Example of auto-generating the versionCode

build.gradle

apply plugin: 'com.android.application'

// version information
def versionMajor = 1
def versionMinor = 0
def versionPatch = 0

android {
    compileSdkVersion 26
    buildToolsVersion "26.0.2"
    defaultConfig {
        applicationId "com.example"
        minSdkVersion 19
        targetSdkVersion 26
        versionCode versionMajor * 10000 + versionMinor * 100 + versionPatch
        versionName "${versionMajor}.${versionMinor}.${versionPatch}"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
}