Example of managing dependencies in separate file

build.gradle

apply from: 'deps.gradle'

// ...

dependencies {
    compile supportLibs
    compile rxJavaLibs
    compile retrofitLibs
    compile okHttpLibs

    compile 'com.jakewharton:butterknife:7.0.0' // view injection
    compile 'com.github.bumptech.glide:glide:3.7.0'
    compile 'com.f2prateek.dart:dart:1.1.0' // activity arguments arguments inject

    testCompile unitTestLibs
    androidTestCompile androidTestsLibs
}

deps.gradle

ext {
    versions = [
            okHttp             : '3.3.1',
            support            : '25.1.0',
            retrofit           : '2.0.2',
            espressoVersion    : '2.2.2',
            BUILD_TOOLS_VERSION: '25.0.1',
            MIN_SDK_VERSION    : 14,
            TARGET_SDK_VERSION : 25
    ]
    supportDeps = [
            appcompatV7      : "com.android.support:appcompat-v7:$versions.support",
            recyclerView     : "com.android.support:recyclerview-v7:$versions.support",
            supportAnnotation: "com.android.support:support-annotations:$versions.support",
            design           : "com.android.support:design:$versions.support",
            gridLayout       : "com.android.support:gridlayout-v7:$versions.support"
            cardView         : "com.android.support:cardview-v7:$versions.support"
            palette          : "com.android.support:palette-v7:$versions.support"
    ]
    rxJava = [
            rxJava   : 'io.reactivex:rxjava:1.1.6',
            rxAndroid: 'io.reactivex:rxandroid:1.2.1'
    ]
    retrofit = [
            retrofit     : "com.squareup.retrofit2:retrofit:$versions.retrofit",
            rxAdapter    : "com.squareup.retrofit2:adapter-rxjava:$versions.retrofit",
            gsonConverter: "com.squareup.retrofit2:converter-gson:$versions.retrofit"
    ]
    okHttp = [
            logger: "com.squareup.okhttp3:logging-interceptor:$versions.okHttp",
            self  : "com.squareup.okhttp3:okhttp:$versions.okHttp"
    ]
    unitTests = [
            supportAnnotation: "com.android.support:support-annotations:$versions.support",
            junit            : 'junit:junit:4.12',
            robolectric      : 'org.robolectric:robolectric:3.0',
            assertj          : 'com.squareup.assertj:assertj-android:1.0.0'
    ]
    androidTests = [
            espressoCore   : "com.android.support.test.espresso:espresso-core:$versions.espressoVersion",
            espressoContrib: "com.android.support.test.espresso:espresso-contrib:$versions.espressoVersion",
            espressoIntents: "com.android.support.test.espresso:espresso-intents:$versions.espressoVersion",
            testRunner     : "com.android.support.test:runner:0.5",
            testRules      : "com.android.support.test:rules:0.5"
    ]

    supportLibs = supportDeps.values()
    retrofitLibs = retrofit.values()
    okHttpLibs = okHttp.values()
    rxJavaLibs = rxJava.values()
    unitTestLibs = unitTests.values()
    androidTestsLibs = androidTests.values() + supportLibs
}