gradle: Skipping task ':compileJava' as it has no source files

By default, Java source files need to go into src/main/java (or src/test/java for test sources). Either adapt your directory structure accordingly, or reconfigure the source directory as follows:

sourceSets {
    main {
        java {
            srcDirs = ["src"]
        }
    }
}

You can also change gradle version to 4.8 in

gradle-wrapper.properties 
distributionUrl=https\://services.gradle.org/distributions/gradle-4.8-bin.zip

then do:

./gradlew build