Android自用-----WindowManager$BadTokenException: Unable to add window -- token null is not for an application

错误产生：

private Context mcontext;

    @Override
    protected void onCreate(Bundle savedInstanceState) {mcontext = getApplicationContext();
        System.out.println("mcontext=" + mcontext);

    }

new AlertDialog.Builder(mcontext)
            .setIcon(android.R.drawable.ic_dialog_alert)
            .setTitle("Warnning")
            .setMessage(
                    "You forget to write the message. Do you want to fill out it ??")
            .setPositiveButton("Yes", positiveListener).setNegativeButton(
                    "No", negativeListener).create().show();

导致报这个错是在于new AlertDialog.Builder(mcontext)，虽然这里的参数是AlertDialog.Builder(Context context)但我们不能使用getApplicationContext()获得的Context,而必须使用Activity,因为只有一个Activity才能添加一个窗体。 

解决方法：将new AlertDialog.Builder(Context context)中的参数用Activity.this（Activity是你的Activity的名称）来填充就可以正确的创建一个Dialog了。

new AlertDialog.Builder(MyActivity.this)
                .setIcon(android.R.drawable.ic_dialog_alert)
                .setTitle("Warnning")
                .setMessage(
                        "You forget to write the message. Do you want to fill out it ??")
                .setPositiveButton("Yes", positiveListener).setNegativeButton(
                        "No", negativeListener).create().show();

 

 问题得以解决！