Toast.makeText(getApplicationContext(),"Please Check your Internet ",Toast.LENGTH_LONG).show();
}
}
private void setProgressBarVisibility(int visibility) {
// If a user returns back, a NPE may occur if WebView is still loading a page and then tries to hide a ProgressBar.
if (progressBar != null) {
progressBar.setVisibility(visibility);
}
}
private void createDialog() {
mDialog = new Dialog(this);
mDialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
mDialog.setContentView(R.layout.dialog_exit);
mDialog.setCanceledOnTouchOutside(true);
mDialog.setCancelable(true);
mDialogyes = (Button) mDialog.findViewById(R.id.yes);
mDialogno = (Button) mDialog.findViewById(R.id.no);
mDialogyes.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(Intent.ACTION_MAIN);
intent.setFlags(intent.FLAG_ACTIVITY_CLEAR_TOP);
intent.addCategory(Intent.CATEGORY_HOME);
startActivity(intent);
finish();
System.exit(0);
}
});
mDialogno.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
mDialog.dismiss();
}
});
}
@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
mDialog.show();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();
if (id == R.id.home) {
} else if (id == R.id.contact) {
} else if (id == R.id.about) {
} else if (id == R.id.share) {
} else if (id == R.id.rate) {
} else if (id == R.id.more) {
}
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
}
答案 0 :(得分：2)
看代码，永远不会调用方法createDialog()，因此mDialog的值始终为空。您需要先初始化对话框（即通过调用createDialog()），然后才能在mDialog.show();覆盖中调用onBackPressed()。
答案 1 :(得分：0)
在调用show方法之前需要对空指针进行检查。示例：
if (mDialog != null) {
mDialog.show();
}
如上所述，您的mDialog为null，因为您需要先调用createDialog方法。无论您从另一个应用程序复制的任何代码都遗漏了某些东西。