How to set my Activity as main activity in android? [duplicate]

In your manifest file , use the below code to declare an activity as a launcher activity:

<activity android:name=".yourActivityName" >
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

From Android Developer docs:

ACTION_MAIN activity: Start up as the initial activity of a task, with no data input and no returned output.

CATEGORY_LAUNCHER: The activity can be the initial activity of a task and is listed in the top-level application launcher`.

In AndroidManifest.xml file inside application tag add an activity tag and remove action MAIN from old activity tag set that as default

 <application...... >
    <activity
        android:name=".DefaultActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.DEFAULT" />
        </intent-filter>
    </activity>
    <activity
        android:name=".NewActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

 </application>