I wouldn't say it is the hardest, but from all the languages that I have worked in, and from a first impression, Java bogs you down with a lot of details.
Compare this 👇
class HelloWorldApp {
public static void main(String[] args) {
System.out.println("Hello, world!");
}
}
...with a Kotlin equivalent
fun main(args : Array<String>) {
println("Hello, world!")
}