Dialog

In the Android tutorial, it gives some examples to Instantiate AlertDialog.Builder like this.

AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());

But getActivity() is a method of the Fragment class, can not apply to everywhere. 

The correct way to do it is use

AlertDialog.Builder builder = new AlertDialog.Builder(this);

or when calling from an inner class, use

AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);

MainActivity here is your main activity name.

This is one example how to build a single selection dialog.

private String[]fruits = new String[]{"荔枝","葡萄","栗子","西瓜"};     private TextView dia_schsres = null; … protected void onCreateDialog() {                 Dialog schDia = new AlertDialog.Builder(ExampleActivity.this);     //AlertDialog.Builder or AlertDialog or Dialog                     schDia.setIcon(R.drawable.ic_launcher).setTitle("Background").                         setItems(fruits,new DialogInterface.OnClickListener() {                             @Override                             public void onClick(DialogInterface dia, int which) {                                 dia_schsres.setText(fruits[which]);                             }                         }).setNegativeButton("Cancel",new DialogInterface.OnClickListener() {                     @Override                     public void onClick(DialogInterface dia, int which) {                         dia.dismiss();                     }                 }).create(); }