How to set dropdown arrow in spinner?

Basically one needs to create a custom background for a spinner. It should be something like this:

spinner_background.xml

<selector xmlns:android="http://schemas.android.com/apk/res/android">

    <item>

        <layer-list>

            <item>
                <color
                    android:color="@android:color/white"/>
            </item>

            <item>
                <bitmap
                    android:gravity="center_vertical|right"
                    android:src="@drawable/ic_arrow_drop_down_black_24dp"/>
            </item>

        </layer-list>

    </item>

</selector>

Then create a custom style for your spinner, where you specify the above selector as background:

<style name="Widget.App.Spinner" parent="@style/Widget.AppCompat.Spinner">
    <item name="overlapAnchor">true</item>
    <item name="android:background">@drawable/spinner_background</item>
</style>

And finally in your app theme you should override two attributes if you want it to be applied all across your app:

<item name="spinnerStyle">@style/Widget.App.Spinner</item>
<item name="android:spinnerStyle">@style/Widget.App.Spinner</item>

And that's pretty much it.

copy and paste this xml instead of your xml

<?xml version="1.0" encoding="UTF-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/back1"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <LinearLayout
        android:id="@+id/linearLayout1"
        android:layout_width="wrap_content"
        android:layout_height="55dp"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentTop="true"
        android:layout_marginTop="20dp" 
        android:background="@drawable/red">

        <Spinner
            android:id="@+id/spinner1"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:dropDownWidth="fill_parent" 
            android:background="@android:drawable/btn_dropdown"
         />

    </LinearLayout>

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="55dp"
        android:layout_alignLeft="@+id/linearLayout1"
        android:layout_alignRight="@+id/linearLayout1"
        android:layout_below="@+id/linearLayout1"
        android:layout_marginTop="25dp"
        android:background="@drawable/red"
        android:ems="10"
        android:hint="enter card number" >

        <requestFocus />
    </EditText>

    <LinearLayout
        android:id="@+id/linearLayout2"
        android:layout_width="wrap_content"
        android:layout_height="55dp"
        android:layout_alignLeft="@+id/editText1"
        android:layout_alignRight="@+id/editText1"
        android:layout_below="@+id/editText1"
        android:layout_marginTop="33dp"
        android:orientation="horizontal" 
        android:background="@drawable/red">

        <Spinner
            android:id="@+id/spinner3"
            android:layout_width="72dp"
            android:layout_height="wrap_content"
            android:background="@android:drawable/btn_dropdown"
             />

        <Spinner
            android:id="@+id/spinner2"
            android:layout_width="72dp"
            android:layout_height="wrap_content" 
            android:background="@android:drawable/btn_dropdown"
            />

        <EditText
            android:id="@+id/editText2"
            android:layout_width="22dp"
            android:layout_height="match_parent"
            android:layout_weight="0.18"
            android:ems="10"
            android:hint="enter cvv" />

    </LinearLayout>

    <LinearLayout
        android:id="@+id/linearLayout3"
        android:layout_width="wrap_content"
        android:layout_height="55dp"
        android:layout_alignParentLeft="true"
        android:layout_alignRight="@+id/linearLayout2"
        android:layout_below="@+id/linearLayout2"
        android:layout_marginTop="26dp"
        android:orientation="vertical"
        android:background="@drawable/red" >
    </LinearLayout>

    <Spinner
        android:id="@+id/spinner4"
        android:layout_width="15dp"
        android:layout_height="18dp"
        android:layout_alignBottom="@+id/linearLayout3"
        android:layout_alignLeft="@+id/linearLayout3"
        android:layout_alignRight="@+id/linearLayout3"
        android:layout_alignTop="@+id/linearLayout3"
        android:background="@android:drawable/btn_dropdown"
      />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true"
        android:layout_below="@+id/linearLayout3"
        android:layout_marginTop="18dp"
        android:text="Add Amount" 
        android:background="@drawable/buttonsty"/>
</RelativeLayout>

As a follow-up to another answer, I was asked how I changed the spinner icon to get something like this:

One pretty easy way is to use a custom spinner item layout:

Spinner spinner = (Spinner) findViewById(R.id.spinner);
ArrayAdapter<String> adapter = new ArrayAdapter<String>(
        this,
        R.layout.view_spinner_item,
        ITEMS
);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(adapter);

In res/layout/view_spinner_item.xml, define a TextView with android:drawableRight pointing to the desired icon (along with any customisations to text size, paddings and so on, if you wish):

<?xml version="1.0" encoding="utf-8"?>
<!-- Custom spinner item layout -->
<TextView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@android:id/text1"
    style="?android:attr/spinnerItemStyle"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:singleLine="true"
    android:textSize="@dimen/text_size_medium"
    android:drawablePadding="@dimen/spacing_medium"
    android:drawableRight="@drawable/ic_arrow_down"
    />

(For the opened state, just use android.R.layout.simple_spinner_dropdown_item or similarly create a customised layout if you want to tweak every aspect of your spinner.)

To get the background & colours looking nice, set the Spinner's android:background and android:popupBackground as shown in that other question. And if you were wondering about the custom font in the screenshot above, you'll need a custom SpinnerAdapter.