A user interface (UI) is the primary means by which a user interacts with an application. The user interfaces of Android applications are made by the creation and manipulation of layout files. Layout files are XML files that exist in app | res | layout.

To create the layout for the HelloApp, we are going to do three things:

1. Add a LinearLayout to our layout file
2. Place the TextView within the LinearLayout and remove the android:text attribute it possesses
3. Add a button to the LinearLayout

Open the activity_hello.xml file if it's not already opened. You will be presented with the layout editor. If the editor is in the Design view, change it to its Text view by toggling the option at the bottom of the layout editor. Now, your layout editor should look similar to that of the following screenshot:

A LinearLayout is a ViewGroup that arranges child views in either a horizontal or vertical manner within a single column. Copy the code snippet of our required LinearLayout from the following block and paste it within the ConstraintLayout preceding the TextView:

<LinearLayout
       android:id="@+id/ll_component_container"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:orientation="vertical"
       android:gravity="center">
</LinearLayout>

Now, copy and paste the TextView present in the activity_hello.xml file into the body of the LinearLayout element and remove the android:text attribute:

<LinearLayout
       android:id="@+id/ll_component_container"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:orientation="vertical"
       android:gravity="center">
  <TextView
       android:id="@+id/tv_greeting"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:textSize="50sp" />
</LinearLayout>

Lastly, we need to add a button element to our layout file. This element will be a child of our LinearLayout. To create a button, we use the Button element:

<LinearLayout
       android:id="@+id/ll_component_container"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:orientation="vertical"
       android:gravity="center">
  <TextView
       android:id="@+id/tv_greeting"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:textSize="50sp" />
  <Button
       android:id="@+id/btn_click_me"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:layout_marginTop="16dp"
       android:text="Click me!"/>
</LinearLayout>

Toggle to the layout editor's design view to see how the changes we have made thus far translate when rendered on the user interface:

Now we have our layout, but there's a problem. Our CLICK ME! button does not actually do anything when clicked. We are going to fix that by adding a listener for click events to the button. Locate and open the HelloActivity.java file and edit the function to add the logic for the CLICK ME! button's click event as well as the required package imports, as shown in the following code:

package com.mydomain.helloapp
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.text.TextUtils
import android.widget.Button
import android.widget.TextView
import android.widget.Toast

class HelloActivity : AppCompatActivity() {

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_hello)
    val tvGreeting = findViewById<TextView>(R.id.tv_greeting)
    val btnClickMe = findViewById<Button>(R.id.btn_click_me)

    btnClickMe.setOnClickListener {
      if (TextUtils.isEmpty(tvGreeting.text)) {
        tvGreeting.text = "Hello World!"
      } else {
        Toast.makeText(this, "I have been clicked!", 
                       Toast.LENGTH_LONG).show()
      }
    }
  }
}

In the preceding code snippet, we have added references to the TextView and Button elements present in our activity_hello layout file by utilizing the findViewById function. The findViewById function can be used to get references to layout elements that are within the currently-set content view. The second line of the onCreate function has set the content view of HelloActivity to the activity_hello.xml layout.

Next to the findViewById function identifier, we have the TextView type written between two angular brackets. This is called a function generic. It is being used to enforce that the resource ID being passed to the findViewById belongs to a TextView element.

After adding our reference objects, we set an onClickListener to btnClickMe. Listeners are used to listen for the occurrence of events within an application. In order to perform an action upon the click of an element, we pass a lambda containing the action to be performed to the element's setOnClickListener method. 

When btnClickMe is clicked, tvGreeting is checked to see whether it has been set to contain any text. If no text has been set to the TextView, then its text is set to Hello World!, otherwise a toast is displayed with the I have been clicked! text.