Simple "Hello World" dialog example in Android studio.

In my case I started new application, and left everything by default.

In content_main.xml (in my case it is located in \app\src\main\res\layout\content_main.xml) I added button:

<Button
	android:layout_width="wrap_content"
	android:layout_height="wrap_content"
	android:text="New Button"
	android:id="@+id/button"
	android:layout_below="@+id/textView"
	android:layout_alignParentLeft="true"
	android:layout_alignParentStart="true"
	android:layout_marginTop="53dp"
	android:nestedScrollingEnabled="false"
	android:onClick="testMessage"
/>

Here notice "android:onClick="testMessage""

Then in MainActivity.java (in my case it is located in \app\src\main\java\com\example\smi\msearch\MainActivity.java) I created method like:

public void testMessage(View view) {
	AlertDialog builder = new AlertDialog.Builder(MainActivity.this).create();
	builder.setTitle("Yo!");
	builder.setMessage("This is a test!");
	builder.show();
}