Alert Dialog Positive Button Issue

问题

I have alert dialog initiated on button click , it had username and password , after alert dialog fired , if you press OK button , it force close and close the app ,

any help will be appreciated.

 public class MainActivity extends Activity {
final Context context = this;
private Button button;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);

     setContentView(R.layout.main);  

    TextView tv=(TextView)findViewById(R.id.introclusion_tv1);
    tv.setTypeface(FontFactory.getBFantezy(getBaseContext()));

    TextView tv1=(TextView)findViewById(R.id.introclusion_tv2);
    tv1.setTypeface(FontFactory.getBFantezy(getBaseContext()));
    tv1.setText(Html.fromHtml(getString(R.string.introclusion)));



button = (Button) findViewById(R.id.button1);

// add button listener
button.setOnClickListener(new OnClickListener() {

    @Override
    public void onClick(View arg0) {

        LayoutInflater li = LayoutInflater.from(context);
        View dialog_layoutView = li.inflate(R.layout.dialog_layout, null);
        AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(context);
        alertDialogBuilder.setView(dialog_layoutView);
        alertDialogBuilder

                .setCancelable(false)
                .setPositiveButton("OK",

                        new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog,int id) {

                                EditText username = (EditText) findViewById(R.id.txt_name);
                                EditText password = (EditText) findViewById(R.id.password);

                                if(username.getText().toString().length() > 0 && password.getText().toString().length() > 0 ) {
                                    if(username.getText().toString().equals("test") && password.getText().toString().equals("test")) {
                                Intent intent = new Intent(MainActivity.this,Text.class);
                                startActivity(intent);
                                finish();}
                                    else{
                                        // get your custom_toast.xml layout
                                        LayoutInflater inflater = getLayoutInflater();

                                        View layout = inflater.inflate(R.layout.custom_toast,
                                                (ViewGroup) findViewById(R.id.custom_toast));

                                        // set a dummy image
                                        ImageView image = (ImageView) layout.findViewById(R.id.image_toast);
                                        image.setImageResource(R.drawable.ic_launcher);

                                        // set a message
                                        TextView text = (TextView) layout.findViewById(R.id.text_toast);
                                        text.setText("Wrong username or password");

                                        // Toast...
                                        Toast toast = new Toast(getApplicationContext());
                                        toast.setGravity(Gravity.CENTER_VERTICAL, 0, 0);
                                        toast.setDuration(Toast.LENGTH_LONG);
                                        toast.setView(layout);
                                        toast.show();}}}    


                        })
                .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();

    }

});

  }

  }

LOGCAT:

  java.lang.NullPointerException
at com.example.demo.MainActivity$1$1.onClick(MainActivity.java:63)
at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:163)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:130)
at android.app.ActivityThread.main(ActivityThread.java:3687)
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:867)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
at dalvik.system.NativeStart.main(Native Method)

dialog_layout.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
android:padding="10sp" >

<EditText
    android:id="@+id/txt_name"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:hint="@string/dialog_uname"
    android:singleLine="true" >

    <requestFocus />
</EditText>

<EditText
    android:id="@+id/password"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:ems="10"
    android:inputType="textPassword" >
</EditText>

回答1:

Change

LayoutInflater li = LayoutInflater.from(context);

to

LayoutInflater li = LayoutInflater.from(arg0.getContext());

and change

AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(context);

to

AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(arg0.getContext());

Your context variable is null because you initialize it before the Activity is created. And you don't need to create a context variable inside of an Activity. If, for whatever reason (maybe its simplicity) you want to use the variable then declare it where you are but initialize it inside of onCreate()

Also, inside the onClick() signature, I would change the variable name to something meaningful but that's not required. Like chanig

public void onClick(View arg0)

to

public void onClick(View v)

Edit

You also need to change these lines

EditText username = (EditText) findViewById(R.id.txt_name);
EditText password = (EditText) findViewById(R.id.password);

to

EditText username = (EditText) dialog_layoutView.findViewById(R.id.txt_name);
EditText password = (EditText) dialog_layoutView.findViewById(R.id.password);

This will tell them to look in dialog_layout. Currently they are looking in the layout that you inflated with setContentView()

来源：https://stackoverflow.com/questions/18861082/alert-dialog-positive-button-issue