How to specify source and target compatibility in Java module?

Question

I have a Gradle project consisting of an Android module (the com.android.library plugin is applied in the build.gradle file) and a Java module (the

Answer 1

(As pointed out by the OP, this answer doesn't really fit the question. I'd like to keep it here at the bottom though, just in case somebody else lands on this page searching for the same problem as me)

For android, the compileJava block that Jeroen Wijdemans suggested, does not work. What does work is specifying in the app build.gradle

// https://developer.android.com/studio/write/java8-support.html
// Configure only for each module that uses Java 8
// language features (either in its source code or
// through dependencies).
compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
}

Currently, intellij 2017.2.6 tells me then that you need to enable Jack. So let's do that as well, by adding the following inside the android block:

    // https://stackoverflow.com/questions/37004069/errorjack-is-required-to-support-java-8-language-features
    jackOptions {
        enabled true
    }

If building now results in errors or warnings, simply rebuilding the whole project might get rid of them. (It did work for me)