Gradle warning: variant.getOutputFile() and variant.setOutputFile() are deprecated

Building on the answer from Larry Schiefer you can change the script to something like this:

android {
    applicationVariants.all { variant ->
        variant.outputs.each { output ->
            def outputFile = output.outputFile
            if (outputFile != null && outputFile.name.endsWith('.apk')) {
                def fileName = outputFile.name.replace('.apk', "-${versionName}.apk")
                output.outputFile = new File(outputFile.parent, fileName)
            }
        }
    }
}

The complete code snippet looks like that one:

// Customize generated apk's name with version number
applicationVariants.all { variant ->
    variant.outputs.each { output ->
        def outputFile = output.outputFile
        if (outputFile != null && outputFile.name.endsWith('.apk')) {
            def manifestParser = new com.android.builder.core.DefaultManifestParser()
            def fileName = outputFile.name.replace(".apk", "-DEBUG-" + manifestParser.getVersionName(android.sourceSets.main.manifest.srcFile) + ".apk")
            output.outputFile = new File(outputFile.parent, fileName)
        }
    }
}

The build variant output API has changed in the latest Android Gradle plugin. It now allows multiple output files (or directories), which is why this method has been marked as deprecated. If you use variant.outputs instead, it will give you a Collection you can then iterate over and get each output file. You'll have to verify the file object is non-null and that it matches your criteria (e.g. has a '.apk' extension.) Then you can create a new File object and add it to the output within the collection.

Android Plugin for Gradle 3.0.0

You can use like this

android.applicationVariants.all { variant ->
    variant.outputs.all {
        outputFileName = "${variant.name}-${variant.versionName}.apk"
    }
}

you can get more on the features and new changes in android documentation https://developer.android.com/studio/build/gradle-plugin-3-0-0-migration.html#update_gradle