implement an Espresso test

app/build.gradle

@@ -10,6 +10,8 @@ android {
         targetSdkVersion 23
         versionCode 1
         versionName "1.0"
+
+        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
     }
     buildTypes {
         release {
@@ -25,5 +27,10 @@ dependencies {
     compile 'com.jakewharton:butterknife:7.0.1'
 
     testCompile 'junit:junit:4.12'
-    testCompile "org.robolectric:robolectric:3.0"
+    testCompile 'org.robolectric:robolectric:3.0'
+
+    androidTestCompile 'com.android.support:support-annotations:23.1.1'
+    androidTestCompile 'com.android.support.test:runner:0.4.1'
+    androidTestCompile 'com.android.support.test:rules:0.4.1'
+    androidTestCompile 'com.android.support.test.espresso:espresso-core:2.2.1'
 }

app/src/androidTest/java/calculator/simplemobiletools/com/simple_calculator/ApplicationTest.java

@@ -3,11 +3,8 @@ package calculator.simplemobiletools.com.simple_calculator;
 import android.app.Application;
 import android.test.ApplicationTestCase;
 
-/**
- * <a href="http://d.android.com/tools/testing/testing_android.html">Testing Fundamentals</a>
- */
 public class ApplicationTest extends ApplicationTestCase<Application> {
     public ApplicationTest() {
         super(Application.class);
     }
-}
+}

app/src/androidTest/java/calculator/simplemobiletools/com/simple_calculator/MainActivityTest.java

@@ -0,0 +1,27 @@
+package calculator.simplemobiletools.com.simple_calculator;
+
+import android.support.test.rule.ActivityTestRule;
+import android.support.test.runner.AndroidJUnit4;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import static android.support.test.espresso.Espresso.onView;
+import static android.support.test.espresso.action.ViewActions.click;
+import static android.support.test.espresso.assertion.ViewAssertions.matches;
+import static android.support.test.espresso.matcher.ViewMatchers.withId;
+import static android.support.test.espresso.matcher.ViewMatchers.withText;
+
+@RunWith(AndroidJUnit4.class)
+public class MainActivityTest {
+    @Rule
+    public ActivityTestRule<MainActivity> mActivityRule =
+            new ActivityTestRule<>(MainActivity.class);
+
+    @Test
+    public void addDigitTest() {
+        onView(withId(R.id.btn_5)).perform(click());
+        onView(withId(R.id.result)).check(matches(withText("5")));
+    }
+}