android expandlistview arrow
<p>I know how to make an expandedlistview, but I don't know what should i use to see the arrow down and up , how please ?</p>

<p> like when i open a group the arrow becomes down and when i close it , the arrow becomes up </p>

<h3> Main xml </h3>

<p><img src="http://i.stack.imgur.com/N4mCa.png" alt="enter image description here"></p>

<pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" &gt;

 &lt;ExpandableListView

    android:id="@+id/ExpList"

    android:layout_height="match_parent"

    android:layout_width="match_parent"

    android:groupIndicator="@null" /&gt; 

&lt;/LinearLayout&gt;
</code></pre>

<h3> Group xml </h3>

<pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="55dip"
    android:background="#FF0000"
    android:orientation="vertical" &gt;


    &lt;TextView
        android:id="@+id/tvGroup"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginRight="15dp"
        android:textColor="#000000"
        android:textSize="17dip" /&gt;

&lt;/LinearLayout&gt;
</code></pre>

<h3> child xml </h3>

<pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="55dip"
    android:background="#FF0000"
    android:orientation="vertical" &gt;


    &lt;TextView
        android:id="@+id/tvChild"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:textColor="#000000"
        android:textSize="17dip" /&gt;

&lt;/LinearLayout&gt;
</code></pre>
