[android help] First Android app not running

First Android app not running

I am following the steps as in android training google and my activity_main.xml is like

    
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal">
            android:layout_weight="1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:hint="@string/edit_message" />
            android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_send" />

I have defined the hint strings

    My First App
    Enter a message
    Send
    mainActivity
    Settings
    Hello world!

But when I click on run button, it shows no activity !

.

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