Use variable version numbers

app/build.gradle

@@ -45,22 +45,29 @@ repositories {
 dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])
 
+    def support_version = "28.0.0-alpha1"
+    def constraintlayout_version = "1.1.2" // 2.0.0-alpha1
+    def lifecycle_version = "1.1.1"
+    def room_version = "1.1.1"
+    def javamail_version = "1.6.0"
+    def jsoup_version = "1.11.3"
+
     // https://developer.android.com/topic/libraries/support-library/revisions.html
-    implementation 'com.android.support:appcompat-v7:28.0.0-alpha1'
-    implementation 'com.android.support.constraint:constraint-layout:1.1.+'
-    implementation 'com.android.support:recyclerview-v7:28.0.0-alpha1'
-    implementation 'com.android.support:design:28.0.0-alpha1'
+    implementation "com.android.support:appcompat-v7:$support_version"
+    implementation "com.android.support.constraint:constraint-layout:$constraintlayout_version"
+    implementation "com.android.support:recyclerview-v7:$support_version"
+    implementation "com.android.support:design:$support_version"
 
     // https://developer.android.com/topic/libraries/architecture/adding-components.html
-    implementation "android.arch.lifecycle:extensions:1.1.+"
-    implementation "android.arch.persistence.room:runtime:1.1.+"
-    annotationProcessor "android.arch.lifecycle:compiler:1.1.+"
-    annotationProcessor "android.arch.persistence.room:compiler:1.1.+"
+    implementation "android.arch.lifecycle:extensions:$lifecycle_version"
+    implementation "android.arch.persistence.room:runtime:$room_version"
+    annotationProcessor "android.arch.lifecycle:compiler:$lifecycle_version"
+    annotationProcessor "android.arch.persistence.room:compiler:$room_version"
 
     // https://javaee.github.io/javamail/
-    implementation 'com.sun.mail:android-mail:1.6.0'
-    implementation 'com.sun.mail:android-activation:1.6.0'
+    implementation "com.sun.mail:android-mail:$javamail_version"
+    implementation "com.sun.mail:android-activation:$javamail_version"
 
     // https://jsoup.org/
-    implementation 'org.jsoup:jsoup:1.11.3'
+    implementation "org.jsoup:jsoup:$jsoup_version"
 }