显示警报对话框 Android

Question

我试图用 android 显示一个简单的对话框，但它在开始时崩溃了。

MainActivity.java :

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        MessageDialog messageDialog = new MessageDialog();
        Dialog dialog = messageDialog.onCreateDialog(savedInstanceState);
        dialog.show();
    }
}

MessageDialog.java :

public class MessageDialog extends DialogFragment {
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {

        FragmentActivity activity = getActivity();
        AlertDialog.Builder builder = new AlertDialog.Builder(activity);
        builder.setMessage("Hello World").setPositiveButton("0K", null);

        return builder.create();
    }
}

错误：

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.*****.*****, PID: 7458 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.*****.*****/com.example.*****.*****.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources$Theme android.content.Context.getTheme()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2924) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2985) at android.app.ActivityThread.-wrap14(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1635) at android.os.Handler.dispatchMessage(Handler.java:102)

Answer 1

你为什么打电话给 "onCreateDialog"？

使用以下代码显示对话框片段：

MessageDialog dialog = new MessageDialog();
dialog.show(getSupportFragmentManager(), "fragment_name");

(第二个参数是分片的tag，以后可以通过"FragmentManager#findFragmentByTag"获取分片)

显示警报对话框 Android

Display AlertDialog Android

android

android-alertdialog