android 动态向下箭头 向上/向下箭头添加到android numberpicker

我做了一个数字选择对话框.它看起来像左边的图像,但没有上/下箭头：

我该如何添加它们？

这是我的代码：

public class NumberPickerFragment extends DialogFragment

implements DialogInterface.OnClickListener {

protected final String CLS = this.getClass().getSimpleName();

@Override

public Dialog onCreateDialog(Bundle savedInstanceState) {

// TODO: Make this more flexible

NumberPicker np = new NumberPicker(getActivity());

np.setMaxValue(250);

np.setMinValue(100);

np.setValue(150);

np.setDescendantFocusability(NumberPicker.FOCUS_BLOCK_DESCENDANTS);

np.setWrapSelectorWheel(false);

return new AlertDialog.Builder(getActivity())

.setTitle(R.string.height)

.setView(np)

.setPositiveButton(android.R.string.ok,this)

.create();

}

@Override

public void onClick(DialogInterface dialog,int whichButton) {

// TODO: Do something with the return value

Log.d(CLS,"Received click");

}

}